Evaporation
Summary: The change of state from liquid to gas that occurs only at the surface of a liquid and can happen at any temperature below the boiling point. Tags: igcse chemistry Created: 2026-07-17
Evaporation is the change of state from liquid to gas that takes place only at the surface of a liquid. Unlike boiling, it can occur at any temperature below the boiling point, which is why puddles dry up and wet clothes dry even on a cool day. According to the kinetic particle theory, the particles in a liquid have a range of kinetic energies, and it is the most energetic particles at the surface that are able to overcome the forces of attraction of their neighbours and escape into the gas phase. Because the particles that leave are the ones with the most energy, the average kinetic energy of the particles left behind falls, so the remaining liquid cools; evaporation is therefore an endothermic process that takes energy from the liquid and its surroundings. This cooling effect explains why sweating cools the body and why ethanol feels cold on the skin. Evaporation happens faster when the liquid is warmer, when air moves across its surface, and when the liquid has a larger exposed surface area.
Evaporation as a Change of State
Evaporation is one of the interconversions between states of matter described by the kinetic particle theory. Like boiling, it converts a liquid into a gas, but it is a surface-only process that does not require the liquid to reach its boiling point.
Evaporation: the change of state from liquid to gas that occurs at the surface of the liquid only, at any temperature below the boiling point, with energy absorbed from the liquid and its surroundings (endothermic).
What happens to the particles
| Stage | Particle behaviour |
|---|---|
| Liquid at any temperature | Particles are close together, irregularly arranged, and slide past one another; they have a range of kinetic energies. |
| At the surface | The most energetic surface particles have enough energy to overcome the attraction of neighbouring particles. |
| Escape | These high-energy particles leave the surface and become gas (vapour) particles. |
| Remaining liquid | The average kinetic energy of the particles left behind falls, so the liquid cools. |
In more detail:
- The particles in a liquid do not all have the same energy; there is a range of kinetic energies, with some particles moving much faster than others.
- The most energetic particles at the surface of the liquid have enough energy to overcome the forces of attraction from the particles around them.
- These high-energy particles escape from the surface and become gas (vapour) particles, spreading out into the space above the liquid.
- Because the particles that escape are the fastest ones, the average kinetic energy of the remaining particles decreases.
- A lower average kinetic energy means a lower temperature, so the remaining liquid cools down. The liquid then absorbs energy from its surroundings, which is why evaporation has a cooling effect on whatever the liquid is in contact with.
This cooling effect has everyday consequences: sweat evaporating from the skin cools the body, and a volatile liquid such as ethanol feels cold when it evaporates from the hand.
Factors that affect the rate of evaporation
| Factor | Effect on rate of evaporation | Explanation in terms of particles |
|---|---|---|
| Temperature | Higher temperature → faster evaporation | More particles have enough kinetic energy to escape from the surface. |
| Movement of air (draught/breeze) | Moving air → faster evaporation | Escaped particles are carried away from the surface, so fewer return to the liquid. |
| Surface area | Larger exposed surface → faster evaporation | More particles are at the surface at any one time, so more can escape. |
This is why washing dries fastest on a warm, breezy day when it is spread out, and why a liquid in a wide shallow dish evaporates faster than the same liquid in a tall narrow container.
Evaporation compared with boiling
Both processes change a liquid into a gas, but they differ in important ways:
| Feature | Evaporation | Boiling |
|---|---|---|
| Where it occurs | At the surface of the liquid only | Throughout the whole liquid |
| Temperature | At any temperature below the boiling point | Only at the boiling point |
| Bubbles | No bubbles form | Bubbles of vapour form throughout the liquid |
| Speed | Slow and gradual | Rapid |
| Which particles escape | Only the most energetic particles at the surface | Particles throughout the liquid gain enough energy to escape |
| Energy supply | Energy taken from the liquid and surroundings, cooling the remaining liquid | Requires continuous heating |
Evaporation in practical chemistry
Evaporation is used in the laboratory as a separation technique: heating a solution in an evaporating basin drives off the solvent and leaves the dissolved solid behind. For example, evaporating sea water leaves solid salt. Slow evaporation at room temperature, or partial evaporation followed by cooling, is used in crystallisation to obtain well-formed crystals of a salt from its solution. Evaporation is a physical change: the particles of the substance are unchanged, and the vapour can be condensed back into the liquid.
Everyday examples of evaporation
- Puddles drying up after rain, even though the water never reaches 100 °C.
- Wet clothes drying on a washing line, fastest on warm, windy days.
- Sweating: sweat evaporating from the skin absorbs energy from the body, cooling it down.
- Perfume or ethanol evaporating quickly from the skin and feeling cold as it does so — volatile liquids (those with weak intermolecular forces and low boiling points) evaporate readily at room temperature.
Sources
- Cambridge IGCSE Chemistry 0620 Syllabus (2023-2025), Cambridge Assessment International Education
Common Misconceptions
| Misconception | Reality |
|---|---|
| A liquid can only evaporate when it is hot or boiling. | Evaporation occurs at any temperature below the boiling point, because some surface particles always have enough energy to escape; it simply happens faster at higher temperatures. |
| Evaporation and boiling are the same process. | Evaporation occurs only at the surface, at any temperature, without bubbles; boiling occurs throughout the liquid, only at the boiling point, with bubbles of vapour forming in the body of the liquid. |