Boiling
Summary: The endothermic change of state from liquid to gas that occurs throughout the liquid at a fixed temperature called the boiling point. Tags: igcse chemistry Created: 2026-07-17
Boiling is the change of state in which a liquid becomes a gas, occurring throughout the whole body of the liquid at a specific, fixed temperature called the boiling point. It is an endothermic process: energy must be continuously supplied for the liquid to keep boiling. At the boiling point, the particles have gained enough kinetic energy to overcome all the forces of attraction (intermolecular forces) holding them together in the liquid, so they escape and spread out to form a gas. Bubbles of vapour form throughout the liquid — not just at the surface — and rise to the top before escaping, which is the visible sign of boiling. While a pure liquid is boiling, its temperature remains constant at the boiling point, because all the energy supplied is used to separate the particles rather than to make them move faster. This distinguishes boiling from evaporation, which happens only at the surface of a liquid and can occur at any temperature below the boiling point.
Boiling as a Change of State
Boiling is one of the interconversions between states of matter described by the kinetic particle theory. It is the reverse of condensation.
Boiling: the change of state from liquid to gas, occurring throughout the liquid at a fixed temperature called the boiling point, with energy absorbed from the surroundings (endothermic).
What happens to the particles
| Stage | Particle behaviour |
|---|---|
| Liquid below the boiling point | Particles are close together, irregularly arranged, and move by sliding past one another; forces of attraction hold them close. |
| Heating the liquid | Particles absorb energy, gain kinetic energy and move faster; the temperature rises. |
| At the boiling point | Particles throughout the liquid have enough energy to overcome all the forces of attraction between them; bubbles of vapour form within the body of the liquid and rise to the surface. |
| Gas formed | Particles are far apart, randomly arranged, and move quickly in all directions, filling the space available. |
Key points about the process:
- Boiling is endothermic: energy must be supplied continuously, usually by heating.
- The energy absorbed at the boiling point is used to completely overcome the intermolecular forces between particles so they can separate as a gas; it does not increase the particles’ average kinetic energy.
- Because the average kinetic energy of the particles does not change during the change of state, the temperature stays constant at the boiling point while the liquid boils. This appears as a flat, horizontal section on a heating curve.
- Bubbles of vapour form throughout the liquid, not only at the surface. These bubbles contain the vapour (gaseous form) of the substance itself — for water, the bubbles contain water vapour, not air and not hydrogen or oxygen.
- Boiling is a physical change: no new substance is formed and the change can be reversed by condensation.
Boiling point
The boiling point is the temperature at which a liquid boils at a given pressure — the temperature at which bubbles of vapour can form throughout the liquid. For a pure substance at standard atmospheric pressure, it is a sharp, fixed value that can be used to identify the substance and test its purity.
| Substance | Boiling point (°C) |
|---|---|
| Oxygen | −183 |
| Ethanol | 78 |
| Water | 100 |
| Sodium chloride | 1413 |
Substances with strong forces of attraction between their particles have high boiling points, because more energy is needed to separate the particles completely. Impurities raise the boiling point of a liquid and cause it to boil over a range of temperatures, so a sharp boiling point at the expected value indicates a pure substance.
Boiling compared with evaporation
Both boiling and evaporation change a liquid into a gas, but they are different processes:
| Feature | Boiling | Evaporation |
|---|---|---|
| Where it occurs | Throughout the whole liquid | At the surface of the liquid only |
| Temperature | Only at the boiling point | At any temperature below the boiling point |
| Bubbles | Bubbles of vapour form in the liquid | No bubbles form |
| Speed | Rapid | Slow and gradual |
| Energy supply | Requires continuous heating | Uses energy absorbed from the surroundings |
Boiling on a heating curve
If a liquid is heated at a steady rate and its temperature is plotted against time, the graph shows:
- A rising line as the liquid warms up (particles gain kinetic energy).
- A horizontal (flat) section at the boiling point, while liquid and gas exist together and the energy supplied is used to overcome the intermolecular forces.
- A rising line again once all the liquid has become gas and the gas warms further.
The temperature of the flat section is the boiling point of the substance.
Boiling Point
The boiling point is the specific temperature at which a liquid boils — that is, the temperature at which the vapour pressure of the liquid equals the external atmospheric pressure. At this temperature, bubbles of vapour form throughout the liquid (not just at the surface). The boiling point of a pure substance is a characteristic physical property and can be used to identify the substance or assess its purity (impurities raise the boiling point). Different substances have different boiling points because of differences in the strength of the forces between their particles: substances with stronger intermolecular forces or bonds have higher boiling points.
Exam point: Boiling occurs at a specific temperature (the boiling point) throughout the liquid, unlike evaporation which occurs at any temperature only at the surface.
Sources
- Cambridge IGCSE Chemistry 0620 Syllabus (2023-2025), Cambridge Assessment International Education
Common Misconceptions
| Misconception | Reality |
|---|---|
| The bubbles in boiling water contain air, or hydrogen and oxygen gas. | The bubbles contain water vapour — the gaseous form of the water itself. Boiling is a physical change, so the water molecules are not broken apart into hydrogen and oxygen. |
| The temperature of a boiling liquid keeps rising the longer it is heated. | The temperature of a pure liquid stays constant at the boiling point while it boils; all the energy supplied is used to overcome the forces of attraction between particles, not to raise the temperature. |