Every script, three topics — a flat page for a quick scan

Here are a spoon, a glass of juice and a balloon of air.

The spoon keeps its shape wherever you put it.

The juice flows into any shape you pour it into.

The air spreads out to fill whatever holds it.

The spoon, the juice and the air behave in three different ways.

So why does the spoon hold its shape when the juice and the air cannot, and what are all three made of?

To find out, we will look at what everything is made of, and why solids, liquids and gases behave as they do.

Okay, so are you ready to find out what a spoon, a glass of juice and a balloon of air are made of?

Here is a flat soccer ball on a balance.

The balance reads 412 grams.

A student pumps the ball up hard and puts it back.

Now the balance reads 417 grams.

The balance measures how much material is in the ball, and scientists call that amount its mass.

Only air went in, so the air has mass, about 5 grams of it.

And the pumped ball is bigger, so the air takes up space.

Anything that has mass and takes up space is called matter.

Air has mass and takes up space, so air is matter.

Here are some more examples.

A stone has mass and takes up space, so a stone is matter.

Water has mass and takes up space, so water is matter.

But now consider the light from a lamp.

Light has no mass and takes up no space, so light is not matter.

And a shadow has no mass and takes up no space, so a shadow is not matter.

So, here's what we've seen. Matter is anything that has mass and takes up space.

So the extra 5 grams on the balance was air, and air is matter.

Okay, now your turn.

Here is a glass with 200 milliliters of water.

A student tips in 10 grams of sugar and stirs.

The grains shrink, and vanish.

The water looks clear.

She sips the water, and the water tastes sweet.

So the sugar is still there.

Let's zoom in.

Where did the sugar go?

It broke into pieces far too small to see.

And those pieces spread all through the water.

Scientists call these tiny pieces particles.

One particle is far too small to see, and far too small to taste.

A grain of sugar you can see is billions of particles packed together.

The water is made of particles too.

And it is not only sugar and water.

A stone is made of particles.

The air in a soccer ball is made of particles.

So, here's what we've seen. All matter is made of particles that are far too small to see.

The sugar is still in the glass, as billions of particles too small to see.

Okay, now your turn.

Here are students in a quiet classroom, seen from above. The windows are shut, and no fan is running.

The teacher opens a bottle of perfume at the front.

Twenty seconds later, the back row can smell it, and nobody carried the smell to them.

So how did the perfume cross the room?

Its particles moved across by themselves.

Perfume is matter, so perfume is made of particles.

Some perfume particles leave the open bottle.

Particles are always moving.

The air is made of particles too, and they are moving.

The perfume particles bump into air particles.

They bounce off and keep moving, in every direction.

So the perfume particles spread through the whole room.

After twenty seconds, some reach the back row.

Now consider a glass of still water, with one drop of food coloring and no stirring.

By the next morning the whole glass is colored.

Particles never stop moving, so the coloring particles moved through the whole glass.

Particles are always moving.

They never stop.

So in the classroom, moving perfume particles spread from the bottle to the back row.

That is how the smell crossed a still room.

Okay, now your turn.

Here are two mugs, each with 250 milliliters of water.

This one is at 10 degrees Celsius, and this one is at 80 degrees Celsius.

A student drops a tea bag into each, and does not stir.

After thirty seconds, the hot water is brown all the way through.

The cold water shows only a faint cloud around the bag.

The tea and the water are the same in both mugs, so what is different about the hot water?

Its particles move faster.

Let's zoom into the water.

Water is made of particles, and particles are always moving.

Heating a substance makes its particles move faster.

Cooling makes them move more slowly.

Tea particles leave the bag and bump into the moving water particles.

Faster water particles knock the tea particles around faster.

So the brown color spreads through the hot mug sooner.

The particles do not grow or shrink; only their speed changes.

So, heating makes particles move faster. Cooling makes them move more slowly.

So in the two mugs, the tea and the water were the same.

Heating only made the hot water's particles move faster, so the color spread sooner.

Okay, now your turn.

Here is a student pouring dry sand into a jar. It takes the jar's shape, just as water would.

She says sand is a liquid, because it pours.

We ask two questions: does it keep its own shape, and does it keep its own volume?

A steel spoon keeps its shape and volume in a cup, a bowl and a pitcher, so it is a solid.

Here is 250 milliliters of juice.

In the glass it stands tall; in a wide bowl it lies flat.

A measuring cup still reads 250 milliliters.

It takes its container's shape but keeps its volume, so the juice is a liquid.

Here is 20 milliliters of air sealed in a syringe.

She pulls the plunger out, and the air fills 60 milliliters.

She pushes it in, and the same air fits into 10 milliliters.

It keeps neither shape nor volume, so air is a gas.

Honey flows slowly, but it takes its jar's shape and keeps its volume, so honey is a liquid.

Back at the sand, one grain keeps its own shape and volume, so it is a solid.

A jar of sand is thousands of tiny solids.

So, here's what we've seen.

A solid keeps its own shape and its own volume.

A liquid keeps its own volume but takes the shape of its container.

A gas keeps neither: it spreads to fill whatever space it is given.

So, when she said sand is a liquid, she was not right. Sand is a solid, in very small pieces.

Okay, now your turn.

Here is a steel spoon in a cup.

Now the spoon goes into a bowl, then into a pitcher.

It stays spoon-shaped every time.

Water poured into the same three containers takes three different shapes.

So what holds the spoon in one shape?

The particles inside it do, so let's zoom into the steel.

In a solid the particles are packed tightly, touching, in fixed positions.

Particles are always moving, and each particle here vibrates on the spot.

Each one stays in its place.

No particle can move past its neighbors.

So the particles keep the same arrangement wherever the spoon goes.

The whole spoon holds one shape, and one volume.

Now consider an ice cube dropped into a glass of juice.

Ice is a solid.

Its particles cannot move past their neighbors.

So the cube keeps its shape.

So, in a solid the particles are packed tightly, touching, in fixed positions. Each particle vibrates on the spot.

That is why a solid keeps its shape and its volume.

So the spoon's particles cannot move past one another, and it stays spoon-shaped in the cup, the bowl and the pitcher.

Okay, now your turn.

Here is 250 milliliters of juice, poured from a tall glass into a wide bowl.

The juice flows and flattens, but a steel spoon in the bowl keeps its shape.

So why can the juice change shape while its volume stays 250 milliliters?

Its particles can slide. Let's zoom in, with a box of the spoon's particles on the left to compare.

In a liquid the particles still touch.

But they have no fixed positions.

The particles slide past one another.

When the juice pours into the bowl, the sliding particles rearrange to match the bowl's shape.

So the liquid takes the shape of its container.

But the particles stay touching, so the juice takes up the same space as before.

So the liquid keeps its volume.

Now consider honey poured slowly into a jar.

Its particles slide past one another too, so the honey takes the jar's shape.

So, in a liquid the particles touch but have no fixed positions. They slide past one another.

That is why a liquid takes the shape of its container but keeps its volume.

So the juice's sliding particles rearranged to fit the bowl and stayed touching.

Okay, now your turn.

Here is a sealed syringe with 20 milliliters of air.

A student pulls the plunger out, and the air fills 60 milliliters.

She pushes it in, and the same air fits into 10 milliliters.

How did the same air fit into less space?

Its particles moved closer together. Let's zoom in.

In a gas the particles are far apart, with empty space between them.

That space is truly empty: nothing is between the particles, not even air.

The particles fly fast in every direction.

Nothing holds them together, so they keep going until they hit a wall.

Now let's watch six air particles in the syringe.

She pulls the plunger out, and the same six spread through the bigger space.

She pushes it in, and the same six crowd into the smaller space.

The particles stay the same size; only the empty space between them grows or shrinks.

So, in a gas the particles are far apart with empty space between them. They fly fast in every direction.

That is why a gas fills any container it is given.

So in the syringe, the same particles spread into 60 milliliters and crowded into 10 milliliters.

Okay, now your turn.

Here are three particle boxes: ice, water and steam.

Ice, liquid water and steam are all water, so all three boxes hold the same water particles.

In the ice, each particle vibrates on the spot.

In the water, the particles slide past one another.

In the steam, the particles fly fast in every direction.

Which particles carry the most energy of movement?

The steam particles do.

A moving particle has energy because it is moving.

Energy of movement is called kinetic energy.

The faster a particle moves, the more kinetic energy it has.

Gas particles fly fastest, so they have the most kinetic energy.

Liquid particles slide, so they have less.

Solid particles only vibrate on the spot, so they have the least.

From least to most kinetic energy, the order is solid, liquid, gas.

The same order holds for any substance, like wax.

Energy of movement is called kinetic energy.

Solid, liquid and gas run from least to most kinetic energy.

So the flying steam particles carry the most kinetic energy, and the vibrating ice particles carry the least.

Okay, now your turn.

Here is a pan of cold water at 20 degrees Celsius on a stove.

The burner heats it.

At 100 degrees Celsius, steam rises off the pan.

In the cold water the particles slid past one another.

In the steam the same particles fly.

Where did the extra kinetic energy, the energy of movement, come from?

It came from the burner.

The faster a particle moves, the more kinetic energy it has.

Heating gives particles energy and speeds them up.

The burner gave energy to the water particles, minute after minute.

The particles that became steam took in the most energy.

So they move fastest, and carry the most kinetic energy.

The same holds for any substance: a gas has taken in more energy than the same substance as a liquid, and a liquid more than a solid.

So, heating gives particles energy and speeds them up.

A gas has taken in the most energy, so its particles move fastest and have the most kinetic energy.

In the pan, the burner heated the water and gave its particles energy.

The particles that took in the most now fly as steam, with the most kinetic energy.

Okay, now your turn.

Here is everything we found out about particles and the three states of matter.

Matter is anything that has mass and takes up space.

And all matter is made of particles that are far too small to see.

Particles are always moving. They never stop.

Heating makes particles move faster. Cooling makes them move more slowly.

To tell the three states apart, ask two questions: does it keep its own shape, and does it keep its own volume?

A solid keeps its own shape and its own volume.

A liquid keeps its own volume but takes the shape of its container.

And a gas keeps neither: it spreads to fill whatever space it is given.

Okay, now your turn.

Here is everything we found out about the particles inside each state.

In a solid the particles are packed tightly, touching, in fixed positions. Each particle vibrates on the spot.

So a solid keeps its shape and its volume.

In a liquid the particles touch but have no fixed positions. They slide past one another.

So a liquid takes the shape of its container but keeps its volume.

In a gas the particles are far apart with empty space between them. They fly fast in every direction.

So a gas fills any container it is given.

Energy of movement is called kinetic energy.

Solid, liquid and gas run from least to most kinetic energy.

Heating gives particles energy and speeds them up.

