Changes of State (AQA GCSE Physics: Combined Science)

• 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 six 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