Melting, Boiling and Evaporation

This lesson explains Melting, Boiling and Evaporation for Cambridge IGCSE Physics 0625. It separates the Core requirements from the additional Supplement work for Extended candidates. Focus on the cause-and-effect explanation and the exact quantities being compared. The goal is to explain the idea accurately and apply it in an unfamiliar context.

The current 2026-2028 syllabus does not require a specific latent heat calculation.

Core: what happens during melting and boiling?

During melting or boiling, energy is transferred into the substance without a rise in temperature while the change of state takes place.

The transferred energy changes the arrangement and separation of the particles rather than increasing their average kinetic energy. Because temperature depends on average particle kinetic energy, the temperature stays constant during the change of state.

At standard atmospheric pressure:

• pure ice melts at 0 °C
• pure water boils at 100 °C

A heating graph can therefore contain:

• sloping sections while temperature changes
• horizontal sections while a change of state occurs at constant temperature

Core: condensation and solidification

Condensation is the change from gas to liquid. Gas particles lose energy, move less freely and come closer together to form a liquid.

Solidification, or freezing, is the change from liquid to solid. The particles lose energy and become arranged in fixed positions, although they continue to vibrate.

Do not say that particles stop moving in a solid. Their motion is vibration about fixed positions.

Core: what is evaporation?

Evaporation occurs when more energetic particles escape from the surface of a liquid.

The particles that escape have more energy than the average particle in the liquid. The particles left behind therefore have a lower average kinetic energy, so the liquid cools.

Evaporation can happen below the boiling temperature. A puddle can evaporate even though it is nowhere near 100 °C.

Supplement: boiling compared with evaporation

Boiling	Evaporation
occurs throughout the liquid	occurs only at the surface
occurs at a fixed boiling temperature for a given pressure	can occur at any temperature
bubbles form within the liquid	no bubbles are required
rapid when the boiling point is reached	can be slow

Three factors named in the syllabus increase the rate of evaporation:

1. Higher temperature: a greater fraction of particles has enough energy to escape.
2. Larger surface area: more particles are exposed at the surface.
3. Greater air movement: escaped vapour is carried away, so evaporation can continue more rapidly.

Original exam-style question

A student places equal volumes of water in two identical shallow dishes. Dish A is placed in still air. Air from a fan moves across Dish B. Both dishes start at the same temperature.

(a) State which dish loses water faster. [1]

Dish B.

(b) Explain your answer. [2]

Moving air carries water vapour away from the surface. This allows more energetic water particles to continue escaping from the liquid, so evaporation is faster.

(c) Explain why the temperature of the remaining water can fall. [2]

The more energetic particles escape. The average kinetic energy of the particles remaining in the water decreases, so the temperature falls.

Original marking guidance

• one mark for Dish B
• one mark for moving air removing vapour from the surface
• one mark for faster continued escape of particles
• one mark for the more energetic particles escaping
• one mark for lower average kinetic energy or lower temperature of the remaining liquid

Reading a heating or cooling graph

When a graph becomes horizontal during a change of state, state all three points where relevant:

1. energy is still being transferred
2. the state or particle arrangement is changing
3. the average kinetic energy and temperature remain constant during the change

For a cooling curve, a horizontal section can show condensation or solidification rather than boiling or melting. Use the direction of energy transfer and the initial state to decide.

Common mistakes

• Using a removed calculation. Do not use a specific latent heat equation for the current 2026 to 2028 syllabus.
• Calling evaporation slow boiling. Evaporation is a surface process and can happen at any temperature.
• Saying temperature is constant because no energy is transferred. Energy is transferred during the state change; it does not increase average kinetic energy.
• Saying solid particles stop moving. They vibrate about fixed positions.
• Forgetting the role of air movement. Moving air removes vapour from above the surface.
• Explaining cooling only as “heat is lost”. Refer to the escape of more energetic particles and the lower average kinetic energy left behind.

Exam technique

Use precise particle language. A strong evaporation explanation follows this chain:

more energetic surface particles escape → the remaining particles have lower average kinetic energy → the liquid’s temperature falls

For boiling, distinguish the fixed boiling temperature and the formation of bubbles throughout the liquid.

How this is examined

Core Papers 1 and 3 can test all the basic changes of state, the melting and boiling temperatures of water, evaporation and evaporative cooling. Extended Papers 2 and 4 can also ask for a detailed comparison of boiling and evaporation, the factors that affect evaporation and a fuller particle explanation of cooling. Papers 5 and 6 may use heating or cooling data to test graph drawing, observation and interpretation skills.