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!

A 3-minute excerpt.

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!


The Transcript
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.

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?

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.

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

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 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.

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.

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 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.