So a gas has taken in the most energy, and its particles move fastest and have the most kinetic energy.

Okay, now your turn.

Here is a student in a kitchen on a hot afternoon.

She leaves an ice cube on a plate, and when she comes back it is a puddle of water.

An hour later, the plate is dry.

She takes a cold can from the fridge, and within a minute drops of water cover it.

Nobody touched the ice, the puddle or the can.

So where did the ice and the puddle go, and where did the drops on the can come from?

To find out, we will look at what the particles do when water changes from one state to another, and back again.

Okay, so are you ready to find out where the ice and the puddle went?

Here is an ice cube on a plate in a warm kitchen.

Thirty minutes later, the plate holds a small puddle of water.

Nobody touched the ice, so what happened to its particles?

The warm kitchen changed how its particles move.

Ice is a solid: its particles are packed tightly, touching, in fixed positions.

Each particle vibrates on the spot.

The warm air heats the cold ice.

Heating gives the particles energy.

The particles vibrate faster and faster.

The particles break out of their fixed positions.

Now the particles touch with no fixed positions and slide past one another.

The solid has become a liquid.

A solid turning into a liquid is called melting.

Now consider a frozen fruit pop in a warm lunchbox.

The warm lunchbox heats the pop and gives its particles energy.

The particles break loose, and the pop melts.

So, heating gives particles energy.

The particles in a solid vibrate harder, break out of their fixed positions and slide past one another.

The solid melts into a liquid.

So on the plate, the warm kitchen gave the ice particles energy, they broke loose, and the ice melted.

Okay, now your turn.

Here is a tray of water in a freezer at minus 18 degrees Celsius.

Three hours later, every cube is solid.

Nobody touched the water, so what happened to its particles?

The cold freezer changed how its particles move.

Water is a liquid: its particles touch with no fixed positions and slide past one another.

The warm water heats the cold freezer air, so the water cools.

Cooling takes energy away from the particles, so they slow down.

The slow particles stop sliding and lock into fixed positions.

Each particle still vibrates on the spot.

The liquid has become a solid.

A liquid turning into a solid is called freezing.

Freezing is melting run backwards.

Now consider melted chocolate in a mold in a fridge.

The chocolate cools in the fridge, and cooling takes energy from its particles.

The particles lock into fixed positions, and the chocolate freezes.

So, cooling takes energy away from particles.

The particles in a liquid slow down, stop sliding and lock into fixed positions.

The liquid freezes into a solid.

So in the tray, the freezer took energy from the water particles, they locked into place, and the water froze.

Okay, now your turn.

Here is a puddle on a playground after rain.

The sun comes out, and by afternoon the puddle has gone.

Nobody mopped it up, so where did the water go?

It went into the air, as a gas.

The sun heats the puddle, giving its particles energy.

The fastest surface particles fly off.

In the air they are far apart and flying, so they are a gas.

Water as a gas is called water vapor.

A liquid turning into a gas from its surface is called evaporation.

Now consider a pan of water at 100 degrees Celsius.

Particles all through the water fly apart into pockets of gas.

Each bubble is water vapor; it rises and bursts.

A liquid turning into a gas all through the liquid, as bubbles, is called boiling.

The bubbles in boiling water are water vapor, not air.

So, heating gives particles energy.

The fastest particles at the surface fly off as a gas: that is evaporation.

At 100 degrees Celsius, water particles fly off all through the liquid as bubbles: that is boiling.

So on the playground, the sun gave the puddle's particles energy, and the fastest flew off.

Okay, now your turn.

Here is a can of soda from the fridge on a warm, humid day.

Within a minute, drops of water cover the can.

The can was dry, so where did the water come from?

It came from the air.

The air holds water vapor: water particles flying about as a gas.

A warmer thing always heats a colder thing.

So the warm air heats the cold can, and the air touching the can cools.

Cooling takes energy away from the particles.

Water vapor particles hit the cold can and slow down.

The slow particles crowd together on the can.

Now the particles touch and slide past one another.

The gas has become a liquid.

A gas turning into a liquid is called condensation.

Condensation is evaporation run backwards.

Now consider a cold car window.

Water vapor from people's breath hits the glass, slows down and crowds together.

The window mists over.

So, cooling takes energy away from particles.

Gas particles that touch a cold surface slow down and crowd together, touching and sliding.

The gas condenses into a liquid.

So the cold can took energy from the vapor particles, and they crowded into drops.

Okay, now your turn.

Here is a block of dry ice in a bowl at a party.

Over the evening the block disappears, and the bowl stays dry.

Ice leaves a puddle, so where did the dry ice go?

It turned straight into a gas.

When ordinary ice warms, its particles break loose and slide, so it melts into a puddle.

Dry ice is not frozen water but frozen carbon dioxide, the gas we breathe out.

When dry ice warms, its particles fly straight off as gas.

The particles never slide as a liquid, so no puddle forms.

A solid turning straight into a gas is called sublimation.

To tell melting from sublimation, ask: did a liquid form?

A snowman shrinks and water runs off: a liquid formed, so it is melting.

A mothball shrinks for months, nothing wet: no liquid formed, so it is subliming.

So, here's what we've seen. Melting is a solid turning into a liquid.

Sublimation is a solid turning straight into a gas, skipping the liquid.

If a liquid formed, the solid melted; if no liquid ever formed, the solid sublimed.

So the dry ice flew straight off as gas, and the bowl stayed dry.

Okay, now your turn.

Here is a candle burning.

Wax near the flame melts and runs down the side.

At the bottom, the wax cools and hardens into lumps.

Is the lump still the same wax, and is the melted wax still wax?

Yes, both are still wax.

Near the flame, heating gave the wax particles energy.

The same particles broke out of their fixed positions, so the wax melted.

At the bottom, cooling took energy away from the wax particles.

The same particles locked back into fixed positions, so the wax froze.

Here are the wax particles as a solid, a liquid, and a solid again, side by side.

The particles never changed; only their arrangement and movement changed.

The same particles make the same substance.

So melted wax is still wax, and hardened wax is still wax.

Now consider water: ice, liquid water and water vapor are the same particles.

When ice melts or water boils, the same particles only move differently.

Ice is water, liquid water is water, and water vapor is water.

So, a change of state never makes a new substance.

The same particles are there before and after; only their arrangement and movement change.

Okay, now your turn.

Here is a sealed plastic bag of 50 grams of ice on a balance.

A student leaves the bag on the counter until all the ice has melted.

Before she weighs the bag of water, let's work out what the balance will read.

The balance measures the mass of everything in the bag.

The bag is sealed, so no particle can get out or in.

Heating gave the ice particles energy, and the ice melted into water.

Every particle that was in the ice is still in the bag, now as water.

The particles did not change, and their number did not change.

The same particles have the same mass, so the balance still reads 50 grams.

Now consider a sealed jar of melted wax on a balance, reading 80 grams.

The wax freezes solid; no particle left or arrived, so the balance still reads 80 grams.

So, the mass of a substance does not change when it changes state.

The same particles are all still there, so the mass is the same before and after.

So in the bag, every ice particle is still there as water, and the bag still reads 50 grams.

Okay, now your turn.

Here is a sealed bag of ice on a balance. The balance reads 45 grams.

The ice melts inside the bag.

A student is asked for the mass of the water, but she has not weighed it.

Can she still give the answer?

Yes, she can.

The mass of a substance does not change when it changes state.

The bag is sealed, so every ice particle is still there as water.

So the mass of the water equals the mass of the ice: 45 grams.

The bag held 45 grams of ice. The ice melted while sealed, so the bag holds 45 grams of water.

Now consider an open pan holding 500 grams of water.

The water boils, and 200 grams leaves as water vapor.

So the mass left is the mass before minus the mass that escaped.

500 grams minus 200 grams leaves 300 grams.

So, when a substance changes state, its mass after equals its mass before.

If some of the substance escapes as a gas, the mass left is the mass before minus the mass that escaped.

So in the bag, the mass does not change, and the water is 45 grams.

Okay, now your turn.

Here is everything we found out about changing state.

Heating gives particles energy; cooling takes it away.

The particles in a solid vibrate harder, break out of their fixed positions and slide past one another. The solid melts into a liquid.

A solid turning into a liquid is called melting.

The particles in a liquid slow down, stop sliding and lock into fixed positions.

The liquid freezes into a solid, and a liquid turning into a solid is called freezing.

Okay, now your turn.

Here is everything we found out about turning into a gas, and back.

Water as a gas is called water vapor.

The fastest particles at the surface of a liquid fly off as a gas: that is evaporation.

At 100 degrees Celsius, water particles fly off all through the liquid as bubbles of water vapor: that is boiling.

Gas particles that touch a cold surface slow down and crowd together, touching and sliding. The gas condenses into a liquid.

A gas turning into a liquid is called condensation.

Sublimation is a solid turning straight into a gas, skipping the liquid.

If a liquid formed, the solid melted; if no liquid ever formed, the solid sublimed.

A change of state never makes a new substance. Only the particles' arrangement and movement change.

So the mass of a substance does not change when it changes state.

If some escapes as a gas, the mass left behind is the mass before minus the mass that escaped.

Okay, now your turn.

Here is a metal spoon.

Ava says the spoon is shiny and hard, and cold to touch.

Ben says the spoon is his favorite, and it cost two dollars.

Both students are telling the truth.

But which of those facts belong to the spoon itself, and how would you measure the ones that do?

Every material has facts like these that belong to it.

To find out, we will look at which facts belong to a material, and how to measure them.

Okay, so are you ready to find out which facts belong to the spoon itself?

Two students describe the same metal spoon.

Ava says the spoon is shiny, hard, and cold to touch.

Ava measures the spoon. It is 15 centimeters long, and on a balance it reads 40 grams.

Ben says the spoon is his favorite, a present, and cost two dollars last year.

Now imagine a stranger finds the spoon: which facts would still be true?

The stranger can see the shine, feel the hardness and the cold, and measure the length and the mass.

The stranger cannot tell the owner, the price, or when Ben bought it.

Those facts are about Ben, not the spoon.

Something you can observe or measure about a material, without changing it into anything else, is called a physical property.

Now consider a red rubber ball: anyone can see its color, so color is a physical property.

Nobody can tell its owner by looking or measuring, so the owner is not a physical property.

So, here's what we've seen. A physical property is something you can observe or measure about a material without changing it into anything else.

So shiny, hard, cold, 15 centimeters and 40 grams are the spoon's physical properties.

Okay, now your turn.

An apple on a balance reads 250 grams.

A big bag of feathers on the same balance also reads 250 grams.

The bag is far bigger, so does it have more mass?

No, it does not.

The balance measures how much material is in the thing on it.

The amount of material in something is called its mass, measured in grams or kilograms.

