phase change

Meaning: A phase change is a transition between solid, liquid and gas: melting, freezing, vaporization, condensation, sublimation and deposition. Heat added or removed during the change breaks or forms intermolecular attractions, so it changes potential energy, not average kinetic energy. For a pure substance at constant pressure, temperature therefore stays constant until the change is complete. Melting, vaporization and sublimation are endothermic; their reverses are exothermic.

Equation: q = mL, where q is heat in J, m is mass in kg, and L is the latent heat of fusion or vaporization in J/kg. With moles, q = nΔH, where n is in mol and ΔH is in J/mol. Within a single phase, use q = mcΔT, where c is specific heat in J/(kg·K) and ΔT is in K.

On the MCAT: On a heating curve, sloped segments are single phases (q = mcΔT) and flat plateaus are phase changes (q = mL). The boiling plateau is longer than the melting plateau because ΔHvap is larger than ΔHfus. On a phase diagram, you read phases, the triple point and the critical point, and predict what happens when T or P changes. Water's solid-liquid line has a negative slope.

Common confusion: Students expect temperature to rise while a pure substance boils or melts because heat is still flowing in. It does not rise; the energy goes into potential energy until the change is complete.