What is latent heat and why is it important for phase changes?

• A. Latent heat is energy absorbed or released during a phase change without a temperature change; it enables the material to change state.
• B. Latent heat is energy that raises the temperature of a substance.
• C. Latent heat is energy released during a chemical reaction.
• D. Latent heat is energy stored in a solid at rest.

Answer: (D)

Latent heat is energy absorbed or released during a phase change without a change in temperature. When a solid melts, the added energy goes into breaking the orderly bonds holding the particles in a lattice, allowing them to move more freely as a liquid. The temperature stays the same at the melting point until the entire solid has melted, because the energy is used for changing the phase, not warming the substance. The same idea applies to boiling and condensation: energy is absorbed to form a gas and released when forming a liquid, all while the temperature remains constant during the transition. The amount of energy per mass involved in a specific phase change is described by the latent heat of fusion or latent heat of vaporization, with the relation Q = mL. This concept explains why phase changes can store or release large amounts of energy without a temperature change. Other ideas—warming a substance raises its temperature (sensible heat), latent heat isn’t about chemical reactions, and it isn’t energy stored in a solid at rest—don’t describe what latent heat actually is.