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Shedding Light on Heat

Episode 2:

Changes of State

The Heat is on! A huge amount of the technology and comforts that we have in our world just wouldn’t

exist if it wasn’t for what we’ve learned about the way heat behaves. So keep cool and use this excellent

series to teach your students everything that they need to know about heat, including its effect on things

and how it transfers from one thing to another.

In Episode 2, Changes of State, we look at the differences between solids, liquids, and gases, and at why

things change state when they absorb or lose heat energy. We also look at the strange example of carbon

dioxide, which doesn’t follow the same rules that that most other substances do!

Contents:

Part A: Introduction. A re-cap of the awesome Kinetic Theory.

Part B: Solids and Liquids: The atoms in a solid are held together in fixed positions. But how? Why

does a solid melt when it is heated? And do water molecules themselves actually change when ice melts

into water?

Part C: Liquids and Gases: You can compress air easily with a bicycle-tyre pump but you can’t do the

same thing if the pump was filled with water. Why not? And what exactly is happening to the water

molecules when liquid water is boiling?

Part D: Sublimation: Carbon dioxide is normally a gas, but if you cool it down to -78.5°C, it turns into a

solid. When it is re-heated, it doesn’t melt!

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Shedding Light on Heat Episode 2: Changes of State

Part A: Introduction

Solids, Liquids, and Gases. Pretty much everything on Earth

is either a solid, a liquid, or a gas. But what’s the difference,

at the atomic level, between these three states of matter?

Well let’s go back to the previous episode for a minute to see

what we already know.

So we’ve seen that in, for example, a solid piece of iron the

atoms are continuously vibrating.

The kinetic theory tells us that when the iron is cold, the iron atoms vibrate relatively slowly but when the

iron is hot (and you should therefore strike, no, that’s just a joke), the atoms are vibrating relatively

quickly. As evidence for this idea that atoms move at different speeds at different temperatures, we saw in

our last episode that food dye diffuses much more quickly in hot water than in cold water. The reason for

this is that the water molecules in hot water collide with the food dye molecules much more quickly and

with more force (and remember it’s all just random movement). This pushes the food dye molecules

around more quickly.

The kinetic theory also helps explain what solids, liquids, and gases are and why things change state. So

what happens exactly when something changes state from say a solid to a liquid, or from a liquid to a

gas? Well, that what we’re going to look at in this lesson. So let’s begin.

Part B: Solids and Liquids

By definition, a solid is composed of atoms, or groups of

atoms called molecules, that are held in fixed positions. The

atoms vibrate furiously in every direction, but they basically

stay where they are. So what holds them in place? To answer

that we need to look at atoms themselves.

Atoms are made up of even smaller particles called protons,

neutrons, and electrons. The protons and neutrons are each

about 2000 times more massive than electrons. The protons

and neutrons form the nucleus of the atom which is in the centre of the atom and the electrons kind of

move around the nucleus at really high speeds. Because of the way that electrons move, every atom is

basically spherical. The protons have what we call a positive charge and the electrons have a negative

charge.

Charge is kind of like magnetism. A

magnet has a North pole and a South pole

and the two poles attract each other

thanks to a magnetic force between them.

The positively charged protons and the

negatively charged electrons attract one

another with what’s called an electrostatic

force and so the atom maintains its form.

However, an electrostatic force of

attraction also exists between atoms,

because the protons of one atom can

attract the electrons of another atom. As a result, all atoms are a little bit sticky.

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Different atoms have a different amount of

stickiness depending on how many protons the

atom has and how the electrons move around

the atom. Molecules like water molecules and

sucrose molecules (sucrose is what we typically

call table sugar) are also a little bit sticky.

So in a solid, the atoms, or in this case the water

molecules, are vibrating but the electrostatic

forces between them are strong enough to keep

them in fixed positions. The molecules vibrate

but they don’t move around.

What happens though if we heat a solid? The

molecules (a) vibrate faster and faster as the

temperature increases and (b) as a result of the

extra vibration, they move a little further apart

which weakens the size of the electrostatic force

between them, just like the force between two

magnets weakens as they get further and further

apart. Eventually the water molecules are

vibrating so fast that the forces holding them in

place are no longer strong enough to hold them in

place and they break free of their fixed positions. The solid becomes a liquid, which is free to slosh and

splash around. This process of a solid turning into a liquid (I’m sure you already know) is called melting

and the temperature at which something melts is called its melting point.

