Latent Heat

When a substance changes phase, the energy added or removed goes into rearranging intermolecular bonds rather than changing the particles’ average kinetic energy. That is why the temperature stays constant during melting, boiling, freezing, or condensation, and why the heat transfer is given by Q = mL. Here, L is the amount of energy needed per unit mass for a specific phase change, so multiplying by the mass gives the total heat involved.

Use this formula whenever a problem involves melting, freezing, boiling, condensing, or any other phase change with no temperature change. It is especially useful in calorimetry, heating-curve questions, and energy calculations for materials such as ice, steam, or refrigerants.

• Using a temperature change formula instead of a latent heat formula during a phase change
• Mixing up the latent heat value for fusion, vaporization, or condensation
• Forgetting to convert mass into kilograms when L is given in J/kg