Evaporation
1. What is Evaporation?
Evaporation is the process by which a liquid changes into a gas (vapour) at a temperature below its boiling point.
For example:
Water in a bowl slowly disappears over time.
Wet clothes dry even when it’s not very hot.
This happens because some liquid particles at the surface have enough energy to escape into the air.
Key features of evaporation
It happens at all temperatures (not only at boiling point).
It happens only at the surface of a liquid.
It is a slow process.
It does not require heating, though heat speeds it up.
Factors affecting evaporation
Evaporation becomes faster when:
Temperature is high (particles move faster)
Surface area is large (more particles can escape)
Wind speed is high (vapour is carried away)
Humidity is low (air can hold more vapour)
2. Why Does Evaporation Cool You Down?
Evaporation causes cooling.
How it works
Particles in a liquid have different energies.
The high-energy particles escape first during evaporation.
When they leave, the average energy of the remaining liquid decreases.
Lower energy means lower temperature.
Everyday examples
Sweating:When sweat evaporates from your skin, it takes heat away from your body, making you feel cool.
Alcohol or perfume on skin:It evaporates quickly and makes your skin feel cold.
Water in an earthen pot:Water seeps out and evaporates, cooling the remaining water inside.
That’s why evaporation is called a cooling process.
3. Evaporation and Energy
Evaporation is directly related to energy transfer.
Energy absorption
Evaporation requires energy.
This energy is taken from:
The liquid itself, or
The surroundings (like your skin or air)
This energy is called latent heat of vaporization.
Latent heat of vaporization
It is the amount of heat energy required to change a liquid into vapour without changing its temperature.
During evaporation:
Temperature stays the same,
But energy is absorbed to break the attraction between particles.
Energy changes summary
Liquid particles absorb energy
They overcome attractive forces
They change into vapour
Surroundings lose heat, so cooling occurs