freezing point

Meaning: At the freezing point, liquid and solid are in equilibrium. Removing energy from the liquid lets intermolecular attractions lock the molecules into a lattice. That energy comes out as latent heat of fusion, so the average kinetic energy, and therefore the temperature, does not change until all the liquid has become solid. Water freezes at 0 °C at 1 atm.

Equation: Q = mLf, where Q is the heat removed during freezing (J), m is the mass (kg), and Lf is the latent heat of fusion (J/kg). Outside the plateau, heat changes temperature by Q = mcΔT, where c is specific heat (J/(kg·K)) and ΔT is the temperature change (K).

On the MCAT: Heating and cooling curves plot temperature against heat added or removed. The flat segments are phase changes at the freezing and boiling points, and the sloped segments are single phases whose slope reflects specific heat. Calculations add up the steps: cool the liquid to the freezing point, freeze it, then cool the solid. Phase diagrams may also appear. For most substances the solid-liquid line slopes up and to the right, so higher pressure raises the freezing point. For water the line slopes the other way, so higher pressure lowers it.

Common confusion: Students expect temperature to keep falling while heat is removed. During the phase change it holds steady while both phases are present, and it falls again only once the sample is entirely solid.