Vapour
A gas can be liquefied by pressure at any temperature below its critical temperature. A gas below its critical temperature is called a vapour. Evaporation is the escape of the fastest particles from the surface of a liquid at any temperature. Boiling takes place at a fixed temperature throughout the liquid, when bubbles of vapour form inside it.
| Evaporation | Boiling | |
|---|---|---|
| Temperature | At any temperature | At the boiling point only |
| Where | At the surface only | Throughout the liquid |
| Speed | Slow | Fast |
| Rate depends on | Temperature, surface area, wind, humidity | Heat supply |
A vapour in contact with its own liquid is saturated when the number of particles leaving the liquid equals the number returning. Its pressure, the saturated vapour pressure (SVP), depends only on the temperature: it rises rapidly as the temperature rises. A liquid boils when its saturated vapour pressure equals the external pressure. That is why water boils at a lower temperature on a mountain (lower pressure) and at a higher temperature in a pressure cooker.
Humidity
Humidity is the amount of water vapour in the air. Absolute humidity is the mass of water vapour in each cubic metre of air. Relative humidity compares the amount of vapour present with the greatest amount the air could hold at that temperature.
Choose the actual vapour pressure and the saturated vapour pressure at the air temperature. Find the relative humidity.
Dew point and hygrometers
If air is cooled, its saturated vapour pressure falls until it equals the vapour pressure present. This temperature is the dew point. Below it vapour condenses as dew, mist or fog. A hygrometer measures humidity. In a wet and dry bulb hygrometer, evaporation cools the wet bulb, so it reads lower than the dry bulb. The drier the air, the bigger the difference.
| Effect | Explanation |
|---|---|
| Wet clothes dry faster on a windy day | Wind carries away water vapour so evaporation continues |
| Sweat feels sticky in humid weather | Air that is almost saturated takes little more vapour, so sweat evaporates slowly |
| Dew forms on grass at night | The grass cools below the dew point |
| Clouds and rain | Rising warm air cools to its dew point and the vapour condenses into droplets |
Key terms
- Saturated vapour pressure
- The pressure of a vapour in equilibrium with its liquid at a given temperature.
- Relative humidity
- The ratio of the vapour pressure present to the saturated vapour pressure, as a percentage.
- Dew point
- The temperature at which air becomes saturated.
- Hygrometer
- An instrument for measuring humidity.
Summary
- Evaporation occurs at any temperature at the surface; boiling at a fixed temperature throughout.
- Saturated vapour pressure depends on temperature; boiling occurs when SVP equals external pressure.
- Relative humidity = vapour pressure ÷ SVP × 100.
- Dew point is the temperature at which air is saturated.