Describe condensation and solidification in terms of particles

Cambridge IGCSE Physics 0625 – 2.2.3 Melting, Boiling and Evaporation

This section covers the five phase‑change processes required by the syllabus. For each process the particle model, the energy changes (including latent heat) and the characteristic temperature behaviour are highlighted.

1. Melting (solid → liquid)

• Particle behaviour: In a solid particles are arranged in a regular lattice and vibrate about fixed positions.
• What happens when heat is supplied: The vibrational kinetic energy (E_k) increases. At the melting point E_k becomes comparable to the attractive (binding) energy E_a. Particles break free from their fixed points but remain close together, forming a liquid.
• Temperature change: During the melting plateau the temperature does not rise. The added heat is used entirely to overcome intermolecular forces.
• Latent heat of fusion (L_f):
  

  Q = m Lf (energy absorbed while the temperature stays constant).

• Example (water): Standard melting point = 0 °C at 1 atm; L_f ≈ 334 kJ kg⁻¹. (The point shifts with pressure – a useful extension for higher‑level questions.)

2. Boiling (liquid → gas – throughout the bulk)

• Particle behaviour: In a liquid particles are close together and move freely. When the liquid reaches its boiling point, the kinetic energy of many particles is sufficient to overcome the intermolecular attractions, and vapour bubbles form within the bulk because the vapour pressure equals the external pressure.
• Temperature change: During the boiling plateau the temperature does not rise. All added heat is used to break the remaining intermolecular forces.
• Latent heat of vapourisation (L_v):
  

  Q = m Lv

• Example (water): Standard boiling point = 100 °C at 1 atm; L_v ≈ 2 260 kJ kg⁻¹. (The boiling point also varies with pressure.)

3. Evaporation (surface → gas)

• Particle behaviour: Only the most energetic molecules at the surface have enough kinetic energy to escape into the surrounding air.
• Occurs at any temperature below the boiling point; the rate increases with temperature because a larger fraction of molecules satisfy the energy requirement.
• Factors that increase the rate:

  • Higher temperature – more molecules have sufficient kinetic energy.
  • Larger surface area – more molecules are exposed.
  • Increased air flow or reduced humidity – the surrounding vapour is removed, allowing more molecules to escape.

• Cooling effect: When the high‑energy molecules leave, the average kinetic energy of the remaining liquid falls, producing a cooling effect (e.g., sweating).
• Energy aspect: No separate latent‑heat term is required in the syllabus, but each escaping molecule must possess kinetic energy ≥ the intermolecular binding energy.

4. Condensation (gas → liquid)

• Particle behaviour: Gas particles are widely spaced and move rapidly (high E_k). On cooling, E_k decreases.
• When condensation occurs: Once E_k falls to the level of the attractive forces (van der Waals, dipole‑dipole, etc.), particles are drawn together and a liquid forms.
• Energy change: The process releases the latent heat of condensation, which is equal in magnitude to the latent heat of vapourisation (L_v).
• Resulting liquid: Takes the shape of its container but has a fixed volume.

5. Solidification (freezing) – liquid → solid

• Particle behaviour: Liquid particles are close together and move rapidly. Cooling reduces their kinetic energy.
• When solidification occurs: When E_k becomes much smaller than the intermolecular binding energy, particles lock into a regular lattice and vibrate about fixed positions.
• Energy change: The latent heat of fusion is released (equal in magnitude to L_f).
• Resulting solid: Has a fixed shape and volume.

Key Summary – Particle Arrangement & Energy Relations

Typical Phase‑Change Diagram (qualitative)

These notes now cover all five required sub‑topics, use the exact wording required by the Cambridge IGCSE 0625 syllabus, and explicitly state the temperature‑plateau behaviour, pressure dependence of melting/boiling points, and the cooling effect of evaporation.