latent heat

Meaning: Latent heat is the heat absorbed or released when a substance changes phase at constant temperature. The energy goes into changing intermolecular attractions (potential energy) instead of average kinetic energy, so the temperature of a pure sample holds steady. Melting and vaporizing absorb heat; freezing and condensing release the same amount. Vaporization needs far more than fusion because the molecules must be separated completely.

Equation: q = mL, where q is heat (J), m is mass (kg), and L is the specific latent heat (J/kg). For water, L of fusion is about 3.34 × 10⁵ J/kg and L of vaporization is about 2.26 × 10⁶ J/kg.

On the MCAT: Heating curves plot temperature against heat added. Sloped segments use q = mcΔT; flat plateaus use q = mL. Expect to sum the segments to take ice to steam, or to compare plateau lengths (the vaporization plateau is longer than the melting plateau). Evaporative cooling, such as sweat removing heat from skin, also applies the idea.

Common confusion: Adding heat does not always raise temperature. During a phase change the heat goes to the transition, and the temperature stays constant until the whole sample has changed. Do not mix up L with specific heat c: c applies within a single phase, and L applies at the transition.