Specific latent heat of vaporisation – change of state from liquid to vapour

The idea

Specific latent heat of vaporisation is the energy needed to change 1 kg of a substance from liquid to vapour at its boiling point, without any temperature change. During boiling, energy is transferred to the particles to overcome the forces holding them together in the liquid, so the particles separate into a gas. The energy supplied does not increase the average kinetic energy of the particles, so the thermometer reading stays constant. The specific latent heat of vaporisation, Lv, is calculated using E = m × Lv, where E is energy in joules, m is mass in kilograms and Lv is in J/kg.

For example: Vaporising 0.50 kg of water requires 0.50 × 2 260 000 = 1 130 000 J. This is much larger than the energy needed to melt the same mass of ice because vaporisation breaks all the intermolecular forces completely.

What earns the marks

• State that specific latent heat of vaporisation is the energy required to change 1 kg of a substance from liquid to vapour at constant temperature.
• Explain that the energy supplied during boiling is used to overcome the forces between particles rather than to raise the temperature.
• Use the equation E = m × Lv correctly, with mass in kilograms and specific latent heat in joules per kilogram.
• Interpret a flat section on a heating curve at the boiling point as the energy absorbed during vaporisation with no temperature change.
• Compare the specific latent heat of vaporisation with the specific latent heat of fusion, recognising that vaporisation usually requires more energy per kilogram.

Common mistakes and how to put them right

• Thinking that the temperature rises while a liquid boils.

  Put it right by explaining that the energy breaks intermolecular forces, so the average kinetic energy and temperature remain constant.

• Using the mass in grams in E = m × Lv.

  Put it right by converting grams to kilograms before substituting into the equation.

• Confusing specific latent heat of vaporisation with specific heat capacity.

  Put it right by noting that specific latent heat involves a change of state at constant temperature, while specific heat capacity involves a temperature change without a change of state.

Exam tips

• Read the question carefully to identify whether it asks about melting, boiling, condensing or freezing before selecting the correct latent heat value.
• Show the equation, substitution and answer with the correct unit, J or J/kg as appropriate, to gain full credit in calculations.
• When describing a graph, name the state change occurring on each flat section and state that temperature remains constant during the change.