Name the changes of state and explain them using the particle model, interpret heating and cooling curves, and account for the flat portions at the melting and boiling points

What this dot point is asking

SEAB wants you to name the changes of state (melting, freezing, boiling, condensation), explain each using the kinetic particle model, and interpret a heating or cooling curve. The most-tested point is why the temperature stays constant during a change of state even while heating continues, which is shown by the flat portions of a heating curve at the melting and boiling points.

The answer

Naming the changes of state

The six changes link the three states:

• Melting: solid to liquid (at the melting point).
• Freezing (solidifying): liquid to solid (at the freezing point, the same temperature as melting).
• Boiling: liquid to gas throughout the liquid (at the boiling point).
• Condensation: gas to liquid.
• Evaporation: liquid to gas at the surface, below the boiling point.
• Sublimation: solid directly to gas (for a few substances).

Melting and boiling absorb energy (heating); freezing and condensation release energy (cooling).

Explaining changes with the particle model

When a solid is heated, its particles vibrate faster. At the melting point they have enough energy to overcome the forces holding them in fixed positions, so they begin to slide and the solid melts into a liquid. On further heating the liquid particles move faster, and at the boiling point they gain enough energy to break away from one another completely and escape as a gas. Cooling reverses this: as particles lose energy they slow down, come closer, and the forces between them pull them into a liquid and then a solid.

Reading a heating curve

A heating curve plots temperature against time as a substance is heated steadily from solid to gas. It has a characteristic shape:

1. Sloping up: the solid warms, particles vibrate faster.
2. Flat (first plateau): at the melting point the solid melts; temperature is constant.
3. Sloping up: the liquid warms.
4. Flat (second plateau): at the boiling point the liquid boils; temperature is constant.
5. Sloping up: the gas warms.

Why the plateaus are flat

During a change of state the energy supplied is used to overcome the forces between particles (to free them or separate them), not to increase their average kinetic energy. Temperature measures average kinetic energy, so while the substance is changing state the temperature stays constant until the change is complete. This is the key explanation examiners look for.

Examples in context

Example 1. Using a melting point as a purity check. A pure substance melts sharply at one fixed temperature, giving a clean flat portion on its heating curve. An impure sample melts over a range and at a lower temperature, so the absence of a sharp plateau signals impurity, linking this topic to separation and purification.

Example 2. Sweating to cool down. When sweat evaporates from skin, the fastest-moving water particles escape, taking energy with them and leaving the rest cooler. This everyday cooling shows evaporation happening at the surface and below the boiling point, without the bulk liquid reaching boiling.

Try this

Q1. Name the change of state from a gas to a liquid. [1 mark]

• Cue. Condensation.

Q2. Explain why the temperature does not rise while a pure solid is melting. [2 marks]

• Cue. The heat energy is used to overcome the forces holding the particles in fixed positions, not to increase their kinetic energy, so the temperature stays constant.

Q3. State two differences between evaporation and boiling. [2 marks]

• Cue. Evaporation occurs only at the surface and at temperatures below the boiling point; boiling occurs throughout the liquid and only at the boiling point.