Both read 250 grams, so both have a mass of 250 grams.

The bag takes up more room, but how much room a thing takes up is a different property.

Now consider two jars the same size: peanut butter and cotton balls.

The peanut butter reads 500 grams, and the cotton balls read 60 grams.

They are the same size, but the peanut butter has far more mass.

A small stone at 300 grams has more mass than a big sponge at 50 grams.

So, mass is the amount of material in something.

A balance measures mass, in grams or kilograms.

So the apple and the feathers both read 250 grams, and both have the same mass.

The bag is bigger, but bigger does not mean more mass.

Okay, now your turn.

Here is a student with a tall thin glass of water and a short wide one.

The tall glass looks fuller.

She pours the tall glass's water into a tall tube with a scale up its side, called a graduated cylinder.

The water settles at 150 milliliters.

She empties it and pours in the wide glass's water: this time, 250 milliliters.

The wide glass held more, so why were her eyes wrong?

Her eyes judged height.

The amount of space something takes up is called its volume.

A high level in a thin glass can be a small volume, and a low level in a wide glass a large one.

Each numbered mark here is 50 milliliters more than the one below.

The small mark halfway between two numbered marks is 25 milliliters more than the lower one.

Let the liquid settle, find its flat top, and read it against the nearest mark.

Write each number with its unit: 150 milliliters and 250 milliliters.

So, volume is the amount of space something takes up.

A graduated cylinder measures the volume of a liquid in milliliters.

So her eyes judged height, and the cylinder measured volume.

Okay, now your turn.

A student stirs a spoonful of sugar into water.

The grains disappear, and the water looks clear.

She stirs a spoonful of sand into a second glass.

The sand sits on the bottom, however long she stirs.

Why do sugar and sand behave so differently?

The sugar breaks apart, and the sand does not.

In the water, the sugar breaks apart into particles far too small to see.

But the water tastes sweet, so the sugar is still there.

Breaking into particles too small to see and spreading through the water is called dissolving.

A material that dissolves in water is called soluble.

The sand grains stay whole and sink to the bottom.

A material that does not dissolve in water is called insoluble.

Salt disappears too, so salt is soluble.

Chalk dust clouds the water, then settles, so chalk is insoluble.

Watching whether a material dissolves does not change it into anything else, so soluble or insoluble is a physical property.

So, here's what we've seen. A material that dissolves in water is called soluble.

A material that does not dissolve in water is called insoluble.

So sugar is soluble, and sand is insoluble.

Okay, now your turn.

A student leaves a steel spoon and a wooden spoon in a pan of hot soup.

After a minute the steel handle is too hot to hold, but the wooden handle is cool.

Why did only the steel handle get hot?

Steel lets heat through easily, so the hot soup heats the handle too.

Wood hardly lets heat through, so the wooden handle stays cool.

A material that lets heat through easily is called a conductor: steel is a conductor.

A material that hardly lets heat through is called an insulator: wood is an insulator.

Copper gets hot too, so copper is a conductor.

Plastic stays cool, so plastic is an insulator.

For electricity, she puts each material in a simple circuit: if the bulb lights, the material lets electricity through.

Copper lights the bulb, and plastic leaves it dark.

Steel lights it too; wood does not.

Steel and copper are metals, and metals conduct both.

Wood and plastic conduct neither.

So, here's what we've seen. A material that lets heat or electricity pass through easily is called a conductor.

A material that does not is called an insulator.

Okay, now your turn.

Here is everything we found out about physical properties.

A physical property is something you can observe or measure about a material without changing it into anything else.

Mass is the amount of material in something. A balance measures mass, in grams or kilograms.

Volume is the amount of space something takes up. A graduated cylinder measures the volume of a liquid in milliliters.

To read it, let the liquid settle, find its flat top, and read it against the nearest mark.

Okay, now your turn.

Here is everything we found out about dissolving and conducting.

A material that dissolves in water is called soluble.

A material that does not dissolve in water is called insoluble.

And soluble or insoluble is a physical property of the material.

A material that lets heat or electricity pass through easily is called a conductor.

A material that does not is called an insulator.

To test for electricity, put the material in a simple circuit; if the bulb lights, the material lets electricity through.

Metals conduct both heat and electricity; wood and plastic conduct neither.

Okay, now your turn.

Here are two blocks, exactly the same size.

A student lifts them, and one is far harder to lift than the other.

She drops a steel bolt into water and it sinks; she drops a block of wood in and it floats.

She drops a small plastic bead into water and it floats, but in cooking oil the same bead sinks.

So why is one block so much harder to lift than the other, and what decides sink or float?

To find out, we will look at the mass and the volume of a thing, and what they decide about sinking and floating.

Okay, so are you ready to find out what decides sink or float?

A student wants the volume of a small stone.

The stone is lumpy, so a ruler is no use.

She has a graduated cylinder, a tall tube marked off in steps, holding forty milliliters of water.

How can the water tell her the stone's volume?

The water will show the space the stone takes up.

She lowers the stone in, and the whole stone goes under.

The water level rises and settles at fifty-five milliliters.

The stone takes up space, so it pushes the water out of its way.

The level rose by exactly the stone's own volume.

Pushing the water aside like this is called displacement.

Fifty-five milliliters minus forty milliliters is fifteen milliliters: the stone displaced fifteen milliliters of water.

One milliliter of water fills a cube one centimeter on each side, so one milliliter is one cubic centimeter.

So the stone's volume is fifteen cubic centimeters.

Now consider a lump of clay. The level rises from forty milliliters to seventy milliliters, so the clay's volume is thirty cubic centimeters.

So, volume of the solid equals end reading minus start reading.

So for the stone, forty milliliters to fifty-five milliliters gives fifteen cubic centimeters.

Okay, now your turn.

Two blocks sit on a table, exactly the same size: one steel, one oak.

A student lifts them, and the steel block is far harder to lift.

On a balance, the steel reads four hundred grams and the oak thirty-five grams.

They are the same size but not the same mass, so what is different about the steel?

It packs more mass into the same space.

Each block has a volume of fifty cubic centimeters.

The steel packs four hundred grams into that space; the oak packs only thirty-five grams.

So the steel packs far more mass into each cubic centimeter.

How much mass a material packs into each cubic centimeter is called its density.

Same volume and more mass means denser: steel is denser than oak.

Now consider a steel ball and a glass ball that both read forty grams.

The steel ball is five cubic centimeters; the glass ball is sixteen cubic centimeters.

The steel packs the same mass into less space, so the steel ball is denser.

So, here's what we've seen. Same volume and more mass means denser.

Same mass and less volume means denser.

Okay, now your turn.

A steel cube has a mass of forty-eight grams and a volume of six cubic centimeters.

An oak block has a mass of fifty-six grams and a volume of eighty cubic centimeters.

Neither the masses nor the volumes match, so comparing cannot decide.

How dense is each one, as a number?

The steel packs forty-eight grams into six cubic centimeters.

If we share the forty-eight grams equally between the six cubic centimeters, each one gets eight grams.

So the steel's density is eight grams per cubic centimeter.

To find a density, divide the mass by the volume: density equals mass divided by volume.

Now for the oak, fifty-six grams divided by eighty cubic centimeters gives zero point seven grams per cubic centimeter.

Now consider an aluminum bar: eighty-one grams divided by thirty cubic centimeters gives two point seven grams per cubic centimeter.

Every time, write the values, the equation, the substitution, the calculation, and the answer with its unit.

So, density equals mass divided by volume, in grams per cubic centimeter.

So the steel at eight grams per cubic centimeter is denser than the oak at zero point seven grams per cubic centimeter.

Okay, now your turn.

A student drops a steel bolt into water, and it sinks.

She drops a block of wood in, and it floats.

Then she drops a small plastic bead in, and it floats.

The same bead in a bucket of cooking oil sinks.

The same bead in two liquids gives two outcomes, so what decides?

Density decides.

Density is mass divided by volume, in grams per cubic centimeter.

Water is one point zero grams per cubic centimeter.

Steel is eight grams per cubic centimeter, denser than water, so the bolt sinks.

Wood is zero point seven grams per cubic centimeter, less dense than water, so the wood floats.

The bead is zero point nine five grams per cubic centimeter, less dense than water, so it floats.

Cooking oil is zero point nine two grams per cubic centimeter, so in oil the bead is denser, and it sinks.

A huge oak log floats and a tiny steel nut sinks: density decides, not mass.

So, an object denser than the liquid sinks. An object less dense than the liquid floats.

So the bead is less dense than the water and denser than the oil.

Okay, now your turn.

A steel cube, forty-eight grams and six cubic centimeters, has a density of eight grams per cubic centimeter.

A student saws the cube in half.

Each half is twenty-four grams and three cubic centimeters.

Is each half less dense than the whole cube?

No, it is not.

Density equals mass divided by volume: twenty-four grams divided by three cubic centimeters is eight grams per cubic centimeter.

Each half has the same density as the whole cube.

Cutting the cube in half halved the mass and halved the volume.

Half the mass shared over half the volume still gives eight grams in each cubic centimeter.

So density belongs to the material, not to the size of the piece.

Now consider a gold ring of one cubic centimeter and nineteen point three grams, and a gold bar a hundred times bigger.

The bar has a hundred times the mass and a hundred times the volume, so both are nineteen point three grams per cubic centimeter.

A steel filing and a steel girder are both eight grams per cubic centimeter.

Here's the pattern. A small sample and a large sample of the same material have the same density.

Okay, now your turn.

Here is everything we found out about density.

To find the volume of an odd-shaped solid, read the water level before and after the whole solid goes under.

The solid pushes aside exactly its own volume of water: that is displacement.

Volume of the solid equals end reading minus start reading.

Each one milliliter of water displaced means one cubic centimeter of solid.

Density is how much mass a material packs into each cubic centimeter.

Same volume and more mass means denser. Same mass and less volume means denser.

Density equals mass divided by volume, in grams per cubic centimeter.

To calculate it, write the values, the equation, the substitution, the calculation, and the answer with its unit.

Okay, now your turn.

Here is everything we found out about sinking, floating and the size of the piece.

To predict sinking or floating, compare the object's density with the liquid's density.

An object denser than the liquid sinks. An object less dense than the liquid floats.

Density decides, not mass: a huge oak log floats and a tiny steel nut sinks.

Density belongs to the material, not to the size of the piece.

So a small sample and a large sample of the same material have the same density.

Okay, now your turn.

Here is a student at a bench with three samples: a glass of water, a glass of orange juice, and a bowl of sand with salt stirred in.

After an hour, the water looks the same all through, but the juice has settled into layers.

The sand and salt make one gray heap that looks like one thing.

So how many kinds of matter are in each sample, and how are they spread?