The same thing happens if you heat up solid tin,

which is made up entirely of tin atoms. The

atoms vibrate faster and faster until the

electrostatic forces can’t hold them in place

anymore and the tin melts, turning into a liquid.

A liquid is a substance where the atoms or

molecules are free to slide around each other

although they’re still in contact with each other.

Liquids have no fixed shape but take the shape

of whatever container they’re in.

Now melting doesn’t change what the

substance is. The water molecules are still

the same water molecules whether they are

in the form of solid ice or of liquid water.

They’re all still H2O molecules, but in the

liquid state, the molecules have enough

energy to break away from their fixed

positions.

Likewise the tin atoms are still tin atoms

when the tin melts. No new substance

forms when a solid melts into a liquid.

Now what happens if we cool a liquid down?

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If a liquid is cooled down, the atoms or molecules

vibrate and move around more and more slowly and at

a certain point they’re no longer vibrating fast enough

to overcome the electrostatic forces between them and

they stick together again in fixed positions. The liquid

becomes a solid. We call this process freezing,

although if the substance is usually a solid, like rock,

then we also call it solidifying. It would seem weird to

say that when lava cools down it freezes and that these

volcanic rocks are frozen lava, but in a sense they are! The temperature at which a solid melts (or which a

liquid freezes) depends on how strong the electrostatic forces between the atoms or molecules are. It’s a

bit like magnets. Some magnets can be separated easily, while other magnets need much more force.

Water needs to be cooled down to 0°C before it starts freezing and, of course, ice that is below 0°C has to

warm up to 0°C before it starts melting. Generally, the melting point and the freezing point of a pure

substance are the same.

Table sugar (which is technically called sucrose) melts at 186°C.

The electrostatic forces between the sucrose molecules are

obviously much stronger than they are between water molecules.

Sulfur melts at 115°C so the electrostatic forces between the sulfur

atoms are stronger than they are between water molecules but not

as strong as the forces between sucrose molecules. Tin melts at

232°C. This is one of the lowest melting points of any metal. Most

metals melt at a much higher temperature because the electrostatic forces between the atoms are much

stronger.

Iron melts at 1538°C. This is liquid iron that is above this

temperature. When it cools to below its melting point it

solidifies.

Now some things, like certain types of plastic, don’t actually

have a definite melting point, they just get softer and softer as

they’re heated until they become liquids. Here you can see what

hot gooey plastic looks like. Plastics can easily be moulded into

whatever shape you want if they’re heated up to the right

temperature.

In a process called injection moulding, small plastic pellets

are heated until they’re soft enough to be injected into a steel

mould. The plastic then cools down, hardens, and the part is

ejected. The process can be repeated again and again.

Injection moulding is just one of many processes used to create a huge number of plastic parts. The

reason plastics don’t have a definite melting point has to do with the size of the molecules that make them

up.

Polyethylene for example, the most commonly used plastic in the world (it’s used to make milk bottles

and cling wrap), is made of long tangled chains of carbon atoms with two hydrogen atoms attached to

each carbon atom. Here I’ve colour-coded the chains. Each chain might be thousands of carbon atoms

long. The atoms are attracted to each other, which ordinarily keeps the plastic solid, but when the plastic

Melting Points of Some Common Plastics

Type of Plastic Melting Point

Polyethylene 115 - 135°C

Polypropylene 130 - 171°C

PVC 100 - 260°C

nylon 190 - 350°C

polystyrene about 240°C

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is heated and the atoms vibrate faster and faster,

this section might break free, but this section

might stay solid. As more and more parts of the

long molecules break free the plastic gets softer

and softer. So instead of having a definite melting

point, polyethylene (and many plastics) melt over

a range of temperatures.

Glass is similar. It too gets runnier and runnier as

it heats up. Above temperatures of about 1500°C,

depending on the type of glass, it’s so runny that

it’s considered a liquid. However, it doesn’t have a definite, clear cut temperature where it melts.

