Explaining changes of state
Changes of state
Heating solids
When solids are heated they expand (get bigger).
The particles in solids are arranged in a fixed pattern.
The particles are held together strongly and are tightly packed.
The particles in the solid vibrate. When the solid is heated, heat energy is transferred to the particles in the solid.
The more energy the particles have, the more they vibrate.
As the particles vibrate more, they take up more space.
The particles are still held in position by the attractive forces between them.
Melting solids
When solids are heated more strongly, they melt. They become liquid. (Heating more strongly means that even more heat energy is transferred to the particles.)
The particles in a solid vibrate more and more as heat energy is transferred to them. The particles vibrate so much that the attractive forces between them are not strong enough to hold them in a fixed pattern. The particles can slide past one another – they can now move, not just vibrate.
The forces are still strong enough for the particles to stay in touch with one another. The more the liquid is heated, the more energy is transferred to the particles and the more the particles vibrate and move.
Boiling liquids
When liquids are heated, they evaporate and boil.
The particles in liquids touch each other. The particles are held together weakly.
The particles move more as heat energy is transferred to them. Some particles have enough energy to break the weak attractive forces holding them together. These particles can move freely and escape as gas particles.
Cooling gases
The particles in a gas are free to move anywhere and spread out. There are no forces holding them. When a gas gets cooler it condenses to form a liquid.
When gas particles reach a cold surface, some of the heat energy from the particles transfers to the surface. The particles move less and get closer together. They form a liquid.
Freezing liquids
When a liquid freezes it becomes a solid.
The particles in a liquid can move and flow past each other. As heat energy is transferred from the particles to the surroundings, the particles move more slowly and the liquid gets cooler.
The cooler the liquid, the less energy the particles have. The less energy the particles have, the less able they are to move or slide past one another. Eventually, the particles have so little energy they cannot move and flow anymore – they can only vibrate. They become arranged in a fixed pattern to form a solid.
Questions
1. Explain why a solid expands when it is heated.
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The particles gain energy and vibrate faster.
2. Use particle theory to explain why solids and liquids cannot be compressed (squashed into a smaller volume).
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High forces of attraction keep the particles tightly packed, so they cannot be compressed.
3. Use particle theory to explain why liquids and gases can flow.
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Weaker forces of attraction allow the particles to move freely and flow.
4. Use particle theory to explain how a liquid changes to a gas.
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Particles receive more energy and escape the forces holding them together.
5. Use particle theory to explain how a liquid changes to become a solid.
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Particles cool down and their forces of attraction increase, locking them in fixed positions.
6. Use particle theory to explain what happens when steam in the bathroom hits a cold surface, such as a mirror.
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Steam particles lose energy when they hit the cold surface and condense into liquid water.
7. Copy this flow chart. The arrows represent the processes involved when matter changes state. Add the name for each process, A–D.
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solid → melting → liquid → evaporation → gas
gas → condensation → liquid → freezing → solid
Think Like a Scientist
Investigation: Modelling Changes of State
In this task, you will model the changes of state by acting as if you are a particle in different physical states. Carry out the steps below on your own, using movement and imagination to reflect particle behavior.
Part 1 – Solid to Liquid
1. Stand still with arms tucked in, close to your body. Imagine you are a particle in a solid, vibrating gently.
2. Now imagine gentle heating. Begin to wiggle and sway more — remain mostly in place, but with more movement.
3. Imagine the solid is melting. Spread your arms slightly and begin moving slowly in place, as if the structure is breaking apart and becoming liquid.
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I started with very limited movement to show solid particles vibrating. As I imagined heat, I moved more and began to “flow,” like liquid particles. It was a helpful model to visualize structure loss during melting.
Part 2 – Liquid to Gas
1. Begin by gently walking in place to model liquid particles moving past each other.
2. Imagine strong heating. Move faster and begin to spread out around the room — this models evaporation.
3. Continue moving more randomly and energetically — you are now modeling boiling, where particles gain enough energy to become gas.
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I moved smoothly for the liquid and faster for gas. The model helped show how particles gain energy and spread apart, though it can't fully show real particle speeds.
Part 3 – Gas to Liquid
1. Move around quickly to show gas particles.
2. Imagine reaching a cold surface. Begin to slow down and move less, coming closer to a central point. This represents condensation — gas to liquid.
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I slowed down and stayed closer together to represent gas condensing to liquid. It was a good model for showing reduced movement and attraction.
Part 4 – Liquid to Solid
1. Begin by moving gently in place like a liquid.
2. Now imagine being placed in a freezer. Gradually slow your movements until you are standing still and close to one spot, as if you have frozen into a solid.
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As I froze, I stopped moving to show particles in a fixed position. The model made the change in movement easy to understand, though it doesn’t show forces between particles.