The change of liquid into vapor without reaching its boiling point is called Evaporation.
While vaporization takes place only at boiling point, evaporation takes place on all temperatures.
Vaporization is a bulk phenomenon, but evaporation is a surface phenomenon. This means that whole bulk of liquid is involved in vaporization. But in case of evaporation, only the particles at the surface of liquid undergo the change of state from liquid to gas.
This explains, drying up of cloth, drying up of freshly mopped floor even during winter season. What happens when water is left in a plate in open? The water evaporates and the plate becomes empty.
Process of Evaporation
When molecules at the surface of water are exposed to heat, their kinetic energy increases. Because of increase in kinetic energy these molecules are able to overcome the force of attraction between the particles. As a result, they escape in the air in the form of vapor. Additionally, some molecules get some of the required kinetic energy from their neighboring molecules. This causes the cooling effect of evaporation.
Factors affecting Evaporation
Evaporation increases with increase in temperature and decreases with decrease in temperature. This means rate of evaporation is directly proportional to the temperature.
Kinetic energy of surface molecules increases with increase in temperature. This helps the molecules to overcome the force of attraction and subsequently, turn into vapor. Reduction in temperature has the opposite effect.
Example: Clothes dry faster in summer than in winter.
Evaporation decreases with increase in pressure and increases with decrease in pressure. This means the rate of evaporation is indirectly proportional to the pressure.
Particles at the surface of liquid require more kinetic energy to escape in air when there is more pressure over it. Hence rate of evaporation decreases with increase in pressure. On the contrary, a reduction in pressure means higher rate of evaporation. Particles need less kinetic energy to escape the effect of low pressure. Therefore, increase in pressure slows down the rate of evaporation and decrease in pressure speeds up the rate of evaporation.
Evaporation increases with increase in surface area and decreases with decrease in surface area. It means rate of evaporation is directly proportional to surface area.
A greater surface area means a greater number of particles on the surface, which are ready to evaporate. A smaller surface area means a smaller number of particles on the surface, available for evaporation.
Example: You need to spread wet clothes on a line. When wet cloth is spread, it allows a greater surface area exposed for evaporation.
It takes longer for water in a pothole to dry. But water on the wet road dries up quickly. A smaller surface area in the pothole results in slower rate of evaporation. Wet road has a larger surface area. So, the wet road dries up quicker than a pothole.
Water kept in a plate evaporates faster than water kept in a tumbler. This happens because a larger surface area of water is exposed in a plate, compared to what happens in case of tumbler. Due to a larger surface area, the water kept in a plate dries up faster than the water kept in a tumbler.
Evaporation decreases with increase in humidity and increases with decrease in humidity present in air. This means rate of evaporation is indirectly proportional to the humidity present in air.
Humidity is the amount of water vapor present in air. You can also say that the amount of moisture in air is called humidity. You may have heard or read about humidity from weather reports on TVs and newspaper. The weather report shows humidity as percentage of moisture in air.
High humidity means presense of more water in air. Low humidity means less amount of water is present in air.
High humidity means the air cannot take additional amount of water. So, it takes longer for clothes to dry in rainy season. Humidity is very high during rainy season.
Low humidity means, the air can easily take up additional amount of water. So, it takes less time for clothes to dry in summer season. Humidity is very low during summer season.
You may have noticed majority of the people sweat profusely in rainy season. In fact, rate of sweating can be more during summer. But the sweat evaporates quickly during summer and hence the skin appears to be dry. During rainy season, the sweat does not evaporate easily because of high humidity and hence the skin appears to be perpetually soaked in sweat.
Evaporation increases with the increase in wind speed and decreases with decrease in wind speed. This means rate of evaporation is directly proportional to the speed of wind.
High speed wind propels away the molecules at surface. As a result, rate of evaporation increases.
Clothes dry up faster on a windy day. This shows the effect of wind speed on evaporation. During winter, people often put wet cloth under a ceiling fan in order to dry them quickly. This is another exmaple of effect of wind speed on rate of evaporation.
Evaporation in Everyday Life
Evaporation plays an important role in water cycle. Water from everywhere keeps on evaporating. This means that water from ocean, river, lake, etc. evaporate and contributes to humidity in the atmosphere.
Sweating is a mechanism to regulate our body temperature. When sweat comes out it evaporates. Evaporation of sweat causes a cooling effect on the body. Thus, sweating gives relief to our body on a hot day.
Pitcher and Air-cooler
Many people keep drinking water in earthen pot. People also use air-coolers during summer. In both the cases, water evaporates from the pores in the pot (earthen pitcher), or from the cooling pads (in cooler).
Evaporation is an endothermic process. This means, temperature from the surrounding is utilised for evaporation. Utilization of temperature from the surroundings results in reduction of temperature of surrounding. Due to this, the water in the pitcher or in the air-cooler becomes cooler than earlier. So, you get pleasantly cold water to drink from a pitcher. Similarly, you get cool breeze to breathe in from an air-cooler.
Production of Salt
Salt is prepared by evporation of saline water. Saline water from sea is collected in shallow pits. This is allowed to evaporate under the sun. Evaporation of saline water leaves heaps of salt crystals. These crystals are further purified for common use.