To find out, we will zoom in on the particles in each sample, and sort samples by what we see.

Okay, so are you ready to zoom in and find out how many kinds of matter are in a glass of juice?

A student pours a glass of distilled water, the kind sold for steam irons, and a glass of orange juice.

After an hour the water is the same all through, but the juice has settled, with pulp at the bottom.

What is different about the water?

The water holds only one kind of matter.

All matter is made of particles that are far too small to see.

Zoomed in, every particle in the distilled water is a water particle.

Zoomed in, the juice holds water particles, sugar particles and pulp, all mingled.

A sample that contains only one kind of matter is called a pure substance.

The distilled water is a pure substance, and the juice is not.

In a copper wire, every particle is a copper particle, so the wire is a pure substance.

In table sugar, every particle is a sugar particle, so sugar is a pure substance.

Sea water looks as clear, but it holds water particles and salt particles.

That is two kinds of matter, so sea water is not a pure substance.

So, here's what we've seen. A sample that contains only one kind of matter is called a pure substance.

Okay, now your turn.

A student pours salt into a bowl of sand and stirs.

The bowl now holds one gray-white heap that looks like one material.

Under a magnifying glass, she still sees salt grains and sand grains side by side.

Is the heap one kind of matter, or two?

It is two kinds of matter.

Salt on its own is a pure substance: every particle is a salt particle.

Zoomed in, salt particles and sand particles sit side by side in the heap, each still itself.

A sample that contains two or more substances mingled together is called a mixture.

Stirring does not turn two substances into one; each keeps its own particles.

Trail mix is nuts, raisins and seeds in one bowl, so trail mix is a mixture you can see.

Distilled water is one kind of particle only, so it is a pure substance, not a mixture.

Air holds several gases mingled together, so air is a mixture you cannot see.

So, here's what we've seen. A sample that contains two or more substances mingled together is called a mixture.

So the heap is a mixture.

Okay, now your turn.

A student spills iron filings into a tray of sand.

Picking the filings out one by one would take all day.

She holds a magnet just above the tray.

The filings jump up onto the magnet, and the sand stays in the tray.

Why could the magnet pull one part out and leave the other?

The iron stayed magnetic in the tray, and the sand did not.

The tray holds iron filings and sand mingled together, so it is a mixture.

Zoomed in, iron particles and sand particles sit side by side, each still itself.

Iron on its own is magnetic; sand on its own is not.

In the mixture, the iron is still magnetic, and the sand is still not.

So in a mixture, each substance keeps its own properties.

The magnet pulls on the iron and not on the sand, so it lifts only the iron.

Other mixtures come apart the same way: in water, sawdust floats and gravel sinks.

And from sugar water, the water evaporates, but the sugar stays.

Here's the pattern. In a mixture each substance keeps its own properties, so the parts of a mixture can be separated.

Okay, now your turn.

A student stirs salt into a glass of water until it disappears.

She stirs sand into a second glass of water.

Are these two mixtures the same kind of mixture?

No, they are mixed in two different ways.

Zoomed in on the salt water, the salt particles are spread evenly among the water particles.

Every sip is the same, so the salt water is the same all through.

Zoomed in on the sandy water, the sand particles sit below the water particles.

So the sandy water is not the same all through.

A mixture that is the same all the way through is called homogeneous: homo means the same.

A mixture that is different in different places is called heterogeneous: hetero means different.

Both are mixtures.

Air is homogeneous, because every breath is the same.

Trail mix is heterogeneous, because every handful differs.

Brass is copper and zinc spread evenly, so it is homogeneous.

So, here's what we've seen. A mixture that is the same all the way through is called homogeneous.

A mixture that is different in different places is called heterogeneous.

So the salt water is homogeneous, and the sandy water is heterogeneous.

Okay, now your turn.

A student is handed four cups: distilled water, the kind sold for steam irons, salt water, sandy water, and table sugar.

She must sort each cup: pure substance, homogeneous mixture, or heterogeneous mixture.

Which two questions should she ask, and in which order?

Question 1 is: is there more than one kind of matter?

No means a pure substance; yes means a mixture, so ask question 2.

Question 2 is: is the mixture the same all the way through?

Yes means homogeneous, evenly mixed; no means heterogeneous, not evenly mixed.

For the salt water, question 1 is yes: it holds salt and water, so it is a mixture.

Question 2 is yes, every sip is the same, so it is a homogeneous mixture.

For the sandy water, question 1 is yes and question 2 no, so it is a heterogeneous mixture.

For the distilled water, question 1 is no, so it is a pure substance.

For the table sugar, question 1 is no, so it is a pure substance too.

So, to sort a sample, ask two questions in order: is there more than one kind of matter? Is it the same all the way through?

Okay, now your turn.

Here is everything we found out about pure substances and mixtures.

All matter is made of particles that are far too small to see.

A sample that contains only one kind of matter is called a pure substance.

Zoom in, and every particle is the same kind.

But a sample that contains two or more substances mingled together is called a mixture.

Each substance keeps its own particles, so stirring does not turn two substances into one.

And in a mixture each substance keeps its own properties, so the parts of a mixture can be separated.

Okay, now your turn.

Here is everything we found out about sorting mixtures and samples.

A mixture that is the same all the way through is called homogeneous, evenly mixed.

A mixture that is different in different places is called heterogeneous, not evenly mixed.

Both are mixtures; the words only say how evenly each is mixed.

To sort a sample, ask two questions in order: is there more than one kind of matter? Is it the same all the way through?

No to the first question means a pure substance; yes means a mixture, so ask the second question.

Yes to the second question means the mixture is homogeneous; no means the mixture is heterogeneous.

Okay, now your turn.

Here is a student making drinks in a kitchen.

She stirs drink mix into one glass of water, and it vanishes; she stirs flour into another, and it clouds and settles.

She tops up a syrup drink with water, and it turns pale, yet no syrup was taken out.

She drops a sugar cube into hot water and one into cold, and the hot one is gone far sooner.

So what does dissolving make, what makes a drink stronger, and how can a mixture be taken apart again?

To find out, we will look at what happens when something dissolves, and how to take a mixture apart.

Okay, so are you ready to find out where the drink mix went?

A student stirs a packet of drink mix into a glass of water.

The drink mix vanishes, and the water turns the same color all the way through.

She stirs flour into a second glass; it turns cloudy, then the flour settles.

Both glasses hold mixtures, so why did only the drink mix seem to disappear?

The drink mix dissolved, and the flour did not.

Zoomed in on the drink mix glass, its particles are spread evenly among the water particles.

Zoomed in on the flour glass, the flour particles are still stuck together in grains on the bottom.

A mixture in which one substance has dissolved evenly into another is called a solution.

Salt stirred into water vanishes and spreads evenly, so salt water is a solution.

Pepper shaken into water floats and sinks and never vanishes, so it is not a solution.

Vinegar poured into water vanishes into it, so it is a solution, with no solid at all.

So, here's what we've seen. A mixture in which one substance has dissolved evenly into another is called a solution.

So the drink mix glass holds a solution, and the flour glass does not.

Okay, now your turn.

A recipe card for hummingbird nectar reads: dissolve fifty grams of sugar in two hundred milliliters of water.

Two substances make this solution: one dissolves, and the other does the dissolving.

Which is which?

The sugar dissolves, and the water does the dissolving.

The sugar breaks into particles too small to see and spreads out among the water particles.

The water spreads the sugar particles out, so the water does the dissolving.

The substance that dissolves is called the solute.

The substance the solute dissolves into is called the solvent.

Solute plus solvent makes a solution.

The solvent is usually a liquid, and there is usually more of it.

In soda water, the carbon dioxide gas is the solute, and the water is the solvent.

In sweet tea with honey, the honey is the solute, and the tea is the solvent.

With food coloring in water, the coloring is the solute, and the water is the solvent.

So, the substance that dissolves is called the solute. The substance it dissolves into is called the solvent.

So in the recipe, the sugar is the solute, and the water is the solvent.

Okay, now your turn.

Three mugs each hold two hundred milliliters of hot water.

A student stirs five grams of instant coffee into the first, ten grams into the second, twenty grams into the third.

All the coffee dissolves, but the third mug is the darkest and strongest.

What is different about the third mug?

It holds the most coffee in the same water.

In every mug the coffee is the solute and the water is the solvent.

The water is the same in every mug, but the third holds four times the coffee.

Each sip from the third mug carries more coffee, so it is stronger and darker.

How much solute is dissolved in a given amount of solvent is called the concentration.

More solute in the same solvent means more concentrated.

Two pitchers each hold thirty grams of sugar: one in a thousand milliliters of water, one in five hundred milliliters.

The pitcher with five hundred milliliters has the same sugar in less water, so it is more concentrated.

The same solute in more solvent means less concentrated.

So, more solute in the same solvent means more concentrated. The same solute in more solvent means less concentrated.

Okay, now your turn.

A glass holds a dark, strong drink: forty grams of syrup in two hundred milliliters of water.

A student tops the glass up to four hundred milliliters with more water.

The color pales and the drink tastes weaker, yet no syrup was taken out.

Why is the drink weaker?

The same syrup is now spread through twice the water.

The syrup is the solute and the water is the solvent.

Before the extra water, zoomed in, the syrup particles sit close together among the water particles.

After the water is added, every syrup particle is still there, spread through twice the water.

Each sip is the same size, but it carries fewer syrup particles.

So it tastes weaker, because the concentration has gone down.

Adding more solvent to a solution is called diluting it.

Diluting takes no solute away: all forty grams are still in the glass.

Salty soup filled up with water is diluted.

Tea with more hot water poured in is diluted too.

If she adds more syrup instead, the drink gets stronger, and that is not diluting.

So, adding more solvent to a solution is called diluting it, and the concentration goes down.

Okay, now your turn.

A student puts an empty beaker on a balance and sets the reading to zero, then pours in water until it reads two hundred grams.

She measures fifteen grams of salt onto a paper.

She tips the salt in and stirs until every grain has vanished.

She puts the beaker back on the balance. What does the salt water read?

It reads two hundred fifteen grams, and here is why.

Zoomed in, the salt particles are spread out among the water particles.

Every salt particle is still in the beaker, so its mass is still there.

The mass of a solution equals the mass of the solvent plus the mass of the solute.

Two hundred grams plus fifteen grams equals two hundred fifteen grams.

Write the values, the equation, the substitution, the calculation, and the answer with its unit.

Twelve grams of baking soda in two hundred fifty grams of water makes two hundred sixty-two grams.

Milliliters measure volume, not mass, so three hundred milliliters of water cannot go into the sum; only grams are added.

So, the mass of a solution equals the mass of the solvent plus the mass of the solute.

Okay, now your turn.

Two cups each hold two hundred milliliters of water and one sugar cube.

