# Vapour pressure

Vapour pressure is the pressure of vapour in balance with its liquid in a closed container. It rises with temperature. A liquid boils when its vapour pressure equals the outside pressure.

Chemistry (university) · Part 03: Intermolecular forces, gases and solutions · Card 3 of 9 · Last updated 2026-10-19

## Analogy

Like a crowded room with a door: molecules keep escaping, and others keep returning, until the traffic is equal both ways.

## Example

**Water vapour pressure**

At 25 °C: about 3.2 kPa
At 100 °C: 101.3 kPa
This equals normal air pressure,
so water boils.

## In practice

Water boils below 100 °C on a mountain because the air pressure is lower, so less vapour pressure is needed.

## Practice

### Flashcards

- **When does a liquid boil?** When its vapour pressure equals the outside pressure.

## Questions people ask

### Why does a liquid with strong intermolecular forces have low vapour pressure?

Strong attractions hold molecules in the liquid, so fewer have enough energy to escape into the gas. Fewer escaping molecules means lower vapour pressure at a given temperature. This is why water has a lower vapour pressure than diethyl ether at room temperature.

### What is the normal boiling point?

The normal boiling point is the temperature at which a liquid's vapour pressure equals 1 atm (101.3 kPa). For water that is 100 °C. At other pressures the boiling temperature changes, which is why pressure cookers raise the boiling point and cook food faster.

## Related concepts

- [Intermolecular forces](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-3/chemistry-university-part-3-concept-1)
- [Hydrogen bonding](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-3/chemistry-university-part-3-concept-2)

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