States of Matter

Explained

Solids, liquids and gases, described precisely

Three things separate the states: how the particles are arranged, how they move, and how much energy they have. Questions here nearly always ask about one of the three, and the commonest way to lose the mark is to answer about a different one.

The three states

• In a solid the particles are close together in a regular arrangement and vibrate about fixed positions. They have the least energy, so a solid has a fixed shape and a fixed volume.
• In a liquid the particles are still close together but randomly arranged, and they can move past one another. A liquid has a fixed volume but takes the shape of its container.
• In a gas the particles are far apart and move rapidly in random directions. They have the most energy, so a gas fills whatever space it is given and can be compressed.

One point deserves care. Going from solid to liquid, the big change is the arrangement, from regular to random, and the ability to move past one another. The particles do not become dramatically more spread out; most substances expand only slightly on melting. The large increase in spacing happens between liquid and gas.

The changes of state

Melting is solid to liquid and freezing is liquid to solid. Boiling and evaporation are liquid to gas, and condensation is gas to liquid. Sublimation is solid straight to gas without passing through the liquid state.

Heating causes the changes that go towards gas, because energy is being supplied to overcome the forces holding the particles together. Cooling causes the changes that go the other way, and those release energy to the surroundings.

All of these are physical changes. The substance is chemically the same afterwards, and the change can be reversed.

Boiling and evaporation are different

Boiling happens at a fixed temperature, the boiling point, and throughout the whole liquid, which is why bubbles form inside it.

Evaporation happens below the boiling point and only at the surface, where the fastest moving particles escape. Because the fastest leave, the average energy of those remaining falls, so evaporation cools the liquid. That is why sweating works.

Heating and cooling curves

Plot temperature against time while heating a solid and the line rises, then goes flat, then rises, then goes flat again.

The flat parts are the changes of state. Energy is still being supplied, but it is being used to overcome the forces between particles rather than to make them move faster, so the temperature does not change. Being able to say that is the standard explanation question attached to this graph.