Boiling, Condensation & Evaporation

1. Definition

A. Boiling

Boiling is the change of state from liquid → gas that happens throughout the liquid at a fixed temperature (the boiling point, for a pure substance at a given pressure).

B. Condensation

Condensation is the change of state from gas → liquid. For a pure substance, it happens at the same temperature as its boiling point.

C. Evaporation

Evaporation is the change of state from liquid → gas that happens at the surface of a liquid and can occur at any temperature.

2. Key Ideas

• During boiling and condensation, temperature stays constant (energy transfer changes potential energy between particles, not average kinetic energy).
• Boiling produces bubbles inside the liquid (not just at the surface).
• Evaporation happens at the surface only, and can cause cooling by evaporation.
• Energy needed for liquid → gas is the latent heat of vaporisation (see Specific latent heat).
• Rate of evaporation increases with higher temperature, larger surface area, stronger air flow, and lower humidity.

3. Detailed Explanations

A. Boiling happens at constant temperature (pure substance)

When a pure liquid reaches its boiling point, added energy does not raise the temperature. Instead, it is used to separate particles (increase their potential energy) by overcoming intermolecular forces.

B. What happens to particles during boiling?

1. Energy is transferred to the liquid (heating).
2. Particles move faster until the boiling point is reached.
3. At the boiling point, particles throughout the liquid have enough energy to separate and form gas bubbles.
4. The bubbles rise and burst at the surface, releasing gas into the air.

C. Why is the temperature constant during boiling and condensation?

Temperature depends on the average kinetic energy of particles. During boiling or condensation, energy transfer changes the particles’ arrangement and potential energy, so the average kinetic energy stays the same and the temperature does not change.

D. Condensation (gas → liquid)

Condensation is the reverse of boiling:

• particles lose energy
• they come closer together as intermolecular attraction becomes important again
• latent heat is released to the surroundings
• temperature stays constant (for a pure substance) during the change of state

E. Boiling vs evaporation (exam comparison)

F. Cooling by evaporation

In a liquid, not all particles have the same speed. The faster (higher-energy) particles near the surface are more likely to escape. When they leave, the average kinetic energy of the remaining particles decreases, so the liquid cools unless energy is supplied from the surroundings.

G. Factors that increase the rate of evaporation (with reasons)

• Higher temperature: more particles have enough kinetic energy to escape.
• Larger surface area: more particles are at the surface.
• Stronger air flow: removes vapour near the surface, maintaining a steep concentration gradient.
• Lower humidity: the lower vapour concentration above the liquid maintains a larger concentration difference, so evaporation is faster.

4. Common Mistakes

• Saying evaporation happens “when the liquid boils” (evaporation can happen at any temperature).
• Thinking boiling happens only at the surface (boiling happens throughout the liquid because bubbles form).
• Saying temperature rises during boiling/condensation (for a pure substance, it stays constant during the change of state).
• Mixing up the direction of energy transfer (boiling absorbs energy; condensation releases energy).
• Saying all bubbles are “air”. Small bubbles before boiling can contain dissolved gases, but bubbles throughout a boiling liquid are mainly vapour of that liquid.

5. Exam Tips

• Use these keywords:
  • boiling: bubbles, throughout the liquid, boiling point, constant temperature
  • evaporation: surface, any temperature, cooling effect
• When asked “why constant temperature?”, mention latent heat and potential energy between particles.
• When comparing boiling vs evaporation, use a clear table or paired points (don’t mix the features).

6. Worked Examples

Example 1: Melting vs boiling (energy and temperature)Core

a. When a solid undergoes melting:
i. Does its temperature rise, fall, or stay constant?
ii. Is energy absorbed, released, or neither?

b. When a liquid is boiling:
i. Does its temperature rise, fall, or stay constant?
ii. Is energy absorbed, released, or neither?

Show Answer

a.
i. Temperature stays constant (for a pure substance).
ii. Energy is absorbed as latent heat.

b.
i. Temperature stays constant (for a pure substance).
ii. Energy is absorbed as latent heat of vaporisation.

Example 2: Boiling vs evaporation (comparison)Core

State two differences between boiling and evaporation.

Show Answer

Two clear differences:

• boiling happens throughout the liquid and forms bubbles; evaporation happens at the surface only
• boiling happens at a fixed temperature (boiling point for a pure substance); evaporation can happen at any temperature

Example 3: Latent heat calculation (boiling water)Core

Water at 100^° C is boiled to steam at 100^° C. The mass is 0.20 kg and lᵥ = 2.26 × 10⁶ J kg⁻¹.
Find the energy transferred.

Show Answer

Use Q = mlᵥ.
Q = (0.20)(2.26 × 10⁶) = 4.52 × 10⁵ J.

Example 4: Evaporative cooling in a draftCore

Water is in a porous bag (canvas) placed in moving air. Explain why the water becomes cooler than the surrounding air.

Show Answer

Moving air increases the rate of evaporation by removing water vapour near the surface. When faster (higher-energy) particles escape, the average kinetic energy of the remaining particles decreases, so the water cools (cooling by evaporation).

Example 5: Why boiling point changes with pressureCore

At high altitude, water boils at a temperature lower than 100circC. Explain why.

Show Answer

Boiling happens when the vapour pressure of the liquid equals the external pressure. At high altitude, atmospheric pressure is lower, so this condition is reached at a lower temperature. Therefore water boils at a temperature below 100circC.

7. Mind Stretchers

Mind stretcher 1: Humidity and sweatingExtension

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

A small amount of water is placed in a sealed container. At first the water evaporates, but later the water level stops changing. Explain.

Show Answer

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 the water level becomes constant.

8. Practice and next step

Write a two-column comparison of boiling and evaporation, including location, temperature condition and particle explanation. Check it in the Thermal Physics topic diagnostic, then continue to specific latent heat.