One cup is at twenty degrees Celsius, the other at sixty degrees Celsius, and nobody stirs.

The hot cup's cube is gone in three minutes; the cold cup's cube takes twelve minutes.

Why does the hot water dissolve the cube faster?

Its particles move faster, and only the temperature differs between the cups.

Heating makes particles move faster.

Zoomed in on the cube, water particles bump into its surface all the time.

Each bump can knock a sugar particle off, and that particle spreads out among the water.

In the hot cup, the water particles move faster, so they hit the cube harder and more often.

So the hot water knocks sugar particles off sooner, and the cube is gone in three minutes.

In the cold cup, slow water particles knock sugar off slowly, so the cube lasts twelve minutes.

Salt dissolves faster in warm water than in cold.

Here's the pattern. The warmer the solvent, the faster the solute dissolves.

So the hot cup's water hit its cube harder and more often, and that cube dissolved first.

Okay, now your turn.

Two cups each hold two hundred milliliters of water at twenty degrees Celsius.

One gets a four-gram cube, the other four grams of loose grains, and nobody stirs.

The grains are gone in two minutes; the cube takes twelve minutes.

The sugar, the water and the temperature are the same, so why do the grains dissolve first?

The grains have far more surface for the water to reach.

Water particles bump into the sugar's surface, and each bump can knock a sugar particle off.

A particle deep inside the cube cannot be reached until the sugar around it has gone.

So dissolving happens only at the surface.

The cube has six faces for the water to reach.

Cut into eight small cubes, it has forty-eight faces: twice the surface, all reached by water.

More surface means more water particles hitting sugar at once, so more sugar comes off at once.

Fine salt dissolves faster than coarse salt.

Both cups hold the same four grams: breaking changes the surface, not the mass.

Here's the pattern. The smaller the pieces of a solute, the faster it dissolves.

Okay, now your turn.

Two cups each hold two hundred milliliters of water at twenty degrees Celsius and one sugar cube.

A student stirs one cup and leaves the other still.

The stirred cube is gone in two minutes; the still one takes twelve minutes.

Why does stirring help?

Stirring keeps bringing fresh water to the cube.

Only the stirring differs, so stirring is the reason.

Water particles knock sugar particles off the cube into the water next to it.

In the still cup, that water soon holds many sugar particles.

That crowded water has little room for new sugar, so fewer sugar particles come off, and the cube lasts twelve minutes.

In the stirred cup, the spoon sweeps that crowded water away, and fresh water arrives.

Fresh water knocks off new sugar quickly, so the cube is gone in two minutes.

Shaking a bottle of salt and water does the same job, so the salt dissolves faster.

Warmer water, smaller pieces and stirring each make a solute dissolve faster.

None of the three changes how much solute there is, only how fast it dissolves.

So, stirring makes a solute dissolve faster, because it brings fresh water to the solute.

Okay, now your turn.

Four mixtures sit on a bench: coffee grounds in water, salt dissolved in water, nails in wood shavings, marbles in sand.

A student must get one part out of each, and no single method works for all four.

How does she pick?

She names the two parts, then finds a property that one part has and the other does not.

Coffee grounds do not dissolve, and they are too big for a paper cone's tiny holes.

Poured through the paper, the water runs through and the grounds stay, and this is called filtering.

Dissolved salt is too small for paper to hold, but water evaporates and salt does not.

Left in a warm pan, the water leaves as a gas and the salt stays, and this is called evaporating.

A magnet pulls iron and not wood, so it lifts the nails out of the shavings, and this is called using a magnet.

Marbles are big and sand grains small, so on a mesh the sand falls through, and this is called sieving.

So, to separate a mixture, find a property that one part has and the other does not, and pick the method that uses it.

Okay, now your turn.

Here is everything we found out about solutions and separating mixtures.

A mixture in which one substance has dissolved evenly into another is called a solution.

The substance that dissolves is called the solute. The substance it dissolves into is called the solvent.

How much solute is dissolved in a given amount of solvent is called the concentration.

More solute in the same solvent means more concentrated. The same solute in more solvent means less concentrated.

Adding more solvent to a solution is called diluting it, and the concentration goes down.

No solute is lost when a solution is diluted.

And the mass of a solution equals the mass of the solvent plus the mass of the solute.

Okay, now your turn.

Here is everything we found out about dissolving faster and taking a mixture apart.

Warmer water, smaller pieces and stirring each make a solute dissolve faster.

The warmer the solvent, the faster the solute dissolves, because faster water particles hit the solute harder and more often.

The smaller the pieces of a solute, the faster it dissolves, because dissolving happens only at the surface.

Stirring makes a solute dissolve faster, because it brings fresh water to the solute.

Warmer water, smaller pieces and stirring change how fast a solute dissolves, not how much solute there is.

To separate a mixture, find a property that one part has and the other does not.

Filtering separates undissolved pieces from a liquid, and evaporating separates a dissolved solute from its solvent.

A magnet lifts out a magnetic part, and sieving separates big pieces from small ones.

Okay, now your turn.

Here is a student in a kitchen with a sugar cube and a spoon.

She presses the cube, and it breaks into grains. She grinds the grains, and they turn into a fine powder.

Each piece is smaller than the last, so could she keep breaking it up forever, or is there a smallest piece of sugar?

The same question goes for the spoon, the water in the tap, and everything else around you.

To find out, we will look at what the smallest piece of a material is, and what a drawing of those pieces tells you about a substance.

Okay, so are you ready to find out whether the breaking ever stops?

A student unrolls a strip of aluminum foil and cuts it in half.

She cuts one half in half again, and again, and again.

The pieces get smaller each time. Could she keep cutting forever?

No, the cutting has an end.

All matter is made of particles that are far too small to see.

After many cuts, each piece is still made of many particles.

Keep cutting in your mind, and in the end one piece is a single aluminum particle.

The smallest piece of aluminum that is still aluminum is called an atom.

Every material is made of atoms.

For aluminum, one particle is one atom.

For water, one particle is a few atoms joined together.

In these drawings an atom is a circle; every atom of one kind has the same size and shade, and the words under the drawing say which kind.

Joined atoms are drawn touching; atoms that are not joined are drawn apart.

So, every material is made of atoms. The smallest piece of aluminum that is still aluminum is one atom.

So on the strip, the cutting ends at one atom; she cannot cut forever.

Okay, now your turn.

Here is a copper kettle.

Every material is made of atoms, and the kettle is made of copper atoms.

Take one copper atom out of the kettle, in your mind. Is that one atom still copper?

Yes, one copper atom on its own is still copper.

One atom is the smallest piece of copper there can be.

Now imagine a piece even smaller than one atom. Is that piece still copper?

No, a piece smaller than one copper atom is not copper anymore.

Now consider a gold earring: one gold atom is still gold.

And a hundred gold atoms are still gold.

A piece smaller than one gold atom is not gold.

Now consider a silver bracelet: one silver atom is still silver.

And anything smaller than one atom is not silver.

Here's the pattern. One atom of copper is the smallest piece that is still copper; anything smaller is not copper.

The same is true of gold and silver: the smallest piece that is still that substance is one atom.

Okay, now your turn.

Here are three drawings of three materials.

In drawing 1 every atom is the same size and shade.

In drawing 2 the atoms sit in pairs, but every atom is still the same kind.

In drawing 3 there are two kinds of atom. Which drawings show only one kind of atom?

Drawings 1 and 2 do, and drawing 3 does not.

Count the kinds of atom in each drawing.

A substance made of only one kind of atom is called an element.

Drawings 1 and 2 show elements; drawing 3 does not.

Drawing 4, small dark circles only, is an element.

Drawing 5, two kinds not joined, is not an element.

The oxygen in the air is an element: every atom is an oxygen atom, in joined pairs.

The carbon in charcoal is an element: every atom is a carbon atom.

Carbon dioxide has carbon atoms and oxygen atoms joined, two kinds, so it is not an element.

So, here's what we've seen. A substance made of only one kind of atom is called an element.

So drawings 1 and 2 show elements, and drawing 3 does not.

Okay, now your turn.

Two drawings show the same kind of atom.

In drawing 1 each atom floats on its own.

In drawing 2 the atoms sit in pairs, touching, and each pair moves together.

Why does each pair in drawing 2 move as one?

The two atoms in each pair are joined together.

Atoms that are joined together are drawn touching.

Two or more atoms joined together make one particle called a molecule.

Each pair in drawing 2 is a molecule; the single atoms in drawing 1 are not.

A molecule can hold atoms of one kind, or atoms of different kinds.

The oxygen in the air comes as molecules of two oxygen atoms joined.

A water molecule is one oxygen atom joined to two hydrogen atoms.

Pairs of one kind, touching, are molecules.

Single atoms apart are not molecules.

Pairs of two kinds, touching, are molecules.

Two kinds of single atom apart are not molecules.

So, here's what we've seen. Two or more atoms joined together make one particle called a molecule.

So the pairs in drawing 2 are molecules, and the single atoms in drawing 1 are not.

Okay, now your turn.

Here is a glass of water. Water is made of two kinds of atom.

Every water molecule has the same pattern: one oxygen atom joined to two hydrogen atoms.

Any water molecule anywhere in the world has the same pattern.

Is water an element?

No, water is not an element, because an element has only one kind of atom and water has two.

In water, the two kinds of atom are joined into molecules, and the pattern never changes.

A substance made of two or more kinds of atom joined together in a fixed pattern is called a compound.

Water is a compound.

Carbon dioxide is a compound too: one carbon atom joined to two oxygen atoms, the same in every molecule.

Two kinds of atom that are not joined make a mixture, not a compound.

One kind of atom, single or joined, makes an element, not a compound.

So, here's what we've seen. A substance made of two or more kinds of atom joined together in a fixed pattern is called a compound.

So water, with two kinds of atom joined in the same pattern every time, is a compound.

Okay, now your turn.

A student is handed four particle drawings with no labels.

Some show one kind of atom, and some show two; in some the atoms are joined, and in some they float apart.

She sorts the four drawings with one question at a time.

Question 1 is: how many kinds of atom does the drawing show?

One kind means an element, and the sort is done; more than one kind means ask question 2.

Question 2 is: are the different kinds of atom joined together?

Joined, in the same pattern every time, means a compound; not joined, just mingled, means a mixture.

Drawing 1 shows pairs of small dark circles, one kind, so it is an element.

Drawing 2 shows gray and small dark circles touching: two kinds, joined, so it is a compound.

Drawing 3 shows open pairs among single gray circles that never touch: two kinds, not joined, so it is a mixture.

Drawing 4 shows one open and one gray circle touching: two kinds, joined, so it is a compound.

So, to sort a particle drawing, ask two questions in order: how many kinds of atom are there? Are the different kinds joined together?