However, all elements (that is, substances made up of only one type of atom) or substances which are

made of small groups of atoms (like water and glucose, a type of sugar) have definite melting points.

Part C: Liquids and Gases

So, the atoms and molecules in a solid are held together by electrostatic forces but if the solid is heated

the atoms or molecules break free of their fixed positions. The solid melts into a liquid. However, the

electrostatic forces are still strong enough to keep the atoms or molecules in contact.

When you continue to heat up a liquid, the atoms or

molecules that make it up continue to vibrate faster and faster.

Eventually the vibration is so fast that the electrostatic forces

are no longer strong enough to keep the atoms or molecules

together and they shoot off into the air. The liquid turns into a

gas. A gas is made up of extremely fast moving atoms or

molecules travelling at literally 1000s of kilometres per hour,

that bounce around all over the place, crashing into each other

and into things that are around them. Of course they don’t move very far between collisions, we’re talking

billionths of a metre.

The fact that we can

easily compress a gas

is strong evidence

that the atoms or

molecules in the gas

have completely

separated from each

other. A simple

bicycle-tyre pump can push the atoms and molecules that make up the air closer together. Even with a

small force, I can easily compress the amount of air in the pump by more than half of its original volume.

The simulation, which I’m controlling with my mouse shows

what’s happening.

Solids and liquids, however, can’t be compressed easily, since

the atoms that make them up are already really close together.

The stones at the bottom of a stone wall, for example, are

weighed down by tonnes of stones above them, but they only

compress by something like millionths of a millimetre.

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The process of a liquid changing to a gas is called “boiling” and the temperature at which a liquid

becomes a gas is called the liquid’s boiling point. When water turns into a gas, the gas is given a special

name: steam. The bubbles in boiling liquid water are steam: little by little the liquid water turns into

gaseous water.

The boiling point of a liquid again depends on the

electrostatic forces between the atoms or molecules

that make it up. When the electrostatic forces are

weaker, it doesn’t take as much energy to separate

the atoms or molecules apart, and so the boiling

point is lower.

When the electrostatic forces are stronger, the

boiling point is higher because it takes more energy

to separate the atoms or molecules apart.

The boiling point of water is 100°C, and of methylated spirits is 78°C. Sulfur boils at about 444°C.

Evaporation is similar to boiling but can occur even at temperatures below boiling point. The water I

poured on the paper towel on the left had completely evaporated after about 1 hour on this warm spring

day. So why does water evaporate?

I said earlier that water molecules in cold water move and vibrate on average more slowly than water

molecules in hot water. They don’t move very far of course because they’re surrounded by other water

molecules.

However, regardless of the temperature, a water molecule will occasionally just randomly be bumped

with enough force to send it flying off into the air (there goes one now) even if the temperature is less

than 100°C. That’s evaporation. (There goes another one.)

Evaporation occurs because water molecules are all moving at different speeds, and the faster ones quite

often gain enough energy (just through random collisions) to break free. The hotter the water, the faster

the evaporation, because, on average, the water molecules are moving faster.

About 1% of the atmosphere around us is actually

water that has evaporated. This water is called water

vapour. Vapour (spelled vapor in US English) and

evaporation (pronounced evaporation of course) are

obviously related words.

If a gas is cooled, the atoms or molecules slow down to

the point where they are aren’t moving fast enough

anymore to overcome the electrostatic force of

attraction between them, and the gas turns back into a

liquid. This process is called condensation and the temperature at which this happens is called the

condensation point. The condensation point is the same as the boiling point.

Quite often, water itself that has condensed is called

condensation. This ice-cream tub cools the water molecules that

were in the air to the point where the molecules stick together

and condense. People will often say something like

“condensation has formed on the plastic”. So the word

condensation can mean both the process and the stuff that has

formed as a result of the process.

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The breath that we exhale contains quite a lot of water molecules that have evaporated from our lungs and

from our mouths. When it’s cold, it’s only about 1°C at the moment, the water condenses as soon as it

leaves our bodies and forms tiny tiny water droplets which we see as a kind of cloud.

