Evaporation Flashcards
Define evaporation (shortly).
The phenomenon of change of liquid into vapours at any temperature below its boiling point is called evaporation.
Define evaporation (long-ly?).
Particles of matter are always moving. At a given temperature in any gas, liquid or solid, there are particles with different amounts of kinetic energy. In liquids, a small fraction of particles at the surface, having higher kinetic energy, is able to break away from the forces of attraction of other particles and gets turned into vapour.
What are the factors by which the rate of evaporation increases?
- Increase in surface area. More surface area = more evaporation
- Increase in temperature. More number of particles get enough kinetic energy to go into the vapour state.
- Decrease in humidity. The air around us cannot hold more than a definite amount of water vapour at a given temperature. If the amount of water vapour is already high, the rate of evaporation decreases.
- Increase in wind speed. With an increase in wind speed, the particles of water vapour move away with the wind, decreasing the amount of water vapour in the surroundings.
In an open vessel, the liquid keeps on evaporating and causes cooling to it’s surroundings. How?
The particles of liquid absorb energy from the surroundings to regain the energy lost during evaporation. This absorption of energy from the surroundings makes the surroundings cold.
What happens when you pour some acetone on your palm?
The particles gain energy from your palm or surroundings and evaporate, causing your palm to feel cool.
After a hot sunny day, why do people sprinkle water on the roof or an open ground?
Because the large latent heat of vaporisation or water helps to cool the hot surface.
Why should we wear cotton clothes in the summer?
During the summer, we perspire more because of the mechanism of our body which keeps us cool. 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 is a good absorber of water and helps in absorbing the 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?
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.