Okay, now your turn.

A student has four samples: a gold nugget, dry ice, frozen carbon dioxide, a steel fork, and sand with bits of shell.

She joins two sorts into one tree: pure or mixture, then element or compound.

Question 1 is: is there more than one kind of matter?

No means a pure substance, so ask whether it has one kind of atom: one kind makes an element, and different kinds joined make a compound.

Yes means a mixture, so ask whether it is the same all the way through.

The gold nugget is one kind of matter with one kind of atom, so it is an element.

The dry ice is one kind of matter with carbon and oxygen atoms joined, so it is a compound.

The steel fork is iron with a little carbon mingled evenly, so it is a homogeneous mixture.

The sand with shell is different in different places, so it is a heterogeneous mixture.

So, to sort any sample, first ask: is there more than one kind of matter?

A mixture then asks: is it the same all the way through? A pure substance then asks: does it have one kind of atom?

Okay, now your turn.

Hydrogen is a gas that burns.

Oxygen is a gas that feeds a fire.

When hydrogen atoms and oxygen atoms join in a fixed pattern, they make water.

Water is a liquid that puts fires out. How can two gases make a liquid that does the opposite?

Water's particles are new particles, not the gases' particles.

Zoom in: a hydrogen molecule is two hydrogen atoms joined.

An oxygen molecule is two oxygen atoms joined.

Every water molecule is one oxygen atom joined to two hydrogen atoms.

A water molecule is not a hydrogen molecule beside an oxygen molecule.

It is a new kind of particle: three atoms joined in a new pattern.

New particles behave in new ways, so water has its own properties.

In a mixture each substance keeps its own particles, so each keeps its own properties.

Mixed, the two gases are still two gases; joined into water, both gases are gone.

Here's the pattern. A compound's particles are new particles, so a compound has its own properties, not the properties of the elements in it.

So the two gases, joined into water molecules, make a liquid that puts fires out.

Okay, now your turn.

Here is everything we found out about atoms, elements, molecules and compounds.

All matter is made of particles that are far too small to see.

Every material is made of atoms. The smallest piece of aluminum that is still aluminum is one atom.

One atom of copper is the smallest piece that is still copper; anything smaller is not copper.

An atom is drawn as a circle, and atoms that are joined together are drawn touching.

A substance made of only one kind of atom is called an element.

Two or more atoms joined together make one particle called a molecule.

Okay, now your turn.

Here is everything we found out about compounds, mixtures and sorting a sample.

A substance made of two or more kinds of atom joined together in a fixed pattern is called a compound.

A sample that contains two or more substances mingled together is called a mixture, and its kinds are not joined.

To sort a particle drawing, ask two questions in order: how many kinds of atom are there? Are the different kinds joined together?

A sample that contains only one kind of matter is called a pure substance.

To sort any sample, first ask whether it holds more than one kind of matter.

A mixture then asks: is it the same all the way through? A pure substance then asks: does it have one kind of atom?

A compound's particles are new particles, so a compound has its own properties, not the properties of the elements in it.

Okay, now your turn.

Here is a pencil mark, and the dark stuff in it is carbon.

Close up, the mark is a box of gray circles: the carbon atoms.

One of those circles is the smallest piece that is still carbon.

Yet scientists can take an atom apart and look at what is inside.

And here are two atoms that look almost the same, yet one is carbon and one is nitrogen.

So what is inside an atom, and what decides which element it is?

To find out, we will look inside the atom, and at what makes one element different from another.

Okay, so are you ready to look inside an atom?

A pencil leaves a dark mark on paper, and the dark stuff in the mark is carbon.

Carbon is an element, so every atom in the mark is a carbon atom.

One carbon atom is the smallest piece that is still carbon, yet scientists can take an atom apart. What is inside?

Let's zoom in on one gray circle and open it up.

Inside it are much smaller particles, of three kinds.

The filled circles marked plus are called protons.

The open circles with no mark are called neutrons.

The small dark dots marked minus are called electrons.

None of the three kinds is carbon; carbon is what the whole atom is.

Now consider an atom of helium. It holds the same three kinds, with fewer of each.

So, every atom contains three kinds of smaller particle: protons, neutrons and electrons.

An atom of carbon and an atom of helium contain the same three kinds; only the numbers differ.

So if you take one carbon atom from the pencil mark apart, you find those three kinds, not smaller pieces of carbon.

Okay, now your turn.

Here is the inside of a lithium atom, a soft, silvery metal.

Some particles sit packed in the middle.

Others sit out on the rings.

Some carry a plus mark, some a minus mark, and some no mark. Which particle sits where?

In the middle, the protons and the neutrons sit packed together.

This tiny packed center of the atom is called the nucleus.

The electrons sit on the rings, outside the nucleus.

Now consider a wool sweater pulled off over your head. Your hair stands on end, because rubbing gave the hair a charge.

Charge comes in two kinds, positive and negative, and one of each cancels the other.

So what do the marks mean?

Each proton carries a positive charge: that is its plus mark.

Each electron carries a negative charge.

A neutron carries no charge at all.

The drawing is a model: the real atom is mostly empty space, and electrons do not run on tracks.

So, protons and neutrons sit together in a tiny center called the nucleus.

Protons carry a positive charge; neutrons carry no charge.

Electrons move around the outside of the nucleus and carry a negative charge.

Okay, now your turn.

Here are two atoms drawn side by side, and they look almost the same.

The left atom is carbon, the soot on a burnt match.

The right atom is nitrogen, the gas that makes up most of the air.

So what inside them tells them apart?

The number of protons does.

Count the protons in the left atom: 6 protons. Every atom with 6 protons is a carbon atom.

Count the protons in the right atom: 7 protons. Every atom with 7 protons is a nitrogen atom.

Only the protons were counted; the neutrons were not.

Here are the first ten elements, from 1 proton to 10 protons.

Find the count in the table and read off the element: 8 protons is oxygen.

And 4 protons is beryllium, 9 protons is fluorine, 2 protons is helium.

So, the number of protons decides which element an atom is.

Count the protons, find that number in the table, and read off the element.

So in the two atoms, 6 protons make carbon and 7 protons make nitrogen; the proton count tells them apart.

Okay, now your turn.

Imagine a carbon atom with 6 protons in its nucleus.

Now imagine one more proton is pushed in: 7 protons. Is it still carbon?

No, the atom is not carbon anymore.

Here is the carbon atom, 6 protons, and one proton goes into the nucleus.

Count again: 7 protons. Row 7 in the table says nitrogen.

So the atom is now nitrogen, not carbon.

Now consider an oxygen atom with 8 protons, and one proton comes out.

Count again: 7 protons, so the oxygen atom has become a nitrogen atom.

Now imagine a neutron is added to a carbon atom instead.

The proton count is still 6, so the atom is still carbon.

Changing the proton count takes enormous force, so it happens inside stars.

On a bench, a carbon atom stays carbon.

So, change the proton count, and you change the element.

Adding or removing a neutron leaves the element the same.

So the carbon atom with one extra proton has 7 protons, and 7 protons make it nitrogen.

Okay, now your turn.

An atom of oxygen has 8 protons, and each proton carries a positive charge.

Yet an ordinary oxygen atom has no overall charge at all. Something must cancel the positives.

How many electrons does the atom have?

It has 8 electrons.

Each proton carries one positive charge, each electron carries one negative charge, and a neutron carries no charge.

One positive charge and one negative charge cancel each other out.

The oxygen atom's 8 protons carry 8 positive charges.

To cancel 8 positive charges, the atom needs 8 negative charges, so it needs 8 electrons.

8 positive charges and 8 negative charges cancel, so the atom has no overall charge.

An atom with no overall charge is called a neutral atom.

In a neutral atom the number of electrons equals the number of protons.

Now consider a neutral lithium atom: 3 protons, so 3 electrons.

And a neutral neon atom: 10 protons, so 10 electrons.

So, in a neutral atom the positive charges and the negative charges cancel, and the number of electrons equals the number of protons.

So the oxygen atom has 8 protons and 8 positive charges, and 8 electrons cancel them.

Okay, now your turn.

Here is everything we found out about what is inside the atom.

One carbon atom is the smallest piece that is still carbon.

Every atom contains three kinds of smaller particle: protons, neutrons and electrons.

Protons are the filled circles marked plus, neutrons are the open circles, and electrons are the small dark dots marked minus.

Protons and neutrons sit together in a tiny center called the nucleus.

Protons carry a positive charge; neutrons carry no charge.

And electrons move around the outside of the nucleus and carry a negative charge.

Okay, now your turn.

Here is everything we found out about what makes an atom one element and not another.

The number of protons decides which element an atom is.

Count the protons, find the number in the table, and read off the element.

Change the proton count, and you change the element.

But adding or removing a neutron leaves the element the same.

An atom with no overall charge is called a neutral atom.

In a neutral atom the number of electrons equals the number of protons.

Okay, now your turn.

Two bottles sit on a shelf: one label reads Co, the other reads CO.

One holds a hard, gray metal, and the other holds a poisonous gas.

Scientists know 118 elements, and here they all are, on one chart.

And from this chart, chemists can say what an element is like, even one nobody has ever held.

So how does one small letter tell Co from CO, and how do you read this chart?

To find out, we will look at how chemists write each element, and how the chart lays them all out.

Okay, so are you ready to learn to read the chart?

Two bottles sit on a shelf: one label reads Co, the other reads CO.

Co is a hard, gray metal, and CO is a poisonous gas.

So what does the small letter mean?

It means the short name is one name, not two.

Scientists know 118 elements, each made of only one kind of atom.

Chemists write each element as a short name of one or two letters: C for carbon, Co for cobalt.

This short name is called the element's chemical symbol.

The first letter is always a capital, and the second, if there is one, is always small.

So every capital letter starts a new symbol.

Co has one capital letter, so Co names one element: cobalt.

But CO has two capital letters, so CO names two elements: carbon and oxygen.

And HF has two capital letters too, so HF names two elements.

But Hg has one capital letter, so Hg names one element.

So, here's what we've seen. Count the capital letters, and you count the elements.

So on the shelf, Co is one element, and CO is two symbols written together.

Okay, now your turn.

Scientists know 118 elements, and the number of protons decides which element an atom is.

So each element has its own count of protons, from 1 to 118.

Imagine 118 cards, one for each element, tipped out onto a desk.

How would you lay them out so you could find any element fast?

You lay them out in order of their proton count: 1, then 2, then 3, and so on.

The cards run along a row, and a new row starts underneath when the row is full.

This chart of every known element, one box each, is called the periodic table.

Look at carbon's box: the small 6 at the top is there because every carbon atom has 6 protons.

On the table, this number of protons is called the element's atomic number.

