Describe evaporation in terms of the escape of more-energetic particles from the surface of a liquid

2.2.3 Melting, Boiling and Evaporation

Learning objectives (Cambridge IGCSE Physics 0625)

• Describe melting and boiling as the input of latent heat while the temperature remains constant.
• State the melting and boiling temperatures of water at 1 atm.
• Describe condensation and solidification using the particle model.
• Describe evaporation as the escape of the most‑energetic particles from the surface of a liquid.
• Explain why evaporation cools a liquid (and a body in contact with an evaporating liquid).
• Compare evaporation with boiling.
• Explain how temperature, surface area and air movement affect the rate of evaporation.

Melting and boiling – energy input at constant temperature

• Melting (solid → liquid):

  • The solid is heated to its melting point.
  • Energy supplied = latent heat of fusion (L_f).
  • Particles gain enough kinetic energy to break the ordered lattice, but the temperature stays at the melting point until the whole solid has become liquid.

• Boiling (liquid → gas):

  • The liquid is heated to its boiling point.
  • Energy supplied = latent heat of vaporisation (L_v).
  • Particles acquire enough energy to overcome intermolecular forces and form vapour bubbles throughout the bulk; the temperature remains constant at the boiling point until all the liquid has boiled away.

Melting and boiling temperatures of water (1 atm)

Condensation and solidification – the reverse processes

• Condensation (gas → liquid)

  • Gas particles lose kinetic energy → move closer together → become a liquid.
  • The temperature of the gas does not fall until the latent heat of condensation has been released.

• Solidification (liquid → solid)

  • Liquid particles lose kinetic energy → motion slows → arrange into a fixed lattice → form a solid.
  • The temperature remains at the melting point until all the liquid has solidified.

Evaporation – escape of the most‑energetic surface particles

Evaporation is a cooling process that occurs at temperatures below the boiling point. It involves the loss of molecules from the liquid’s surface into the surrounding air.

Particle‑model explanation

• Particles in a liquid have a range of kinetic energies described by the Maxwell‑Boltzmann distribution.
• Particles in the bulk are constantly colliding with neighbours, which prevents them from escaping.
• Surface particles have fewer collisions on the upward side. If a surface particle possesses kinetic energy greater than the attractive intermolecular forces, it can break free and become a vapour molecule.
• Thus, evaporation is the net loss of the most energetic particles from the surface.

Energy relationship

The average kinetic energy of a particle is related to temperature by

\(\displaystyle \langle E{\text{kin}} \rangle = \frac{3}{2}\,k{\mathrm{B}}\,T\)

where \(k_{\mathrm{B}} = 1.38 \times 10^{-23}\,\text{J K}^{-1}\) (Boltzmann’s constant).

Why evaporation cools a liquid (and an object in contact with it)

• Each escaping particle carries away its kinetic energy.
• The remaining liquid therefore has a lower average kinetic energy, which we perceive as a temperature drop.
• When a wet surface (e.g., a cloth) is in contact with skin, the most energetic water molecules leave the cloth. The cloth – and the skin underneath – lose heat, giving the familiar cooling sensation of sweating or evaporative cooling pads.

Factors that affect the rate of evaporation

1. Temperature of the liquid – Higher temperature → larger fraction of particles with enough energy to escape.
2. Surface area – A larger exposed surface provides more sites for particles to leave.
3. Air movement (wind) – Moving air removes vapour molecules from the surface, reducing the chance of re‑condensation.
4. Humidity of the surrounding air – Low humidity means fewer vapour molecules are already present, so the net escape rate is higher.
5. Nature of the liquid – Liquids with weaker intermolecular forces (e.g., alcohol) evaporate more readily than those with strong forces (e.g., water).

Evaporation vs. Boiling – comparison

Suggested diagram

Key points to remember

• Melting and boiling involve the input of latent heat of fusion and latent heat of vaporisation respectively, while the temperature stays constant.
• Water melts at 0 °C (273 K) and boils at 100 °C (373 K) at 1 atm.
• Condensation and solidification are the reverse processes – particles lose kinetic energy and move closer together.
• Evaporation is the escape of the most energetic surface particles; it occurs below the boiling point.
• Each escaping particle removes kinetic energy, so the remaining liquid (and any object in contact) cools.
• The rate of evaporation increases with temperature, surface area and air movement, and decreases with high humidity or strong intermolecular forces.
• Unlike boiling, evaporation happens only at the surface and produces a noticeable cooling effect.