Changes of State (OCR GCSE Physics A (Gateway))

Changes of State

• There are six changes of state that can occur between solids, liquids and gases:

  • Melting - When a solid turns into a liquid (e.g. ice to water)

  • Evaporating - When a liquid turns into a gas

  • Condensing - When a gas turns into a liquid

  • Freezing - When a liquid turns into a solid

  • Subliming - When a solid turns straight into a gas

  • Desubliming - When a gas turns straight into a solid

Conservation of Mass

• When a substance changes state, the number of molecules in that substance doesn’t change and so neither does its mass

  • This is known as conservation of mass

• During a change of state, the internal energy does change

  • Internal energy increases with an increase in temperature (from solid to gas)

Physical vs Chemical Changes

• Changes of state are physical changes

  • Physical changes are usually reversible

  • This means that the material recovers its original properties if the change is reversed

• Physical changes are different to chemical changes because chemical changes are often not 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

Diagram showing the arrangement and motion of different states of matter. These physical changes are reversible