The makers ended each row so that elements that behave alike landed in one column.

So, the periodic table is a chart of every known element, one box per element.

The boxes run in order of atomic number, and the atomic number is the number of protons.

So the cards, laid out in order row by row, make the periodic table.

Okay, now your turn.

A student is asked for the symbol of calcium, and for the number of protons in one calcium atom.

She has never memorized calcium, but the periodic table lies in front of her.

Where does she look, and what does she read?

Every box holds three things: the atomic number at the top, the symbol in the middle, and the name underneath.

She finds the name calcium: its box sits in row 4, second from the left.

She reads the symbol in the middle: Ca.

She reads the atomic number at the top: 20.

The atomic number is the number of protons, so one calcium atom has 20 protons.

Now consider zinc: she finds the name, reads Zn, reads 30, so 30 protons.

And chlorine: she finds the name, reads Cl, reads 17, so 17 protons.

So, to look an element up, find its name on the periodic table, read the symbol in the middle of its box, and read the atomic number at the top.

So her answer is Ca, atomic number 20, and 20 protons, all read off the table.

Okay, now your turn.

Here on a bench are a strip of copper, a lump of sulfur and a chip of silicon.

The copper is shiny, bends without breaking, and lights a bulb.

The sulfur is dull, crumbles, and leaves the bulb dark.

The silicon looks shiny, but shatters like glass, and the bulb glows faintly.

How many kinds of element are on the bench?

There are three kinds.

To sort them, ask three questions: is it shiny, does it conduct electricity, and does it bend without breaking?

Copper answers yes, yes, yes: an element like copper is called a metal.

But sulfur answers no, no, no: an element like sulfur is called a nonmetal.

But silicon answers yes, a little, and no: an element with some of a metal's properties and some of a nonmetal's is called a metalloid.

Scientists call the three properties luster, conductivity and malleability.

So, a metal is shiny, conducts electricity, and bends without breaking.

A nonmetal is dull, does not conduct electricity, and crumbles or is a gas.

A metalloid looks shiny but shatters, and conducts electricity only a little.

So copper is a metal, sulfur is a nonmetal, and silicon is a metalloid.

Okay, now your turn.

Here is the periodic table with the metals and the nonmetals shaded.

Metals fill the left side and the middle, and nonmetals sit at the right.

A heavy zigzag line runs between them, like a staircase.

Often there is no sample to test, only a name on the table.

So where do the metalloids sit?

They sit along the zigzag line.

The heavy zigzag line is called the staircase, and it is where the metals end and the nonmetals begin.

Vanadium sits to the left of the staircase, so vanadium is a metal.

But bromine sits to the right of the staircase, so bromine is a nonmetal.

But arsenic sits with the staircase running down its side, so arsenic is a metalloid.

And potassium, far to the left, is a metal, and iodine, to the right, is a nonmetal.

There are two exceptions: hydrogen, at the top left, is a nonmetal, and aluminum touches the staircase but is a metal.

So, read the element's position against the staircase.

Metals sit to the left of the staircase, nonmetals to the right, and metalloids along it.

So the metalloids sit in the darkest band, along the staircase itself.

Okay, now your turn.

Lithium, sodium and potassium sit one above another in the first column of the periodic table.

All three are soft metals a knife can cut.

And all three fizz in water.

Helium, neon and argon sit one above another in the last column: three gases that join with nothing.

Is it a coincidence that column-mates behave alike?

No, it is no coincidence.

A column of the table is called a group, numbered 1 to 18 across the top.

A row of the table is called a period, numbered 1 to 7 down the side.

Lithium, sodium and potassium are group 1; helium, neon and argon are group 18.

The makers stacked look-alikes in one column, so elements in the same group share properties.

Now consider period 3: sodium, silicon and chlorine are a metal, a metalloid and a nonmetal.

A period runs through all three kinds, so a row does not predict what an element is like.

So, a column of the table is called a group, and a row is called a period.

Elements in the same group share properties.

So the two columns are groups, and their shared properties are no coincidence.

Okay, now your turn.

Francium sits at the bottom of group 1.

Nobody has ever seen a lump of francium: only a few atoms have ever been gathered.

Yet chemists say with confidence that francium is a soft metal that reacts with water.

How can they know that about a sample nobody has held?

They read francium's group, because elements in the same group share properties.

Its group-mates are lithium, sodium, potassium, rubidium and cesium.

Each is a solid that conducts electricity and reacts with water.

Francium's row is blank, so they copy its group-mates' answers across.

Francium sits far to the left of the staircase, so francium is a metal.

Now consider radon, at the bottom of group 18.

Its five group-mates are gases that do not react with water, so radon should be one too.

Read the group, not the row. Iodine's row-mates silver and tin conduct electricity.

But its group-mates chlorine and bromine do not, and neither does iodine.

So, to predict what an element is like, read its group and copy its group-mates' properties across.

So francium's group-mates are soft metals that react with water, and francium is too.

Okay, now your turn.

Here is everything we found out about the periodic table.

Every element has a chemical symbol of one or two letters: the first a capital, the second small.

Count the capital letters, and you count the elements.

The periodic table is a chart of every known element, one box per element.

The boxes run in order of atomic number, and the atomic number is the number of protons.

To look an element up, find its name on the periodic table, read the symbol in the middle of its box, and read the atomic number at the top.

Okay, now your turn.

Here is everything we found out about the kinds of element and where they sit.

A metal is shiny, conducts electricity, and bends without breaking.

A nonmetal is dull, does not conduct electricity, and crumbles or is a gas.

And a metalloid looks shiny but shatters, and conducts electricity only a little.

Metals sit to the left of the staircase, nonmetals to the right, and metalloids along it.

There are two exceptions: hydrogen, at the top left, is a nonmetal, and aluminum touches the staircase but is a metal.

A column of the table is called a group, and a row is called a period.

Elements in the same group share properties.

But a row runs through all three kinds, so a row does not predict what an element is like.

So, to predict what an element is like, read its group and copy its group-mates' properties across.

Okay, now your turn.

A water bottle is labeled H₂O.

A brown bottle in the bathroom is labeled H₂O₂.

The two labels are one small 2 apart.

But you drink the first, and the second bleaches hair.

Chemists write every substance with letters and small numbers like these.

So how does one small number turn a drink into a bleach?

To find out, we will look at what the letters and the numbers on a label say, and how to count with them.

Okay, so are you ready to crack the code on the bottles?

A water bottle is labeled H₂O, and a fire extinguisher is labeled CO₂.

Here is one water particle drawn as circles: one large open circle joined to two small dark ones.

The label writes the same particle as a letter, a small 2 and another letter.

What does each letter say?

Every element has a chemical symbol, so H is hydrogen and O is oxygen.

Each atom lines up under its own letter.

And what does the small 2 count?

It sits low after the H and counts the atoms of the element just before it: 2 hydrogen atoms.

A small number written low after a symbol is called a subscript.

The O has no subscript, so the O counts one oxygen atom.

This way of writing a substance is called its chemical formula.

Now consider CO₂: C is carbon and O is oxygen.

The 2 after the O counts 2 oxygen atoms.

So, a chemical formula writes one particle of a substance.

Each symbol names one element in the particle, and a subscript counts the atoms of the element just before it.

So each of the two labels writes one particle.

Okay, now your turn.

A food-science book lists glucose, the sugar in grapes, as C₆H₁₂O₆.

One glucose particle is built from many atoms: how many of each element, and how many in all?

The formula tells you, in three steps.

A formula writes one particle, and each subscript counts that element's atoms.

One: take the symbols one at a time: C, then H, then O.

Two: read the subscript after each symbol: that subscript is that element's count.

C₆ is 6 carbon atoms, H₁₂ is 12 hydrogen atoms, and O₆ is 6 oxygen atoms.

Three: add the counts for the total: 6 plus 12 plus 6 is 24 atoms in one glucose particle.

Now consider methane, CH₄: the C with no subscript is 1 carbon atom.

The 4 after the H is 4 hydrogen atoms: 5 atoms in all.

So, to count the atoms in a formula, take the symbols one at a time, read each subscript as that element's count, and add the counts for the total.

So glucose is 6 carbon atoms, 12 hydrogen atoms and 6 oxygen atoms: 24 atoms in one particle.

Okay, now your turn.

H₂O is water, and you drink it.

H₂O₂ is hydrogen peroxide: it bleaches hair and stings in a cut.

The two formulas differ by one small 2: why does one small number make a different substance?

Because the 2 builds a different particle.

A subscript counts the atoms in one particle.

Count H₂O: 2 hydrogen atoms and 1 oxygen atom, 3 atoms.

Now compare the hydrogen peroxide particle, drawn beside the water particle: the peroxide particle holds one more oxygen atom.

Count H₂O₂: 2 hydrogen atoms and 2 oxygen atoms, 4 atoms: a different particle.

A substance's properties come from its particles, so a different particle is a different substance.

H₂O₂ is not water with extra oxygen mixed in; hydrogen peroxide is its own substance.

Now consider CO, a poisonous gas, and CO₂, the gas you breathe out.

The CO₂ particle holds one more oxygen atom: a different particle, so a different substance.

So, change a subscript, and you have built a different particle.

And a different particle is a different substance.

So the two bottles are one small 2 apart, and you drink one while the other bleaches hair.

Okay, now your turn.

Two water particles sit side by side, and chemists want to write the two particles, not one.

You cannot change the subscripts: a changed subscript builds a different particle, not water.

So how do you write two water particles?

Chemists write a full-size number in front of the formula: 2H₂O.

A full-size number written in front of a formula is called a coefficient, and it counts whole particles.

A subscript counts the atoms in one particle: one water particle holds 2 hydrogen atoms and 1 oxygen atom.

So two particles hold 4 hydrogen atoms and 2 oxygen atoms.

So a coefficient multiplies every count in the formula: 6 atoms in all.

Now consider three carbon dioxide particles, written 3CO₂.

Three times 1 carbon atom is 3 carbon atoms, and three times 2 oxygen atoms is 6 oxygen atoms: 9 atoms in all.

There are two different 2s: the full-size 2 in front counts particles, and the small, low 2 after the H counts hydrogen atoms.

So, a coefficient counts whole particles, and it multiplies every count in the formula.

So two water particles are written 2H₂O, with 6 atoms in all.

Okay, now your turn.

The medicine in milk of magnesia is magnesium hydroxide, Mg(OH)₂.

Parentheses wrap the O and the H, and a small 2 follows the closing parenthesis.

What does that 2 count?

It repeats the whole group inside the parentheses.

A subscript counts the element just before it, and a coefficient counts whole particles.

The parentheses hold a group of atoms: one oxygen atom and one hydrogen atom.

The 2 comes just after the group, so it repeats the whole group: two OH groups.

