Describe condensation and solidification in terms of particles

Cambridge IGCSE Physics 0625 – 2.2.3 Melting, Boiling and Evaporation

Condensation

Condensation is the change of state from gas to liquid. In terms of particle behaviour:

• Gas particles have high kinetic energy and are widely spaced.
• When a gas cools, its kinetic energy decreases: \$E_k \downarrow\$.
• Reduced kinetic energy allows attractive forces (van der Waals, dipole–dipole) to pull particles closer.
• Particles lose enough energy to form a liquid: \$Ek \approx Ea\$ (attractive energy).
• Resulting liquid has a fixed volume but variable shape.

Solidification (Freezing)

Solidification is the change of state from liquid to solid. Particle behaviour during this transition:

• Liquid particles move rapidly but are still close together.
• Cooling reduces kinetic energy: \$E_k \downarrow\$.
• When \$E_k\$ becomes comparable to the lattice binding energy, particles arrange into a fixed, repeating lattice.
• Particles vibrate around fixed positions: \$Ek \ll Ea\$.
• The resulting solid has a fixed shape and volume.

Key Phase Change Diagram