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Boiling, Condensation & Evaporation

Key idea: Compare boiling, evaporation and condensation, explain constant temperature using latent heat and the particle model, and practise O Level questions.

  • SEC G3 Physics 2027
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Learning objectives

  • Compare physical properties of solids, liquids and gases
  • Explain state properties using particle arrangement, motion, forces and separation
  • Infer random molecular motion from a Brownian-motion experiment
  • Relate temperature rise to increased average kinetic energy of particles
  • Explain gas pressure using particle collisions with container walls
  • Explain heating from higher to lower temperature until thermal equilibrium
  • Describe conduction in solids through particle vibration and mobile electrons
  • Describe convection in fluids through density changes and bulk motion
  • Explain that energy transfer by electromagnetic radiation needs no material medium
  • Explain how surface colour, texture, temperature and area affect radiation transfer rate
  • Apply conduction, convection and radiation in everyday systems
  • describe internal energy as an energy store that is made up of the total kinetic energy associated with the random motion of the particles and the total potential energy between the particles in the system
  • Define heat capacity and specific heat capacity
  • Apply energy transferred = mass × specific heat capacity × temperature change
  • describe melting/solidification and boiling/condensation as processes of energy transfer without a change in temperature
  • Explain the difference between boiling and evaporation
  • Define latent heat and specific latent heat
  • Apply energy transferred for a change of state = mass × specific latent heat
  • Explain latent heat using particle behaviour
  • Sketch and interpret a cooling curve

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.

Heating curve for a pure substanceA temperature-time graph rises through solid, liquid, and gas regions, with constant-temperature plateaus during melting and boiling.TimeTemperaturesolid warmsmelting: solid + liquidliquid warmsboiling: liquid + gasgas warmsenergy transferred to substance →
Schematic heating curve for a pure substance at fixed pressure. Slopes and plateau lengths are not to scale; a plateau shows two states present at constant temperature.

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)

FeatureBoilingEvaporation
Where it happensThroughout the liquidAt the surface only
TemperatureOnly at the boiling point (pure substance)At any temperature
What you observeBubbles form in the liquidNo bubbles (usually)
Cooling effectNot the key ideaCan cool the liquid

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

Modelled example 1

Melting vs boiling (energy and temperature)

Core

Problem

For a pure substance, compare temperature and energy transfer during melting and boiling.
Study the worked solution

  1. Describe melting

    Method

    Keep temperature constant and absorb energy.

    Reason

    Latent energy changes particle arrangement and potential energy rather than average kinetic energy.

    Working

    Melting: constant temperature; energy absorbed.

  2. Describe boiling

    Method

    Keep temperature constant and absorb latent heat of vaporisation.

    Reason

    Energy separates particles into the gas state throughout the liquid.

    Working

    Boiling: constant temperature; energy absorbed.

Guided practice 2

Latent heat calculation (boiling water)

About 4 min

Problem

0.20 kg of water at 100°C becomes steam at the same temperature. Use lᵥ = 2.26 × 10⁶ J kg⁻¹ to find the energy transferred.

Use the constant-temperature state-change equation

Unit: J

Hints

Hint 1: choose vaporisation
The change is liquid to gas, so use lᵥ.
Hint 2: multiply by mass
Q = mlᵥ.
View solution step by step

  1. Select and apply the equation

    Method

    Multiply mass by specific latent heat of vaporisation.

    Reason

    There is a state change without a temperature change.

    Working

    Q = (0.20)(2.26 × 10⁶) = 4.52 × 10⁵ J

Common misconception 3

Evaporative cooling in a draft

Find and correct the mistake

Learner response

Water in a porous bag cools in moving air. 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

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 4

Boiling vs evaporation (comparison)

2 marks

Examination question

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

Write paired comparisons

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, while evaporation can occur at any temperature.

    Reason

    Evaporation involves sufficiently energetic surface particles before boiling begins.

    Working

    Fixed boiling point versus any temperature.

Challenge 5

Why boiling point changes with pressure

Minimal support

Pressure-context transfer

At high altitude, water boils below 100°C. Explain this using external pressure.

Connect the changed condition to boiling

Hints

Hint 1: compare atmospheric pressure
Atmospheric pressure is lower at high altitude.
Hint 2: use the boiling condition
Boiling occurs when vapour pressure equals external pressure.
View solution step by step

  1. State the pressure condition

    Method

    Use equality between liquid vapour pressure and external pressure.

    Reason

    This condition allows vapour bubbles to persist throughout the liquid.

    Working

    Boiling condition: vapour pressure = external pressure.

  2. Apply high altitude

    Method

    Reach the lower external pressure at a lower temperature.

    Reason

    Less vapour pressure is required than at sea level.

    Working

    Lower atmospheric pressure → lower boiling temperature.

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 check, then continue to specific latent heat.

Continue with the next resource in this course.

Course and syllabus information
Course
SEC G3 Physics
Edition
SEC G3 Physics 2027