GCSE Physics: Latent Heat and Heating Curves

The particle model topic of GCSE Physics introduces specific latent heat alongside specific heat capacity, and exam questions often combine the two. This material concentrates on using latent heat and on reading the story of a heating or cooling curve, with the equations given, as they are on the real papers.

You start with the definition: specific latent heat is the energy needed to change the state of one kilogram of a substance with no change in temperature. The latent heat of fusion applies to melting and freezing, and the latent heat of vaporisation to boiling and condensing. You then use E = mL in both directions: the energy to melt a block of ice or boil away a mass of water given in grams, and the latent heat found from a measured energy and mass. A two-stage problem asks for the energy to heat water to its boiling point and then boil part of it, and another question checks that you can tell when to use ΔE = mcΔθ (a change of temperature) and when to use E = mL (a change of state).

Some heating and cooling curves are described in words and others come with the graph drawn, as on the real papers. You explain why the temperature stays constant while a substance melts or boils: the energy supplied increases the potential energy of the particles as the bonds between them are broken, not their kinetic energy, so the temperature does not rise. You explain why the boiling section lasts longer than the melting section, and what happens to the energy when a liquid freezes. Internal energy, the total kinetic and potential energy of the particles, links these ideas together, and it explains why steam at 100 °C causes a worse burn than water at the same temperature.

At Higher tier, and in separate physics for example in AQA's specification, you also explain why compressing a gas does work on it and raises its internal energy and temperature, the reason a bicycle pump gets warm, and why a fast compression warms a gas more than a slow one.

The material offers three formats. The quiz has 12 questions mixing single-answer and multiple-answer items, with worked explanations. The flashcards give quick revision of definitions and equations. The written work is a printable sheet of 8 longer questions, including a heating-curve description, multi-stage energy calculations, a method for measuring the latent heat of ice and a kettle problem, which you answer by hand and upload for feedback on each answer.

• Define specific latent heat and distinguish fusion from vaporisation
• Use E = mL to find energy, mass or latent heat, converting grams to kilograms
• Combine E = mL with ΔE = mcΔθ in multi-stage problems and choose the right equation
• Explain the flat sections of heating and cooling curves in terms of particle energy
• Describe internal energy and explain why steam causes worse burns than boiling water
• (Higher) Explain why doing work on a gas raises its temperature

Practice material written by Zestly, based on the DfE GCSE combined science and physics subject content (particle model of matter: changes of state, specific latent heat, internal energy; pressure in gases), with AQA GCSE Physics 8463 used as the example specification.