States of Matter: Solids, Liquids, Gases, and Changes
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
The states of matter describe different ways that particles can be arranged and move in a substance. Solids, liquids, and gases are the three states you meet most often at school, and the particle model helps explain why each one has different properties.
The particle model
In a solid, particles are packed closely and held in relatively fixed positions. They still vibrate, but they do not move freely past one another. That is why a solid usually keeps both its shape and its volume unless a force changes it.
In a liquid, particles remain close together but can move around one another. A liquid keeps roughly the same volume, yet it changes shape to fit its container. In a gas, particles are much farther apart and move freely in all directions. A gas spreads out to fill the available space and can be compressed much more easily than a liquid or solid.
The particle model makes the states easier to compare because the particles themselves do not need to change identity when the state changes. Water molecules in ice, liquid water, and water vapour are still water molecules. What changes is their energy, movement, spacing, and organisation.
How heating and cooling change state
When a solid is heated, its particles gain energy and vibrate more strongly. At the melting point, enough energy is available for the ordered structure to break down so particles can move around one another. The solid becomes a liquid. Cooling reverses the process at the freezing point.
Further heating can turn a liquid into a gas. Evaporation happens at the surface and can occur below the boiling point because some surface particles have enough energy to escape. Boiling happens throughout the liquid when vapour bubbles can form within it. Condensation is the reverse change, when gas particles lose energy and form a liquid.
Some substances can also move directly between solid and gas. Sublimation is the change from solid to gas without becoming liquid first, while deposition is the reverse. These changes broaden the simple picture and show that phase changes depend on conditions such as temperature and pressure.
Energy, temperature, and phase changes
Keep these ideas together when you revise:
- •Melting changes a solid into a liquid.
- •Freezing changes a liquid into a solid.
- •Evaporation and boiling change a liquid into a gas.
- •Condensation changes a gas into a liquid.
- •Sublimation changes a solid directly into a gas.
One detail that often surprises learners is that temperature can stay constant during a change of state even while heating continues. The added energy is being used to change the attractions and arrangement between particles rather than simply making their random motion faster. After the phase change is complete, further heating can raise the temperature again.
Pressure can shift when changes of state happen because pressure affects how closely particles are pushed together. This is especially important for gases. School diagrams often show one simple melting or boiling point, but real phase behaviour depends on both temperature and pressure. The particle model remains useful because it gives you a way to reason from movement and spacing instead of memorising a disconnected table.
The takeaway
The states of matter become much simpler when you imagine particles instead of memorising labels. Solids have closely packed particles in fixed positions, liquids have close particles that can move, and gases have widely spaced particles moving freely. Add or remove energy, and the arrangement can change while the substance itself remains the same.