Condensation
Summary: The exothermic change of state from gas to liquid in which particles lose kinetic energy and come close enough for intermolecular forces to hold them together. Tags: igcse chemistry Created: 2026-07-17
Condensation is the change of state in which a gas becomes a liquid. It is an exothermic process: energy is released to the surroundings as the gas condenses. When a gas is cooled, its particles lose kinetic energy and slow down, and eventually they no longer move fast enough to overcome the forces of attraction between them. The particles then come close enough together for the intermolecular forces to act, pulling them into the close, irregular arrangement of a liquid. For a pure substance, condensation occurs at the same fixed temperature as boiling — the boiling point — and while the gas is condensing the temperature remains constant, because the energy released as the particles come together balances the energy lost to the surroundings. Condensation is commonly seen when a gas or vapour meets a cold surface, such as water vapour from the air forming droplets on a cold window, mirror or drinks can.
Condensation as a Change of State
Condensation is one of the interconversions between states of matter described by the kinetic particle theory. It is the reverse of boiling (and of evaporation).
Condensation: the change of state from gas to liquid, occurring at the boiling point temperature, with energy released to the surroundings (exothermic).
What happens to the particles
| Stage | Particle behaviour |
|---|---|
| Gas above the boiling point | Particles are far apart, randomly arranged, and move quickly in all directions; the forces between them are negligible. |
| Cooling the gas | Particles lose energy to the surroundings; their kinetic energy decreases and they slow down. |
| At the condensation temperature | Particles are moving slowly enough for the forces of attraction to take effect when they come close together; the intermolecular forces pull the particles towards one another. |
| Liquid formed | Particles are close together, irregularly arranged, and slide past one another; the forces of attraction hold them close. |
Key points about the process:
- Condensation is exothermic: energy is transferred from the gas to the surroundings as the particles come together into the lower-energy liquid arrangement.
- During condensation, the particles lose kinetic energy and slow down until the intermolecular forces can hold them close together.
- For a pure substance, condensation occurs at the same temperature as boiling — the boiling point. Boiling and condensation are the same change of state in opposite directions: pure water boils at 100 °C, and pure steam condenses at 100 °C.
- While a pure gas is condensing, the temperature stays constant, because the energy released as the particles come together balances the energy being lost to the surroundings. On a cooling curve this appears as a flat, horizontal section at the boiling point.
- Condensation is a physical change: no new substance is formed, the particles themselves are unchanged, and the liquid can be boiled or evaporated back into a gas.
Condensation on cold surfaces
A gas condenses when it is cooled, so condensation is often seen where a vapour meets a cold surface. The cold surface removes energy from the gas particles that strike it; the particles slow down, the attractive forces pull them together, and a film or droplets of liquid form. Everyday examples include:
| Example | What is happening |
|---|---|
| Droplets on a cold window or mirror | Water vapour in warm air loses energy to the cold glass and condenses into liquid water. |
| Water on the outside of a cold drinks can | Water vapour in the surrounding air condenses on the chilled metal surface. |
| ”Steam” seen above a boiling kettle | Invisible water vapour condenses into a mist of tiny liquid droplets as it cools in the air. |
| Cloud and dew formation | Water vapour in the air cools and condenses into droplets of liquid water. |
Condensation in practical chemistry
Condensation is essential to distillation. In simple distillation, a solution is boiled and the vapour of the solvent passes into a water-cooled condenser, where it loses energy to the cold surface and condenses back into a pure liquid (the distillate), which is collected. In fractional distillation, each vapour condenses after separating according to its boiling point. In both techniques, it is the exothermic condensation of the vapour on the cold condenser surface that turns the gas back into a collectable liquid.
Condensation on a cooling curve
If a pure gas is cooled at a steady rate and its temperature is plotted against time, the graph shows:
- A falling line as the gas cools (particles lose kinetic energy, temperature decreases).
- A horizontal (flat) section at the boiling point, while gas and liquid exist together and the energy released as the particles come together balances the energy lost to the surroundings.
- A falling line again once all the gas has condensed and the liquid continues to cool.
The temperature of the flat section is the same temperature at which the substance boils, confirming that boiling and condensation occur at the same fixed temperature for a pure substance.
Comparing condensation with the other changes of state
| Change of state | Direction | Energy change |
|---|---|---|
| Melting | Solid → liquid | Endothermic (energy absorbed) |
| Boiling / evaporation | Liquid → gas | Endothermic (energy absorbed) |
| Condensation | Gas → liquid | Exothermic (energy released) |
| Freezing | Liquid → solid | Exothermic (energy released) |
Condensation and boiling are exact opposites: boiling absorbs energy to separate the particles completely, while condensation releases the same quantity of energy as the particles come back together.
Sources
- Cambridge IGCSE Chemistry 0620 Syllabus (2023-2025), Cambridge Assessment International Education
Common Misconceptions
| Misconception | Reality |
|---|---|
| The water that appears on a cold surface comes from inside the glass or can, having leaked or soaked through. | The droplets come from water vapour already present in the surrounding air; the vapour loses energy to the cold surface and condenses into liquid water on the outside. |
| Condensation absorbs energy because the gas has to be cooled. | Condensation is exothermic: the gas releases energy to the surroundings as its particles slow down and the intermolecular forces pull them together. Cooling removes energy, and the change of state itself gives energy out. |