The temperature at which a liquid becomes a solid at normal atmospheric pressure.

• A. Melting
• B. Boiling
• C. Condensation
• D. Freezing

Answer: (D)

Freezing is the temperature at which a liquid becomes a solid at normal atmospheric pressure. As a liquid cools, molecular motion slows until the temperature reaches the freezing point, at which the molecules arrange into a rigid lattice and the substance changes phase. This transition releases latent heat of fusion as the liquid becomes solid. For pure liquids, the freezing point is the same as the melting point, so there’s a single temperature where liquid and solid phases are in equilibrium at 1 atm. In mixtures or solutions, impurities disrupt crystal formation, so freezing occurs over a range and the apparent freezing point is depressed. For example, water freezes at 0°C at 1 atm. This concept is distinct from melting (solid to liquid), boiling (liquid to gas), and condensation (gas to liquid).