Both evaporation and boiling convert a liquid into a gas, but they differ in several important ways.
Key differences
| Evaporation | Boiling | |
|---|---|---|
| Temperature | Occurs at any temperature below the boiling point | Occurs only at the boiling point |
| Where it occurs | At the surface only | Throughout the liquid |
| Bubbles | No bubbles form | Bubbles of vapour form within the liquid |
| Rate | Slow and gradual | Rapid |
| Energy source | From the liquid itself (cooling effect) | From an external heat source |
Factors affecting the rate of evaporation
| Factor | Effect |
|---|---|
| Temperature | Higher temperature increases the rate (more particles have enough energy to escape) |
| Surface area | Larger surface area increases the rate (more particles are at the surface) |
| Air movement | Moving air (wind, fan) increases the rate (removes vapour so more particles escape) |
| Humidity | Lower humidity increases the rate (less vapour in the air, so particles escape more easily) |
Why evaporation causes cooling
The most energetic particles at the surface escape as gas. The particles left behind have a lower average kinetic energy, so the temperature of the remaining liquid decreases.
This explains everyday cooling: sweat evaporating from skin, a wet cloth on a forehead, feeling cold after swimming.
Worked example
Explain why clothes dry faster on a windy day than on a still day at the same temperature.
Wind removes water vapour from the surface of the clothes. This reduces the humidity near the surface, allowing more water molecules to escape from the surface into the air. The rate of evaporation increases, so the clothes dry faster.
Common errors and how to correct them
Saying evaporation only happens at high temperatures. Evaporation occurs at any temperature. Even at room temperature, some particles at the surface have enough energy to escape.
Confusing evaporation with boiling. Evaporation is a surface process that happens below the boiling point. Boiling happens throughout the liquid at the boiling point with bubble formation.
How to apply this in an exam
If asked to compare, give at least three clear differences using a table format. If asked to explain why evaporation causes cooling, focus on the escape of the most energetic particles reducing the average kinetic energy.
Need help with this concept?
A 0625 specialist can work through the student's current question and help identify whether the difficulty is the concept, the calculation or the exam technique.