The energy needed for a substance to change state is called latent heat.…

The idea

When a solid melts or a liquid boils, energy is transferred to the particles without raising the thermometer reading. This hidden energy is called latent heat. It increases the internal energy stored by the particles because their potential energy increases as bonds or intermolecular forces are overcome, while their average kinetic energy, and therefore temperature, stays constant. For example, ice at 0°C absorbs energy and becomes water at 0°C; the temperature does not rise until all the ice has melted. Similarly, steam at 100°C condenses to water at 100°C, releasing latent heat. The flat sections on a heating curve show these changes of state, where energy supplied changes internal energy but not temperature.

What earns the marks

• Latent heat is the energy transferred to or from a substance during a change of state at constant temperature.
• During melting or boiling, energy supplied increases the internal energy stored by the particles.
• The energy is used to overcome forces between particles rather than to increase their average kinetic energy.
• Because average kinetic energy does not change, the temperature remains constant during the change of state.
• A heating curve shows a horizontal plateau at the melting point and boiling point, demonstrating constant temperature while energy is supplied.
• For cooling, latent heat is released when a gas condenses or a liquid freezes, again with no temperature change.

Common mistakes and how to put them right

• Thinking that supplying energy always raises temperature.

  Put it right by explaining that during a change of state the energy increases internal energy but not average kinetic energy, so temperature stays constant.

• Believing that temperature rises gradually during melting.

  Put it right by referring to the flat plateau on a heating curve at the melting point.

• Confusing latent heat with specific heat capacity.

  Put it right by stating that specific heat capacity relates to temperature change, whereas latent heat relates to change of state at constant temperature.

Exam tips

• When describing a heating curve, name the change of state at each flat section and state that temperature remains constant.
• Use the phrase 'energy supplied changes the energy stored by the particles, not the temperature' to address both parts of the statement.
• If asked to explain why temperature stays constant, link it to overcoming intermolecular forces and increasing potential energy rather than kinetic energy.