Changes of State (AQA GCSE Physics): Revision Note
Exam code: 8463
Changes of State
When a substance changes state:
The number of molecules in that substance does not change
Therefore its mass does not change
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
There are five changes of state that can occur between solids, liquids and gases:
Melting - A solid turns into a liquid (e.g. ice to water) when energy is transferred to the system
Boiling - A liquid turns into a gas (evaporating) when energy is transferred to the system
Condensing - A gas turns into a liquid when energy is transferred away from the system
Freezing - A liquid turns into a solid when energy is transferred away from the system
Subliming - A solid turns into a gas when energy is transferred to the system
Diagram showing the arrangement and motion of different states of matter
Examiner Tips and Tricks
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
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