Heat and change of physical state - Concept of Evaporation

Factors affecting the rate of evaporation:

• Surface area: The rate of evaporation increases by increasing the surface area of the liquid.
• Temperature: The rate of evaporation increases with an increase in temperature.
• Humidity: A decrease in humidity increases the rate of evaporation.
• Wind speed: An increase in wind speed increases the rate of evaporation.

Evaporation causes cooling:

During the process of evaporation, the particles of liquid absorb energy or latent heat of vaporisation from the surroundings to get converted to the gaseous state. This absorption of energy from the surroundings makes the surroundings cold.
For example, perspiration or sweating in our body keeps the body temperature constant by taking away the extra heat from the body as the latent heat of vaporisation.

What happens when you pour some acetone (nail polish remover) on your palm? The particles gain energy from your palm or surroundings and evaporate, causing the palm to feel cool. After a hot sunny day, people sprinkle water on the roof or open ground because the large latent heat of vaporisation of water helps to cool the hot surface. Can you cite more examples from daily life where we can feel the cooling effect due to evaporation?

Why should we wear cotton clothes in summer?

During summer, we sweat more because of the mechanism of our body, which keeps us cool. We know that during evaporation, the particles at the surface of the liquid gain energy from the surroundings or body surface and change into vapour. The heat energy equal to the latent heat of vaporisation is absorbed from the body, leaving the body cool. Cotton, being a good absorber of water, helps in absorbing sweat and exposing it to the atmosphere for easy evaporation.

Why do we see water droplets on the outer surface of a glass containing ice-cold water? Let us take some ice-cold water in a tumbler. Soon we will see water droplets on the outer surface of the tumbler. The water vapour present in the air, on coming in contact with the cold glass of water, loses energy and gets converted to a liquid state, which we see as water droplets

• Evaporation
• Rate of Evaporation
• Factors Affecting Evaporation
• How Does Evaporation Cause Cooling?