Boiling – IGCSE Chemistry Definition

Boiling is a change of state from liquid to gas that takes place at a fixed temperature throughout the liquid. Examiners frequently test whether candidates can distinguish boiling from evaporation, and the heating/cooling curve question on Paper 4 relies on understanding why the temperature plateaus during boiling.

The 0620 definition

Boiling is the change of state from liquid to gas at a specific, constant temperature called the boiling point. It occurs throughout the liquid, not just at the surface.

For water at standard atmospheric pressure, the boiling point is 100 degrees C.

What happens to the particles

In a liquid, particles are close together but can slide past one another. At the boiling point, particles throughout the liquid gain enough energy to overcome all intermolecular forces and escape as gas. This requires energy input, but the temperature does not rise — all the energy goes into breaking intermolecular forces.

Boiling vs evaporation

Feature	Boiling	Evaporation
Temperature	At the boiling point only	At any temperature below the boiling point
Where it occurs	Throughout the liquid	At the surface only
Bubbles	Yes — bubbles of vapour form inside the liquid	No bubbles
Rate	Rapid, vigorous	Slow, gradual
Energy input	Continuous external heating	Particles at the surface with enough energy escape

Heating curves

On a heating curve (temperature vs time graph), the boiling section appears as a flat horizontal line. During this plateau:

• Heat energy is still being supplied
• Temperature stays constant
• Energy is used to overcome intermolecular forces, not to increase kinetic energy
• The substance exists as both liquid and gas at this stage

The length of this plateau depends on the strength of the intermolecular forces. Substances with stronger intermolecular forces have higher boiling points and longer plateaus for the same rate of heating.

Worked exam question

The graph below shows temperature against time as a pure substance is heated from solid to gas. (a) Identify the section of the graph that represents boiling. (1) (b) Explain why the temperature remains constant during this section. (2)

Mark scheme

(a) The second horizontal/flat section (at the higher temperature plateau) [1]

(b) Energy is being used to overcome/break intermolecular forces (between particles) [1]; not to increase the kinetic energy of the particles / so temperature does not rise [1]

Common exam mistakes

• Confusing boiling with evaporation. Boiling requires a fixed temperature and occurs throughout the liquid. Evaporation occurs at any temperature and only at the surface.
• Saying “bonds are broken” during boiling without specifying intermolecular forces. Covalent bonds within molecules are not broken during a change of state — only the forces between molecules are overcome.
• Forgetting that a plateau on a heating curve means two states coexist. During the boiling plateau, the substance is part liquid and part gas.