Evaporation and Cooling

Evaporation starts at the surface

Evaporation is the change from liquid to gas at the liquid’s surface. It can happen below the boiling point: a wet shirt can dry without its water boiling. In school comparisons, “at any temperature” means that evaporation does not require the liquid to reach its boiling point; liquid must still be present.

Particles in a liquid have a range of speeds. Some surface particles have enough kinetic energy, and a suitable direction of motion, to escape the attraction of neighbouring particles. The particles become vapour; they do not shrink or change chemical identity.

Why evaporation can cool a liquid

Scroll diagram horizontally to read all labels.

Escaping particles tend to have higher kinetic energy than the average. Their escape removes energy and can reduce the average kinetic energy of the remaining particles, so the liquid’s temperature falls.

Energy can also enter from the surroundings. A wet surface may therefore remain at a steady temperature while evaporation continues: the incoming energy replaces energy removed by evaporation. “Evaporation causes cooling” describes an energy-transfer mechanism, not a guarantee that temperature keeps falling indefinitely.

Comparing boiling and evaporation

Evaporation itself does not produce bubbles in the liquid. If you see bubbles, consider boiling or another cause such as dissolved gas escaping.

Factors affecting net evaporation

Compare each factor with the other conditions kept the same. Vapour particles can also return to the liquid; net evaporation means more leave than return.

• Higher liquid temperature: more particles have enough kinetic energy to escape.
• Larger exposed surface area: more particles are at the surface and able to escape.
• Moving air: carries vapour away from above the liquid, reducing the rate at which particles return.
• Lower humidity: at the same temperature and air flow, fewer water-vapour particles are near the surface to return to the liquid.

Humidity describes water vapour in air, so this last comparison applies to water. Keep the air temperature and flow comparable when explaining why water dries more slowly in humid conditions.

Worked comparisons

Common misconception 1

Evaporative cooling in a draft

Find and correct the mistake

Learner response

Water in a porous bag cools in moving air. Compare with still air at the same temperature, keeping the other conditions the same. A student says the air directly pushes thermal energy out of the water. Diagnose the explanation using evaporation and particles.

Identify what moving air removes

Effect of moving air

Removes water vapourShrinks water particlesStops evaporation

Check my attempt

View solution step by step

1. Explain the faster evaporation

   Method

   State that moving air removes water vapour near the surface.

   Reason

   This maintains a larger concentration difference and allows more particles to escape.

   Working

   Air flow → lower vapour concentration near surface → faster evaporation.

2. Explain the cooling

   Method

   Remove higher-energy surface particles and lower the remaining average kinetic energy.

   Reason

   Temperature is linked to average particle kinetic energy.

   Working

   Average kinetic energy decreases → water temperature decreases.

Examiner practice 2

Boiling vs evaporation (comparison)

2 marks

Examination question

State two differences between boiling and evaporation. [2 marks]

Write paired comparisons

Give two paired differences

View solution step by step

1. Compare location

   1 mark

   Method

   State that boiling occurs throughout the liquid with bubbles, while evaporation occurs only at the surface.

   Reason

   The two processes differ in where liquid particles enter the gas state.

   Working

   Throughout + bubbles versus surface only.

2. Compare temperature condition

   1 mark

   Method

   State that a pure liquid boils at its boiling point at a given pressure, while evaporation can occur below that temperature.

   Reason

   Evaporation involves sufficiently energetic surface particles before boiling begins.

   Working

   Fixed boiling point at given pressure versus no boiling-point requirement.

Self-mark with the mark scheme

Compare your response with each mark point. Select a point only when your response contains that evidence.

Self-mark the paired location and temperature differences.

Mark points

1. 1 mark: Compares throughout/bubbles with surface only.
2. 1 mark: Compares the boiling-point requirement with evaporation below the boiling point.

Calculate my mark

Think it through

Mind stretcher 1: Humidity and sweatingExtension

Compare air at the same temperature and air flow. On a hot day, why does sweating cool you less effectively when the air is very humid?

Show Answer

High humidity means the air already contains a lot of water vapour, so evaporation from the skin is slower. With less evaporation, less energy is taken from the skin, so cooling is less effective.

Mind stretcher 2: “Evaporation stops” in a closed containerExtension

Water is placed in a sealed container held at constant temperature. Some liquid remains when its level stops changing. Does evaporation stop? Explain.

Show Answer

Evaporation continues at the surface, and vapour particles also return to the liquid by condensation. Water vapour builds up in the container, increasing humidity until the air becomes saturated. Eventually the rate of evaporation equals the rate of condensation (dynamic equilibrium), so there is no net change in water level. If all the liquid evaporated, this two-state equilibrium would not apply.

Explain why a draft can help a wet cloth dry, then practise in the Thermal Properties check. For energy during a complete state change, use specific latent heat.