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.
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Analogy
Like a crowded room with a door: molecules keep escaping, and others keep returning, until the traffic is equal both ways.
Example
At 25 °C: about 3.2 kPaAt 100 °C: 101.3 kPaThis 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.