Freezing
Freezing is the phase change from liquid to solid that happens when a liquid loses enough heat for its particles to lock into fixed positions.
Definition
Freezing, or solidification, is the phase change from liquid to solid that occurs when a liquid releases enough thermal energy for attractive forces to lock its particles into fixed positions, happening at the freezing point, which for a pure substance equals its melting point, and during the process the temperature holds constant while heat is given off, making freezing the reverse of melting. It is an exothermic transition that requires nucleation, meaning a small ordered cluster of particles must form before crystal growth can proceed, so a very pure liquid can supercool below its normal freezing point until a nucleus finally appears, and dissolved solutes lower the freezing point through the colligative relationship $\Delta T_f = iK_f m$, where $K_f$ is the solvent's cryoscopic constant.
Example
Water freezes into ice cubes in a freezer and melted wax freezes into a hard candle as it cools, and freezing does not have to feel cold at all, since liquid iron freezes into solid iron above $1500^\circ\text{C}$. Water releases $334\text{ J}$ of heat per gram as it freezes at $0^\circ\text{C}$, which is why farmers spray fruit trees before a frost, letting the freezing water's own released heat protect the buds; dissolving $1.00\text{ mol}$ of $NaCl$ in $1.00\text{ kg}$ of water gives $i = 2$ and $\Delta T_f = 3.72^\circ\text{C}$, so the solution freezes at $-3.72^\circ\text{C}$, and very pure water in a clean container can supercool to $-40^\circ\text{C}$ before freezing spontaneously.
Key Insight
Dissolved substances lower the freezing point of a liquid, which is why seawater freezes near $-1.9^\circ\text{C}$ and antifreeze keeps a car's coolant liquid to $-37^\circ\text{C}$ or lower. Rapid freezing produces many small crystals while slow freezing produces fewer, larger ones, the very difference between smooth ice cream and grainy ice cream, and between fine-grained and coarse-grained igneous rock.