So Mg(OH)₂ holds 1 magnesium atom, 2 oxygen atoms and 2 hydrogen atoms: 5 atoms in one particle.

Now consider Al(OH)₃: the 3 repeats the OH group three times: 1 aluminum atom, 3 oxygen atoms and 3 hydrogen atoms, so 7 atoms in all.

With a coefficient in front, as in 2Mg(OH)₂, count one particle first, parentheses and all.

Then multiply every count by the coefficient: 2 magnesium atoms, 4 oxygen atoms and 4 hydrogen atoms: 10 atoms.

So, a subscript after a closing parenthesis repeats the whole group inside the parentheses.

When a coefficient sits in front, count one particle first, then multiply every count by the coefficient.

Okay, now your turn.

Here is everything we found out about chemical formulas.

Every element has a chemical symbol of one or two letters: the first a capital, the second small.

A chemical formula writes one particle of a substance.

Each symbol names one element in the particle.

A small number written low after a symbol is called a subscript.

A subscript counts the atoms of the element just before it, in one particle.

A symbol with no subscript counts one atom.

To count the atoms in a formula, take the symbols one at a time, read each subscript as that element's count, and add the counts for the total.

Okay, now your turn.

Here is everything we found out about changing the numbers in a formula.

Change a subscript, and you have built a different particle.

A substance's properties come from its particles, so a different particle is a different substance.

A full-size number in front of a formula is called a coefficient, and it counts whole particles.

A coefficient multiplies every count in the formula.

A subscript after a closing parenthesis repeats the whole group inside the parentheses.

And when a coefficient sits in front as well, count one particle first, then multiply every count by the coefficient.

Okay, now your turn.

Here is a kitchen table. A sheet of paper lies on it, torn in two, and an ice cube has melted into a puddle.

By the back door is an iron nail that has been outside for a month: orange, flaky, crumbling.

Something changed in all three. But is the stuff at the end still the stuff you started with?

One question sorts every change like these.

To find out, we will look at changes that make new stuff, and changes that do not.

Okay, so are you ready to find the one question that tells them apart?

A student tears a sheet of paper into two pieces.

She lets an ice cube melt in a glass.

And she stirs a spoon of sugar into water until the grains vanish.

In each case, is the substance you end with the same substance you started with?

Yes, it is, every time.

Each torn piece is still paper: zoom in, and the same paper particles are there.

Look at the glass: the liquid is water, the same water particles, now rearranged.

She sips the sugar water: it tastes sweet, so the sugar is still there, spread among the water particles.

She bends a copper wire into a hook.

It is still copper.

But if an iron nail stays in the rain for a month, orange flakes grow that are not iron.

A change in which no new substance forms is called a physical change.

So, here's what we've seen. A physical change is a change in which no new substance forms.

The size, the shape or the state changes, but the particles are the same particles, arranged or moving differently.

So the torn paper, the melted ice and the dissolved sugar are three physical changes.

Okay, now your turn.

An iron nail left out for a month turns orange-brown and flaky.

Iron is shiny gray, bends without breaking, and a magnet pulls it hard.

The flakes are dull, crumble in your fingers, and a magnet barely pulls them.

They are not iron in a new shape. What are they?

They are a new substance, and the table shows it: every property differs.

Iron atoms in the nail joined with oxygen atoms from the air.

The joined atoms make particles of a new substance, called rust.

Rust is not iron with oxygen stuck on top: the atoms joined into new particles.

A change in which a new substance forms is called a chemical change.

Ask the same question of every change: did a new substance form?

Wood burning to ash, baking soda fizzing with a new gas, an egg frying solid: each gets a yes.

Ice melting to water, salt dissolving: each gets a no.

So, a chemical change is a change in which a new substance forms.

The new substance has its own properties, different from the substance you started with.

So the rusting nail is a chemical change.

Okay, now your turn.

A spoon of baking soda goes into a beaker of vinegar, and it fizzes.

A match burns with a yellow flame and warms your fingers.

A cut apple turns brown.

Two clear liquids poured together turn cloudy white.

What do these scenes have in common?

In each, a chemical change happened, and something you could notice changed.

Nobody can see the atoms join, yet a cook, a mechanic and a doctor spot chemical changes every day.

The apple turned brown: a color change.

Bubbles poured off the vinegar: a gas forming.

A white solid appeared where two liquids met: a solid forming, called a precipitate.

The match warmed your fingers: a temperature change.

And the match burned with a flame: light given out.

Boiling water bubbles too, and boiling is a physical change.

So, five signs tell you a chemical change may have happened: a color change, a gas forming, a solid forming from two liquids, a temperature change, and light given out.

Each sign is a clue, not proof: each can appear without a chemical change.

So the cook, the mechanic and the doctor watch for these five signs, then check for a new substance.

Okay, now your turn.

Read these four sentences.

A tablet dropped into water fizzes.

A pale green leaf turns brown.

The mixture in the beaker gets warm on its own.

Two clear liquids poured together turn cloudy white.

Each sentence shows one sign of a chemical change. Which sign is which?

Look for the word that tells what was seen.

Fizzes: bubbles of gas left the water, so the sign is a gas forming.

Turns brown: the leaf's color changed, so the sign is a color change.

The mixture gets warm on its own, so the sign is a temperature change.

Turns cloudy white: a solid formed where two liquids met, so the sign is a solid forming, a precipitate.

Some scenes show two signs at once: a log burning in a fireplace glows, and the room gets warmer.

So, to match a described change to its sign, look for the word that tells what was seen.

So the four sentences show a gas forming, a color change, a temperature change and a solid forming.

Okay, now your turn.

Water boils in a kettle, and bubbles pour off it.

Baking soda meets vinegar, and bubbles pour off that too.

One is a physical change and one is a chemical change. How can you tell which is which?

Bubbles are a clue, not proof, so ask the question that decides: did a new substance form?

Inside the kettle's bubbles is water as a gas: the same water particles as in the liquid.

No new substance formed, so boiling is a physical change.

Inside the vinegar's bubbles is carbon dioxide, and there was none in the vinegar or baking soda.

A new substance formed, so the fizzing is a chemical change.

Whether the change can be undone is not the test.

If you rip a newspaper in half, nobody can mend it.

Yet the pieces are still newspaper: a physical change.

Wax melting is still wax, a physical change; wax burning to soot is a chemical change.

So, to decide whether a change is physical or chemical, ask one question: did a new substance form?

So the kettle's water bubbles are a physical change, and the vinegar's carbon dioxide bubbles are a chemical change.

Okay, now your turn.

A test asks: is frying an egg physical or chemical? Explain.

A student writes chemical and stops.

That earns half the points; the evidence earns the rest. What does a full answer say?

A full answer has three lines.

Line one says what you observed.

Line two says what that shows about whether a new substance formed.

Line three names the change.

For the egg: I observed the clear, runny white turn white and solid.

That shows a new substance formed: the solid has properties the runny white did not.

So the change is a chemical change.

For a physical change, the evidence is that the substance is still there. Now consider a melting ice cube: I observed that the liquid looks, pours and tastes like water.

It is still water, so no new substance formed, and the change is a physical change.

'It cannot be undone' is not the test; 'it fizzed' counts only if something fizzed.

So, to justify the call, say what you observed, say what it shows about a new substance, then name the change.

So on the test, the egg white turned solid, so a new substance formed: a chemical change.

Okay, now your turn.

Here is everything we found out about physical and chemical change.

A physical change is a change in which no new substance forms.

The size, the shape or the state changes, but the particles are the same particles, arranged or moving differently.

A chemical change is a change in which a new substance forms.

The new substance has its own properties, different from the substance you started with.

Five signs tell you a chemical change may have happened: a color change, a gas forming, a solid forming from two liquids, a temperature change, and light given out.

A solid that forms when two liquids meet is called a precipitate.

And each sign is a clue, not proof, because each can appear without a chemical change.

Okay, now your turn.

Here is everything we found out about telling the two changes apart.

To match a described change to its sign, look for the word that tells what was seen.

To decide whether a change is physical or chemical, ask one question: did a new substance form?

The signs are clues; the new-substance question decides.

And whether the change can be undone is not the test.

To justify the call, say what you observed, say what it shows about a new substance, then name the change.

Okay, now your turn.

Here is a pad of steel wool on a balance, and the balance reads 10.0 grams.

After a week damp in the air, the pad turns brown and crumbly.

Back on the balance it reads 11.2 grams, and nobody added anything.

And here is a tree that grew from 2.3 kilograms to 76.7 kilograms while its soil barely changed.

So where does the extra mass come from, and can mass ever go missing?

To find out, we will look at what happens to the atoms when one substance turns into another, and what happens to the mass.

Okay, so are you ready to find out where a rusting pad's extra mass comes from?

Suppose you leave a pad of steel wool damp in the air for a week.

On day one the balance read 10.0 grams; a week later the pad is brown and crumbly, and it reads 11.2 grams.

Nobody added anything: where did the extra 1.2 grams come from?

The extra 1.2 grams came from the air.

The shiny pad is iron atoms, and the air holds pairs of oxygen atoms.

Watch the oxygen atoms land on the iron atoms and join on.

Iron atoms joined to oxygen atoms make a new substance, iron oxide: rust.

As each oxygen atom joins, the reading climbs.

Chemists write the change on one line: iron and oxygen react to make iron oxide.

Now consider a copper pan heated over a flame.

A black coat grows, and the pan gains mass.

Oxygen atoms from the air joined the copper atoms.

So, here's what we've seen. The atoms in a new substance come from the substances that were there before.

No atom is made from nothing, and no atom is lost.

So the pad's extra 1.2 grams is the mass of the oxygen atoms that joined on.

Okay, now your turn.

Here are three changes drawn before and after: ice melting, sugar dissolving, and hydrogen burning in oxygen.

Which of these three changes makes a new substance?

Hydrogen burning does. Watch hydrogen burn: each pair of hydrogen atoms lets go, and each pair of oxygen atoms lets go.

Now each oxygen atom joins two hydrogen atoms: a water molecule, a new substance.

Count the circles: the same 8 hydrogen atoms and 4 oxygen atoms before and after.

Now watch ice melt: the water molecules move, but every atom keeps its partners.

A change in which atoms rearrange into new substances is called a chemical reaction.

Now consider charcoal burning: each carbon atom joins two oxygen atoms, a chemical reaction.

And when sugar dissolves, the sugar molecules spread among the water molecules, still whole, so that is not a reaction.

Each reaction goes on one line: hydrogen plus oxygen makes water.

So, here's what we've seen. In a chemical reaction the atoms let go of their old partners and join new ones.

So new substances form.

Of the three drawings, only hydrogen burning shows atoms changing partners.

Okay, now your turn.