Changes of State (Edexcel IGCSE Physics)

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

• Unlike chemical changes, changes of state (a type of physical change) 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 - When a solid turns into a liquid (e.g. ice to water)
  • Boiling - When a liquid turns into a gas (evaporating)
  • Condensing - When a gas turns into a liquid
  • Freezing - When a liquid turns into a solid
  • Subliming -When a solid turns into a gas

Diagram showing the arrangement and motion of different states of matter 

• Heating a system will change the energy stored in a system by increasing the kinetic energy of its particles
  • The temperature of the material, therefore, is related to the average kinetic energy of the molecules

• This increase in kinetic energy (and therefore energy stored in the system) can:
  • Cause the temperature of the system to increase
  • Or, produce a change of state (solid to liquid or liquid to gas)

• The higher the temperature, the higher the average kinetic energy of the molecules and vice versa
  • This means they move around faster

As the container is heated up, the gas molecules move faster with higher kinetic energy. The energy stored within the system - the internal energy - therefore increases

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