Freezing

Freezing (solidification) is an exothermic phase change where a liquid turns into a solid at its freezing point ¹. For pure substances, this equals the melting point.

Kinetic Particle Mechanism

Under IGCSE Kinetic Particle Theory ¹:

• Liquid: Particles are close together but slide randomly past each other.
• Cooling: Thermal energy escapes, decreasing particle kinetic energy and speed.
• Freezing Point: Attractive forces overcome thermal motion, locking particles into a fixed lattice as a solid.

[Liquid Flow]         Cooling         [Solid Lattice]
 (o)  (o) (o)     ---------------->     (o)(o)(o)(o)
   (o)  (o)        Freezing Point       (o)(o)(o)(o)

Cooling Curve & Latent Heat Release

Temperature remains constant at the freezing plateau while solidification occurs ²:

Temp ^
     | \ Liquid
F.P. +===========+ (Freezing Plateau)
     |            \
     +-------------\-> Time

Temperature stays constant ($\Delta T = 0$) because lattice formation releases the specific latent heat of fusion ($\Delta E = m \cdot l_f$) as heat is removed ².

Freezing Point Depression

Impurities disrupt lattice ordering, lowering the freezing point over a temperature range (e.g. salting icy roads) ¹.

Related

• Exothermic Change - Process releasing thermal energy to the surroundings.
• Intermolecular Force - Attractive forces that bind particles into a solid lattice.
• Condensation - Exothermic gas-to-liquid phase transition releasing latent heat.
• Lattice - Rigid framework established as liquids freeze.
• Melting - Reverse endothermic transition.
• Pure Substance - Pure materials possessing fixed, identical freezing and melting points.
• State of Matter - Classical states of matter.
• Chemistry - Core science area.