Actual clouds are made up of tiny water

droplets that have condensed from all the water

that has evaporated mostly from the oceans.

The air is made of about 80% nitrogen gas and

about 20% oxygen gas. If you cool down air to

minus 196°C the nitrogen gas will condense and

you’ll end up with liquid nitrogen which is what

you’re seeing here. As it heats back up, liquid

nitrogen boils very quickly and turns back into

a gas.

Part D: Sublimation

This is dry ice. It’s solid carbon dioxide (CO2) made by

cooling carbon dioxide gas down to minus 78.5°C. Carbon

dioxide gas is unusual in that it doesn’t condense into a liquid

when it’s cooled down, it turns directly from a gas into a

solid.

When the dry ice warms up again, it doesn’t melt like water

ice does, it turns directly from a solid to a gas. It’s just one of

those things in nature. Here I’ve placed some dry ice and

some water ice into two beakers and I’ve placed the two beakers onto a hot plate. The water ice is

melting, but the dry ice is said to be “subliming”, not melting.

Sublimation is the name given to the process of

a substance changing state directly from a solid

to a gas without first turning into a liquid. Solid

water (that is, water ice) melts at 0°C while

solid carbon dioxide sublimes at -78.5°C. The

opposite of sublimation is deposition. Solid

carbon dioxide is called dry ice because it looks

like water ice, but it doesn’t from a liquid. The

solid carbon dioxide turns directly into gaseous

carbon dioxide.

About 15 minutes later, the water ice had completely melted into liquid water and the dry ice had

completely sublimed into carbon dioxide gas which had then spread out into the air of the room that we

were filming in.

If I place two small pellets

of dry ice into a conical

flask and place a balloon

over the opening, I can

trap the carbon dioxide

gas that forms as the dry

ice sublimes. The balloon

keeps getting bigger and bigger as more and more carbon dioxide is produced. Whenever a solid or a

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liquid changes state into a gas, the gas takes up much much more space than the original solid or liquid

(when the gas is at normal atmospheric pressure of course).

This because in a gas,

the atoms or

molecules spread out

and they’re not in

contact with each

other except when

they collide. In fact,

just 1 litre of dry ice

expands out to over 800 litres of carbon dioxide gas when it fully sublimes. About 13 minutes later, the

balloon had ballooned right out and couldn’t take any more.

Here you can see what’s left of the two dry ice pellets that I started with.

Dry ice is so cold that it cools the air around it to the point where all the water vapour in the air first

condenses and then freezes on the beaker.

Dry ice is used for many industrial processes and it’s commonly used to

create mist in theatres by dropping it into water. The clouds or mist that

you can see are actually made of water droplets. The dry ice cools the

air around it, so the water in the air condenses and forms, basically,

clouds of water droplets. The actual carbon dioxide gas being produced

isn’t visible. Yep, dry ice is pretty cool stuff. Literally.

So the Kinetic Theory of Matter combined with the fact that atoms are sticky thanks to being made up of

positively charged protons and negatively charged electrons, neatly explains what solids, liquids and

gases are and how and why substances can change state.

But things don’t just change temperature and state when they’re heated up or cooled down, they can also

change size. They can literally expand and therefore take up more space, and they can contract. This

affects everything from the natural world of oceans and rivers, to the modern industrial world of bridges

and buildings. And so, it’s the expansion and contraction caused by heating and cooling that we’ll be

looking at in our next episode. See you then.

CREDITS:

Written, directed, and presented by Spiro Liacos

The simulations of the solids, liquids, and gases were created by

PhET Interactive Simulations

University of Colorado Boulder

https://phet.colorado.edu

https://phet.colorado.edu/sims/html/states-of-matter-basics/latest/states-of-matter-basics_en.html

cooling metal by liquid nitrogen by Konstantin Soin. Licensed under a Creative Commons License.

Precious Plastic - Building machines by davehakkens. Licensed under a Creative Commons License.

Precious Plastic - Build the injection by davehakkens. Licensed under a Creative Commons License.

Extrude beams from plastic waste by davehakkens. Licensed under a Creative Commons License.

Plastic Injection Molding by engineerguy. Licensed under a Creative Commons License.