Changes of State (OCR Gateway GCSE Physics)

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 increase in temperature (from solid to gas);

• 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

• Changes of state are physical changes
  
  • Physical changes are 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 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

ANSWER:   D

• • A is true because the mass lost from the beaker is due to those water molecules evaporating
  • B is true because evaporation causes the most energetic particles to leave the beaker
    • The total number of particles in the beaker decreased

  • C is true because additional water molecules were added to the air, without a significant change in the volume of the air
  • D is not true because no mass is lost during evaporation - it is only changed from a liquid to gas state