Changes of state and specific latent heat: If a change of state happens:

The idea

A change of state, such as melting, boiling, condensing or freezing, happens at a constant temperature even though energy is still being transferred. The energy supplied during melting or boiling breaks the forces between particles rather than raising the temperature, so the substance's internal energy increases while the thermometer reading stays fixed. This hidden energy is called specific latent heat: specific latent heat of fusion for melting or freezing, and specific latent heat of vaporisation for boiling or condensing. Use ΔE = m L, where L is the specific latent heat in J/kg.

For example: Melting 0.5 kg of ice with L = 334 000 J/kg needs 0.5 × 334 000 = 167 000 J.

What earns the marks

• State that temperature remains constant during a change of state while energy is transferred.
• Explain that supplied energy increases the internal energy of the particles and breaks intermolecular forces rather than increasing kinetic energy.
• Distinguish specific latent heat of fusion from specific latent heat of vaporisation and identify which applies to a given change.
• Use ΔE = m L to calculate energy, mass or specific latent heat, keeping mass in kilograms.
• Interpret heating and cooling curves, identifying flat sections as changes of state and sloping sections as temperature changes.
• Describe the reverse processes, condensation and freezing, as releasing energy at constant temperature.

Common mistakes and how to put them right

• Assuming the temperature rises during melting or boiling.

  Put it right: Recognise that the flat section of a heating curve shows constant temperature.

• Using ΔE = m c Δθ for a change of state.

  Put it right: Use ΔE = m L instead because there is no temperature change.

• Mixing up fusion and vaporisation.

  Put it right: Fusion is solid–liquid change, vaporisation is liquid–gas change.

Exam tips

• On a heating curve, label flat regions with the change of state and sloping regions with the equation ΔE = m c Δθ.
• When a question describes melting or boiling, immediately consider ΔE = m L rather than a temperature-change calculation.
• Explain energy in terms of particles and forces to access the higher-tariff reasoning marks.