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[dropcap]I[/dropcap]In liquids, there are always a few particles with a lot of energy and some with very low energy. 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 converted into gaseous state. When such particles leave the liquid surface, they are considered evaporated.
[pullquote-left]“Thus, the process by which a liquid changes into its gaseous state at any temperature below its boiling point is called evaporation.”[/pullquote-left]However, the process of evaporation involves the vaporization of a liquid, but only happens at the surface of the liquid. It is also different from boiling because boiling affects the whole volume of liquid whereas evaporation affects just the top surface. It should also be noted that evaporation is a naturally occurring part of the earth’s water cycle.
- evaporation of water from lake, rivers, sea (earth’s water cycle) ;
- evaporation of sweat from human and animal skin (sweating when you are hot);
- evaporation of water from clothes, soil, a flower vase, aquarium…. (Cloths drying on a line, a floor drying after it is mopped, your hair drying after you wash it, paint drying);
- evaporation of water through stomata of leaves (a plant wilting);
- evaporation of water from the tongue of a dog when it pant (lick you finger, blow on it and feel it cool);
- evaporation of dew from plants within the first few hours of the morning;
- evaporation of any stagnant water (a puddle drying out, splashes drying up at the edge of a pool);
- evaporation of water from vegetables while cooking ;
- evaporation of alcohol (the chemical solvent) if left uncovered.
Factors affecting Evaporation –
The rate of evaporation increases with–
(i) Surface area – The larger the exposed surface area, the higher the rate of evaporation.We know that evaporation is a surface phenomenon. If the surface area is increased, the rate of evaporation increases as more heat and wind can come into contact at any one time. Example – while putting wet clothes for drying up we spread them out.
(ii) Temperature – Evaporation takes place all the time and at any temperature. The higher the temperature, the higher the rate of evaporation. For e.g. – • When the temperature of water is increased (e.g. by putting in sunlight), the water molecules gain more kinetic energy, move faster and escape at a faster rate. • When we dry one handkerchief in a shade and another in the sunlight, the handkerchief in the sunlight dry up faster.
(iii) Humidity -Humidity refers to amount of water vapour present in the air. The air around us cannot hold more than a definite amount of water vapour at a given temperature. When humidity is high, it is more difficult for water to evaporate. The higher the humidity, the lower the rate of evaporation. Example – Clothes do not dry easily in rainy days.
(iv) Wind speed – The stronger the wind, the higher the rate of evaporation. When evaporation takes place, a thin layer of air just above the surface gets nearly saturated. When there is wind, the water vapour is removed as soon as it is formed. This makes space for more evaporation.Example – Clothes dry faster on a windy day. With the increase in wind speed, the particles of water vapour move away with the wind, decreasing the amount of water vapour in the surrounding.
How does evaporation cause cooling?
How do fans make you feel cooler? or Why do you feel cool when the nurse applies spirit before giving an injection? or When you pour some acetone (nail polish remover) on your palm, why do you feel cool?
In the absence of a source of heat, evaporating liquid absorbs heat from its surroundings. In doing so, it cools whatever is near it. This is how evaporation causes cooling. Fans actually add heat to a room. Suppose, you have a perfectly insulated room and you put an electric fan in it, then the room will get warmer. All the electricity that is driving the fan turns directly into heat.So a fan does not cool the room at all. What a fan does is create a wind chill effect. (The windchill factor is the temperature that a person feels because of the wind. For example, if a thermometer reads 35°F outside and the wind is blowing at 25 miles per hour (mph), the windchill factor causes it to feel like it is 8°F. In other words, your 98°F body loses heat as though it is 8° outside.) By blowing air around, the fan makes it easier for the air to evaporate sweat from our skin, and as sweat droplets evaporate, they draw heat away from your skin and you feel cool. The more evaporation, the cooler you feel. In an open container, the liquid keeps on evaporating. The particles of liquid absorb energy from the surrounding to regain the energy lost during evaporation. This absorption of energy from the surroundings make the surroundings cold. 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.
Why should we wear cotton clothes in summer?
How do we feel when we come under a huge umbrella / a shade after walking for few minutes in scorching sunlight? Why do we start sweating too much especially if we are wearing clothes made of synthetic or any other fabric besides cotton?
Sweating is our body’s natural way to cool down. During summer our bodies tend to perspire more in order to beat the heat. The particles at the surface of the liquid gain energy from the surroundings or body surface and evaporates. 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. It helps in absorbing the sweat and expose it to the atmosphere for easy perspiration. This make us feel cool. Moreover, cotton clothes allow better air circulation as they have large holes through which air can pass easily.
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