Changes of state and specific latent heat

GCSE Physics revision notes, key terms and practice questions.

Changes of state

  • The changes of state are melting, freezing, boiling (and evaporating), condensing and sublimating (solid straight to gas).
  • When a substance changes state, its mass is conserved.
  • Changes of state are physical changes: if the change is reversed, the material gets back its original properties. They are not chemical changes.

Internal energy

  • Internal energy is the energy stored inside a system by the particles that make it up. It is the total kinetic energy and potential energy of all the particles.
  • Heating a system increases the energy of its particles. This either raises the temperature or produces a change of state.

Specific latent heat

  • During a change of state, the energy supplied changes the internal energy (by breaking bonds between particles) but not the temperature.
  • Specific latent heat is the energy needed to change the state of 1 kg of a substance with no change in temperature.
  • E = m L: energy (J) = mass (kg) × specific latent heat (J/kg).
  • Specific latent heat of fusion: changing between solid and liquid. Specific latent heat of vaporisation: changing between liquid and vapour.

Heating and cooling graphs

  • A heating graph shows temperature against time. Where the line is flat, the substance is changing state, and energy is being used to break bonds.
  • The first flat section is melting (at the melting point); the second is boiling (at the boiling point).
  • On a cooling graph, the flat sections show condensing and freezing, where energy is released.

Key terms

Internal energy
The total kinetic and potential energy of all the particles in a system.
Physical change
A change, such as a change of state, that can be reversed.
Specific latent heat
The energy needed to change the state of 1 kg of a substance without changing its temperature.
Specific latent heat of fusion
The energy to melt (or freeze) 1 kg of a substance.
Specific latent heat of vaporisation
The energy to boil (or condense) 1 kg of a substance.
Sublimation
A change directly from solid to gas.
Evaporation
A liquid changing to a gas at its surface, below the boiling point.

Practise Changes of state and specific latent heat: 10 questions