Evaporation can be defined as the process through which a liquid (generally water) is converted into a gas/vapour without being heated to its boiling point. The key factors that affect the rate of evaporation of a liquid are:
- Temperature: The greater the temperature of the liquid and its surroundings, the faster the rate of evaporation.
- Surface area occupied by the liquid: Since evaporation is a surface phenomenon, the greater the surface area occupied by the liquid, the quicker it undergoes evaporation.
- Humidity of the surroundings: The greater the humidity of the atmosphere surrounding the water, the slower the rate of evaporation.
- Air circulation or wind speed: The presence of a breeze or another source of air circulation directly contributes towards the rate of evaporation.
Each of these factors affecting evaporation have been explained in detail below.
When the temperature of the liquid (water, in this case) is increased, it results in an increase in the kinetic energy of the individual molecules that constitute the liquid. This increase in energy makes it easier for the liquid molecules to overcome the intermolecular forces of attraction (that holds the liquid together) and escape into the atmosphere as a gas. It can be noted that the temperature of the surrounding can also contribute towards faster evaporation since hot environments can transfer their heat to the liquid.
Surface Area Occupied by the Liquid
Evaporation is a surface phenomenon, which implies that the process only occurs at the surface of the liquid. During evaporation, the molecules of the liquid that are present at its surface overcome the intermolecular forces of attraction to break away from the liquid and escape into the atmosphere as a gas (or vapour).
Liquids are known to not have any definite shape – they assume the shape of their container. Therefore, the greater the surface area of the container (or the greater the surface occupied by the liquid), the greater the number of liquid molecules present at the surface. The larger the number of liquid molecules are at the surface, the larger the number of molecules that will break away from the liquid and become a gas at a given point of time, which contributes to an increase in the rate of evaporation.
Humidity of the Surrounding
There is a limit to how much water vapour the atmosphere can hold. This limit is proportional to the temperature (which implies that hot air can hold more water vapour than cold air). Evaporation can be viewed as an equilibrium process through which the amount of water vapour in the atmosphere reaches an equilibrium with the amount of water in the surface of the liquid. Therefore, the greater the amount of water vapour in the atmosphere over the liquid, the slower the rate of evaporation. Also, it can be noted that an increase in temperature with constant humidity will contribute to an increase in the rate of evaporation since hot air can hold a greater amount of water vapour.
Air Circulation or Wind Speed
Evaporation increases the humidity of the atmosphere that immediately surrounds the liquid. This humid air takes some time to dissipate into the rest of the atmosphere. The presence of a breeze, a powerful wind, or some other form of air circulation can speed up this process and make the environment of the liquid less humid. Therefore, by decreasing the humidity of the liquid’s surrounding, a powerful breeze or wind can increase the rate at which the liquid evaporates.
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