Solids, Liquids and Gases
Three states, one explanation. Everything a solid, a liquid or a gas does comes from how tightly its particles are packed and how much they move.
The three states
Solid, liquid and gas are the three states of matter, and the whole difference between them comes down to how the particles are arranged.
The particles themselves never change. Ice, water and steam are the same water particles throughout, sitting in three different arrangements, and if a child leaves this topic holding on to only one sentence it should be that one. Almost every muddle further down the page starts with someone quietly assuming that melting turns ice particles into water particles.
Solids
In a solid the particles are packed tight in a fixed pattern, held there by the pull between them. They are not still. Each one vibrates on the spot, sometimes fast enough that you can feel it as warmth, but none of them can get past its neighbours to swap places.
Because of that:
- A solid keeps its own shape. A brick stays brick-shaped in any container.
- A solid keeps its own volume.
- A solid cannot be squashed much, because there is almost no space between the particles.
Liquids
In a liquid the particles are still touching, close enough that there is barely any gap left to squeeze out, but they are no longer locked into a pattern and can slide past one another freely. That single difference explains everything a liquid does.
- A liquid takes the shape of whatever it is poured into.
- A liquid keeps its own volume. Pouring 200 ml into a wide bowl still gives 200 ml.
- A liquid cannot be squashed much either. The particles are already touching.
Gases
In a gas the particles have broken away from each other completely and are flying about in every direction, fast, colliding and bouncing off. They spend almost all their time nowhere near one another, which means most of what you are calling a gas is actually empty space.
- A gas fills its container completely, taking both its shape and its volume.
- A gas can be squashed, because there is space between the particles to squash into. That is what a bicycle pump does.
- A gas spreads out on its own, which is why you smell food from another room.
All three compared
Two questions tell you the state
Does it keep its own shape? Only a solid does.
Does it keep its own volume? Solids and liquids do. A gas does not.
Keeps both, it is a solid. Keeps volume only, it is a liquid. Keeps neither, it is a gas.
Run those two questions on anything and you will place it correctly, including the awkward cases that are designed to catch people out. Ask them in that order, shape first, and the answer falls out.
Common mistakes
1. Calling powders liquids
Sand and sugar pour, settle flat and take the shape of whatever you tip them into, which is exactly what the first test asks about, so calling them liquids is a reasonable answer to a reasonable question. The problem is where you are looking. Zoom in on one grain of sand and it keeps its own shape completely. A powder is not a liquid, it is a very large number of very small solids.
2. Thinking the particles change
Believing that ice particles become different, water-shaped particles when they melt. Nothing new is made and nothing is thrown away. The same particles gain enough energy to break out of the fixed pattern and start sliding over each other, and that is the entire event.
3. Assuming all gases are invisible
Assuming that anything cloudy drifting through the air must be a gas. The white plume above a kettle is the clearest example of the trap, because it is not gas at all but millions of tiny droplets of liquid water that have already cooled and condensed. The real gas, water vapour, sits in the clear gap between the spout and the plume, and you cannot see it.
Practice questions
Seven questions. One attempt each, and the explanation appears once the answer is in.
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For parents
Get your child to be the particles. Stand shoulder to shoulder and vibrate on the spot for a solid, shuffle about while staying in contact for a liquid, then break away and run around the room for a gas. It sounds silly. It also survives a term of forgetting, because the picture a child pulls up in a test is usually the one they acted out rather than the one they read.
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