2.2.3 Melting, boiling and evaporation
- Syllabus
- 0625–2026–2027
- Topic
- 2.2.3
- Level
- —
Learning objectives
2.2.3.1Melting and boiling in terms of energy• Describe melting and boiling in terms of energy input without a change in temperature2.2.3.2Melting and boiling temperatures for• Know the melting and boiling temperatures for water at standard atmospheric pressure2.2.3.3Condensation and solidification in• Describe condensation and solidification in terms of particles2.2.3.4Evaporation in terms of the escape of• Describe evaporation in terms of the escape of more-energetic particles from the surface of a liquid2.2.3.5Evaporation causes cooling of a liquid• Know: evaporation causes cooling of a liquid2.2.3.6Differences between boiling and• Describe the differences between boiling and evaporation2.2.3.7Temperature, surface area and air• Describe how temperature, surface area and air movement over a surface affect evaporation2.2.3.8Cooling of an object in contact with• Explain the cooling of an object in contact with an evaporating liquid
Energy changes state while temperature stays constant
During melting and boiling, a substance absorbs energy while its temperature remains constant until the change of state is complete.
| Change | Energy transfer | What changes while temperature is constant |
|---|---|---|
| melting: solid → liquid | energy enters the substance | particles become less strongly held and can move past one another |
| boiling: liquid → gas | energy enters the substance | particles overcome attractions and separate widely as gas |
Temperature depends on average particle kinetic energy. During the change of state, the input increases particle separation and potential energy rather than average kinetic energy, so the temperature does not rise. After the change is complete, further energy input can raise the temperature again.
A flat section on a heating curve does not mean that no energy is entering. It means energy is changing the state instead of increasing the temperature. This course objective is qualitative; no specific-latent-heat calculation is required.
Water changes state at 0 °C and 100 °C
At standard atmospheric pressure, pure water melts and freezes at 0 °C, and boils and condenses at 100 °C.
| Temperature | Processes in opposite directions | Coexisting states during the change |
|---|---|---|
| 0 °C | melting ↔ freezing | ice and liquid water |
| 100 °C | boiling ↔ condensation | liquid water and water vapour |
These values assume standard atmospheric pressure and pure water. The numbers are phase-change temperatures, not the lowest and highest temperatures a thermometer can read.
Condensation and solidification reorganise particles
Condensation changes a gas to a liquid; solidification changes a liquid to a solid. In both changes, the substance releases energy as attractions constrain the particles more strongly.
| Change | Particle account |
|---|---|
| condensation: gas → liquid | particles come much closer together; attractions keep them close while they still move randomly past one another |
| solidification: liquid → solid | particles become held in fixed positions in an ordered, closely packed arrangement; they continue to vibrate |
At the phase-change temperature, average kinetic energy and temperature remain constant while potential energy decreases and energy is transferred out. Particles do not disappear, shrink or stop moving.
Evaporation is escape from a liquid surface
Evaporation occurs when more-energetic particles at the surface of a liquid have enough energy to overcome attractions and escape into the gas above the liquid.
Liquid particles have a range of kinetic energies. At any moment, some surface particles are energetic enough to leave, even when the liquid is below its boiling point. Particles deeper in the liquid cannot escape directly through the surface.
Evaporation is not a stream of bubbles through the liquid. It is a surface process and can occur at many temperatures, not only at the boiling point.
Evaporation cools the remaining liquid
Evaporation causes the temperature of the remaining liquid to decrease.
More-energetic particles are more likely to escape → they carry away more than the average kinetic energy → the particles left behind have a lower average kinetic energy → the liquid's temperature falls.
Cooling is about the average energy of the particles that remain. It is not because moving air directly slows every particle in the liquid.
Distinguish boiling from evaporation
Boiling and evaporation both change liquid to gas, but they occur in different ways.
| Feature | Boiling | Evaporation |
|---|---|---|
| where | throughout the liquid | only at the surface |
| temperature | at the boiling point for the stated pressure | can occur at many temperatures |
| visible behaviour | vapour bubbles form within the liquid and rise | no vapour bubbles form throughout the liquid |
| effect during the process | with continued energy input, temperature stays constant while boiling | preferential escape often cools the remaining liquid |
Steam-like mist above a liquid is not evidence that evaporation occurs throughout it. The key test is whether vapour bubbles form within the body of the liquid.
Control the rate of evaporation
Evaporation is faster when the liquid is warmer, when its exposed surface area is larger, and when air moves more quickly over the surface.
| Change | Effect on rate | Particle reason |
|---|---|---|
| raise temperature | increases | a larger fraction of particles has enough energy to escape |
| increase exposed surface area | increases | more particles are at the surface and able to escape at once |
| increase air movement | increases | vapour particles are carried away, so fewer return to the liquid |
A wide dish evaporates faster than a narrow container holding the same volume at the same temperature. Wet clothes dry fastest when spread out in warm, moving air.
Moving air does not speed evaporation by adding kinetic energy directly to the liquid. Its main role is to remove vapour from above the surface.
An evaporating liquid cools an object in contact
An object in contact with an evaporating liquid cools because energy transfers from the object to the liquid and is carried away as particles escape.
Evaporation removes energetic liquid particles → the liquid needs energy to continue evaporating → energy transfers from the warmer object into the liquid → the object's internal energy and temperature decrease.
Sweat cools skin, a volatile liquid used on skin feels cold, and a wet cloth can cool a container. Faster evaporation usually increases the cooling rate because energy is removed from the object more quickly.
Do not stop at ‘the liquid cools’. To explain cooling of the object, state the energy-transfer direction: from the object into the evaporating liquid and then away with the vapour.