Changes of State

Unlike chemical changes, physical changes like changes of state are reversible

In a solid:
- The molecules are very close together and arranged in a regular pattern
- The molecules vibrate about fixed positions
In a liquid:
- The molecules are still close together (no gaps) but are no longer arranged in a regular pattern
- The molecules are able to slide past each other
In a gas:
- The molecules are widely separated - about 10 times further apart in each direction
- The molecules move about randomly at high speeds

Solids liquids gases, IGCSE & GCSE Physics revision notes

Diagram showing the arrangement and motion of different states of matter

Here are some common misconceptions; check that you don't have them:

The molecules themselves melt and separate
- The only way the atoms in a molecule can separate is through a chemical reaction (and this requires much more energy than changing its state!)
- When a substance undergoes a chemical change (a chemical reaction) a new substance or substances are formed
- In a physical change, like changes of state, the substance doesn't change. Water molecules are still water molecules whether the water is in the form of a solid (ice), a liquid (water) or a gas (water vapour)
The mass of the substance changes
- The mass of the substance is determined by the mass of the individual particles and how many particles there are
- The number of particles in a substance does not change when it changes state
- Therefore, the mass of the substance does not change
- However, gas particles can escape if the container is open when a liquid changes into a gas, in which case the mass of the remaining liquid would decrease as it evaporates or boils

Changes of State (AQA GCSE Physics)