VAPOUR AND HUMIDITY
Vapour and humidity takes place when a liquid evaporates in a given temperature.
EVAPORATION OF THE LIQUIDS
Evaporation: Is the gradual change of states from liquid to gas that occurs at the surface of a liquid.
❖ Evaporation depends on the nature of liquids.
For example
❖ Liquids with strong force of attraction between their molecules will take more time to evaporate.
Examples of such liquids are water, diesel and lubricating oil.
❖ Liquids with weak forces of attraction between their molecules will evaporate very quickly. Such
liquids are said to be volatile. They include methylated spirit and petrol.
FACTORS AFFECTING EVAPORATION
The main factors that affect the rate of evaporate of liquids are:-
1) Temperature
2) Surface area
3) Concentration
4) Rate of flow of air (wind)
1) TEMPERATURE
➢ An increase in temperature increases the rate of evaporation.
Thus the liquid particles move faster as the temperature increases hence the rate of evaporation increases.
2) SURFACE AREA
➢ The rate of evaporation increases with increase in the surface area of the liquid.
Thus larger surface area allows more particles to leave the liquid.
For example, water in the puddle dries out more quickly than that in a cup.
3) CONCENTRATION
➢ If the surrounding air already has a high concentration, the evaporation will be slow.
For example, clothes will dry fast in dry air, but slowly in humid air, this is because humid air already has high concentration of water vapour molecules.
4) RATE OF FLOW OF AIR (WIND)
➢ The rate of flow of air (wind) is directly proportional to the rate of evaporation.
SATURATED AND UNSATURATED VAPOUR PRESSURE
VAPOUR PRESSURE
➢ This is the pressure created by the vapour of a substance that forms above a liquid of the same
substance.
SATURATED VAPOUR PRESSURE (SVP)
➢ This is a state in which the gaseous phase of a substance coexists in equilibrium with the liquid
phase at a given temperature.
o Saturated vapour pressure (SVP) takes place when the rate of evaporation equals to the rate of condensation. At the saturated vapour pressure, the humidity formed.
o Saturated vapour pressure is measured in Pa or kPa.
o The saturated vapour pressure is a measure of the liquid’s rate of evaporation at a given temperature. A substance with high vapour pressure at room temperature is said to be volatile.
