melting point

Meaning: The melting point is the temperature at which a pure solid and its liquid coexist in equilibrium at a given pressure, usually 1 atm; it equals the freezing point. Heat added there goes into the enthalpy of fusion, so a heating curve stays flat while both phases are present. Stronger intermolecular forces or bonding generally give a higher value: ionic and network covalent solids melt high, while molecular solids held by dispersion forces melt low.

Equation: Tm = ΔHfus / ΔSfus, where Tm is the melting temperature (K), ΔHfus is the molar enthalpy of fusion (J/mol), and ΔSfus is the molar entropy of fusion (J/(mol·K)). It follows from ΔG = 0 at equilibrium.

On the MCAT: Expect heating curves with a plateau, rankings of compounds by structure (ionic charge, hydrogen bonding, molar mass), and phase diagrams where the solid-liquid line gives the melting point at each pressure. In lab passages, an impure sample melts at a lower temperature and over a broader range than the pure compound.

Common confusion: Students carry the usual trend, that higher pressure raises the melting point, over to water. Water's solid-liquid line slopes down because ice is less dense than the liquid, so higher pressure lowers its melting point.