Boiling and Condensation

1. Definition

A. Boiling

Boiling changes liquid to gas throughout the liquid, with bubbles of vapour forming inside it. A pure substance has a fixed boiling point at a given pressure.

B. Condensation

Condensation changes gas to liquid and releases energy to the surroundings. In the pure-substance model, liquid and its vapour coexist at the same transition temperature at fixed pressure: the boiling temperature for that pressure.

Evaporation

Evaporation is surface escape from a liquid and can occur below its boiling point. Compare its mechanism in Evaporation and cooling.

2. Key Ideas

• Boiling absorbs energy; condensation releases energy.
• During the constant-temperature transition of a pure substance at fixed pressure, energy changes particle arrangement and potential energy, while average kinetic energy remains constant.
• During boiling, vapour bubbles can form and persist inside the liquid. They are not simply bubbles of air.
• Ordinary condensation of water vapour from air, such as droplets on a cold bottle, can occur far below 100°C.

3. Detailed Explanations

Boiling at constant temperature

A pure liquid warms as energy increases the particles’ average kinetic energy. At its boiling point, while liquid and vapour coexist at fixed pressure, further energy changes the arrangement and increases potential energy as particles enter the gas state. Temperature stays constant until the liquid has vaporised.

Heating curve for a pure substance

Temperature rises as solid warms to a melting plateau at 20 °C, then liquid warms to a boiling plateau at 80 °C, then gas warms. The temperatures and times are illustrative, not measured.

Scroll across the graph to read all labels.

View figure data

Values for Heating curve for a pure substance

Illustrative time (min)	Temperature
0	0
2	20
4	20
6	80
8	80
10	120

Values for Heating curve for a pure substance

Illustrative time (min)	Temperature
0	0
2	20
4	20
6	80
8	80
10	120

B. What happens to particles during boiling?

1. Before boiling, heating generally increases average particle kinetic energy and temperature.
2. At the boiling condition, vapour bubbles can form and persist within the liquid. This does not mean that every particle has the same speed or energy.
3. Energy continues to be absorbed as liquid becomes gas; average kinetic energy does not rise during the constant-temperature transition.
4. Bubbles rise and release vapour at the surface.

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

In the pure-substance model at fixed pressure, the two states coexist at a definite temperature. During boiling, particle potential energy increases. During condensation it decreases, and energy leaves for the surroundings. The average kinetic energy stays constant during either transition.

D. Condensation (gas → liquid)

As vapour condenses, particles become much closer together in the liquid and release energy. They continue moving; condensation does not make them motionless.

For water changing between liquid and pure steam at standard atmospheric pressure, the transition is about 100°C. Water vapour mixed with air is a different situation. If air next to a cold surface cools enough for condensation, liquid droplets can form at ordinary room temperatures. The dew point describes the temperature at which that air becomes saturated when cooled at constant pressure.

Boiling compared with evaporation

The paired comparison distinguishes where each process happens and its temperature condition.

Cooling by evaporation

Use the particle explanation to explain why preferential escape of higher-energy surface particles can cool a liquid.

Factors affecting evaporation

Temperature, exposed area, air flow and humidity are compared in the evaporation lesson.

4. Common Mistakes

• Treating every bubble as vapour. Small bubbles before boiling can contain dissolved gases; sustained boiling bubbles are mainly vapour of the liquid.
• Saying heating must raise temperature during boiling. Under the fixed-pressure, pure-substance conditions, energy changes state instead.
• Saying water vapour can condense only at 100°C; droplets can form from water vapour in cooled air at much lower temperatures.
• Saying boiling breaks water molecules into hydrogen and oxygen. A state change leaves the substance’s chemical identity unchanged.

5. Exam Tips

Describe the direction of energy transfer, state change, and average kinetic or potential energy as appropriate. State “pure substance at fixed pressure” when explaining a constant-temperature boiling or condensation plateau. If the pressure changes, the transition temperature can change too.

6. Worked Examples

Latent heat calculation: boiling water

Use the complete worked calculation in Specific latent heat.

Evaporative cooling in a draft

Use the complete activity in Evaporation and cooling.

Boiling vs evaporation (comparison)

Use the complete activity in Evaporation and cooling.

7. Mind Stretchers

Humidity and sweating

Use the complete activity in Evaporation and cooling.

“Evaporation stops” in a closed container

Use the complete activity in Evaporation and cooling.

8. Practice and next step

Explain both a boiling plateau and droplets on a cold bottle, stating the conditions for each. Practise in the Thermal Properties check, and use specific latent heat to calculate state-change energy.