The three states of matter and the particle model

What's coming out of a boiling kettle in the clear gap right at the spout, where you can see nothing at all? A few centimetres further out, a thick white cloud appears, and nothing was added to the air in between. It's tempting to call the white cloud the steam, and the clear gap plain air. But that clear gap is the hottest part of the plume, which is a strange thing for plain air to be. Answering it takes a picture of water that your eyes can't give you.

Everything around you is made of particles, pieces of matter far too small to see. The same particles can be arranged in three ways, the three states of matter: solid, liquid and gas. Drawing matter this way is called the particle model. In a solid, the particles are packed together in a regular pattern, each one touching its neighbours. Each particle vibrates about a fixed position, and that's why a solid keeps its shape. In a liquid, the particles are still close together and touching, but the regular pattern has gone, and they're arranged at random. They slide past one another, which is why a liquid flows and takes the shape of its container. Now a prediction. In a gas, what has happened to the gaps between the particles? In a gas, the particles are far apart and arranged at random, and they move quickly in all directions. The space between gas particles is empty. There's nothing in it at all, not even air. With all that space, a gas spreads out to fill any container it's in, and it can be squashed into a smaller one. That gives each state one movement word. Solid particles vibrate, liquid particles slide, and gas particles move quickly in all directions. Now a quick check on those three movement words, with no state named for you. In salty water, which word fits the particles: vibrate, slide, or move quickly in all directions? Slide. Salty water is a liquid, so its particles slide past one another, and vibrating about a fixed position belongs to a solid only. One examiner's report on a Foundation paper describes answers that were confused about particle movement and talked about particles vibrating. The fix is to name the state first, then use that state's own movement word, and keep vibrate for solids. The states differ in energy too. Gas particles have the most energy, liquid particles have less, and solid particles have the least. Heating a substance gives its particles more energy, so they move faster. What heating never does is make a particle bigger. Heat a substance and its particles stay the same size: what changes is the gaps between them and their speed. Now a different one. A balloon swells in hot sunshine: did each air particle get bigger? No. Each particle stayed the same size. The particles gained energy and moved faster, so they spread further apart and stretched the balloon. One examiner's report on a Foundation paper names states of matter among the questions students answered particularly well. If the three states make sense to you, you're in good company, and what's left is precision.

A change of state is the same particles moving from one of those three arrangements into another. Melting is solid to liquid. As a solid is heated, its particles vibrate more and more, until they have enough energy to break free of their fixed positions. The temperature where that happens is the melting point, and for water it's nought degrees C. Freezing is melting run backwards, liquid to solid. The particles lose energy and settle into fixed positions, at that same temperature. Boiling is liquid to gas. The particles gain enough energy to escape from one another completely, all through the liquid. That happens at one temperature, the boiling point, and for water it's one hundred degrees C. Now predict this. As a gas cools into a liquid, what do its particles do? The particles lose energy, slow down and come close together again. That change is called condensing. Evaporating also turns a liquid into a gas, but only from the surface and below the boiling point, which is why a puddle dries on a cool day. Every change of state can be run backwards. Cool what you melted and it freezes again, so the substance you started with is never lost. A question about a change of state names a before and an after, so the answer has two halves: the particles before, and the particles after. Try this one. Copper sulfate solution is left in a warm evaporating basin, and as the water evaporates, blue crystals form. Describe how the particles are arranged and how they move, first in the solution, then in the crystals, and say which has more energy. Try it on paper before I show you the answer. Before, while it's still a liquid, they're close together and arranged at random, and they slide past one another. In the crystals, the particles sit in a regular pattern, touching, and each one vibrates about a fixed position. And the particles have more energy in the solution than they do once they're held in the crystals. One examiner's report on a Higher paper describes students who wrote about the particles in the solid but not the solution, which held them to two of the three marks available. The same report notes a noticeable number who also wrote about evaporation and a gas, which the question never asked for, so that time was spent on nothing. So the habit is to fill both halves the question names, then stop, with no sentence about a state it didn't mention.

Every change of state is a physical change, and the other kind of change a substance can go through is a chemical change. In a physical change, no new substance is made. The particles are the same ones, only rearranged. In a chemical change, at least one new substance is made. To tell them apart, ask two questions in this order: is a new substance made, and can you get the original back. Run them on ice melting. It's still water, so no new substance is made, and freezing gets the ice back, so melting is physical. And because the particles are the same ones, the mass doesn't change. Ten grams of ice melts into ten grams of water. Now run them on bread turning into toast. The brown layer is a new substance, and nothing turns toast back into bread, so toasting is chemical. A Foundation examiner's report sets out wrong reasons students gave for cooking an egg being a chemical change, and one of them reads: "it's being put over heat that makes it a chemical change." Why doesn't being put over heat prove a chemical change? Heat melts ice as well, and melting is physical, so heat on its own proves nothing. The fix is a reason that answers one of the two questions: a new substance was made, or there's no getting the original back. A candle makes a good test case. The wax just below the flame melts, and later sets hard again, while the wax drawn up the wick burns and gives off carbon dioxide and water. Melting wax and burning wax: which is physical, which is chemical, and two reasons each? The melting is physical. No new substance is made, because it's still wax, and it goes back to solid wax when it cools. The burning is chemical. New substances are made, and there's no way to turn those gases back into a candle. Each change got two different reasons, one from each of the two questions, and that pairing is what makes an explanation rather than a guess.

Now four to finish. Runny chocolate sets hard in a fridge: which change of state is that?

Freezing. And it happens well above nought degrees C, because chocolate sets at its own temperatures, not at water's.

Now a second case. Breath fogs a cold window: which change, and what did the particles do? Gas to liquid, so that's condensing. The particles lost energy, slowed down and came close together as tiny drops.

Now one about rust. A nail left outside goes rusty: physical or chemical, and how do you know? Chemical. Rust is a new substance, and drying or cooling the nail will never turn it back into shiny iron.

Now the rule itself, in your own words. What is a physical change? A change where no new substance is made, so the original can be got back, and every change of state is one.

Which brings us back to the kettle. The clear gap at the spout is water in the gas state, and water as a gas can't be seen. A few centimetres out, that gas cools and condenses into tiny drops of liquid water, and those drops are the white cloud. So the cloud you can see is liquid, and the clear gap is the freshly boiled gas, which is why it's the hottest part.

If you could explain that kettle to someone else now, give this video a thumbs-up, because this one's done. If it's still hazy, leave it un-thumbed and come back to it later in the week.

Next in the chapter is The limits of the simple particle model, a Higher tier video, and if you're taking Foundation, the one to watch after this is Mixtures, and choosing a separation technique. Hopefully that's next in your feed, but if not, search for ScholaFly C H oh three, oh two.

Every other video is on scholafly.com too, easy to search by video code or by keyword. Whatever you're revising, go there and have a look, or, watch the next video.