Latent heat describes which of the following?

• A. The heat required to raise temperature by 1 degree
• B. The heat required to change phase without a temperature change
• C. The rate of heat transfer
• D. The heat produced by combustion

Answer: (B)

Latent heat is the energy required for a substance to change its phase without changing its temperature. When ice at 0°C melts, heat goes into breaking the solid structure and transforming it into liquid water, but the temperature stays at 0°C until all the ice has melted. Similarly, when liquid water at 100°C boils, heat is used to form steam, so the temperature remains at 100°C while the liquid turns into vapor. Conversely, during freezing or condensation, energy is released as the substance changes phase, with no temperature change until the phase transition completes. In HVAC contexts, this energy transfer is crucial because the refrigerant absorbs or releases large amounts of latent heat during evaporation or condensation, producing cooling or heating without a rise or fall in the refrigerant’s temperature. This is distinct from sensible heat, which would change the temperature of the substance without a phase change.