In this post, we will discuss about evaporation and the factor affecting its rate of evaporation.
What is Evaporation?
Evaporation is generally defined as a process in which a liquid state changes into vapour state.
Evaporation is a form of vaporisation that usually occurs on the surface of liquids and involves the change of the liquid particles into the gaseous state. Therefore, this process involves a change in the phase of matter of liquid.
The drying of wet clothes is one of the typical example of evaporation.
Evaporation is an endothermic process. Endothermic process is the one in which heat is absorbed. Thus, during evaporation, the liquid absorbs heat from the surroundings or from any other body in contact with it. For instance, during the drying of clothes in the sun, the water in the clothes absorbs heat energy of the sunlight.
How does evaporation differ from vaporisation?
The main points of difference between evaporation and vaporisation are:
| S No. | Evaporation | Vaporisation |
|---|---|---|
| 1. | Evaporation takes place at all temperature. The temperature may change during evaporation. | Vaporisation takes place only at a fixed temperature called the boiling point of the liquid. The temperature during vaporisation does not change. |
| 2. | Evaporation is a slow and silent process. | Vaporisation is a fast and violent process. |
| 3. | Evaporation takes place only on the surface of the liquid. | Vaporisation takes place over the entire mass of the liquid. |
| 4. | The rate of evaporation depends upon the surface area of the liquid, wind speed, humidity and temperature. | The rate of boiling does not depend upon the surface area, wind speed, humidity and temperature. |
What are the factors affecting the rate of evaporation?
Temperature: The rate of evaporation of a liquid increases with a rise in temperature. That’s why evaporation is faster on hot summer days as compared to winter days or cloudy days.
Surface area: The rate of evaporation of a liquid increases when it is spread over a larger area. That’s why, a liquid taken in a plate evaporates faster than when taken in a cup. The spread-out clothes dry faster due to faster evaporation of water from the surface of the clothes.
Humidity: When the surroundings contain very small amount of water vapour then the evaporation takes place at a faster rate. That’s why, clothes dry up faster on a dry day as compared on a wet or rainy day. The reason behind this is that on a rainy day, the air contains water vapours in a large amount. So, it will not be able to accept any more water vapour. This results in slower rate of evaporation.
Nature of the liquid: The rate of evaporation also depends upon the nature of the liquid. Some liquids evaporate fast, while others evaporate at a slow rate. For example, petrol, ether, alcohol, chloroform, etc evaporate much faster as compared to water. Generally, liquids having smaller latent heat of vaporisation evaporates faster. In other way, the liquids in which the intermolecular forces are weak, evaporates faster. The liquids which evaporates fast are called as volatile liquids.
Wind speed: When wet clothes are spread, the air near it gets saturated with water vapour. It results in slower evaporation from the wet clothes. With an increase in wind speed, the moist air present near the wet clothes get replaced by dry air. This will increase the rate of evaporation and hence faster drying of clothes.
Why does evaporation causes cooling?
Evaporation is an endothermic process. So, a liquid absorbs heat during evaporation. This heat may be provided by the liquid itself or by the surroundings. When the evaporating liquid takes the required heat from the other parts of the liquid, the rest of the liquid cools down.
On the other hand, if the liquid takes heat from the surroundings, it causes cooling of the surroundings. For example, on a hot day, we perspire. When this sweat evaporates, it absorbs the required heat from our body, and we feel coo. In a refrigerator, the refrigerant is allowed to evaporate in a cooling coil, called evaporator. This cooling oil is fitted in the cooling chamber of the refrigerator, The vapour of the refrigerant is then liquefied with the help of a compressor.
Applications of evaporative cooling?
1. We sweat in order to cool our bodies. Perspiration is essentially evaporation. Water from our body evaporates, taking energy from our body in the process and thus results in the lowering of our body temperature.
2. During the summer, we wear cotton clothes. Cotton, being a good absorber of water allows more sweat to be in contact with the atmosphere, consequently helping in more evaporation. It is for this reason that we feel cooler when we wear cotton clothes.
3. Water is stored in earthen pots to make it cool. The pores of the earthen pot, just like the pores of cotton cloth provide a larger surface area for more evaporation.
4. An air cooler is more effective on hot, dry days. The basic principle behind the working of an air cooler is evaporative cooling. As on a hot, dry day, the temperature is high and humidity is low, the evaporation rate is higher. The water takes energy from the air and gets converted to vapour. This makes the air cooler.
How can the phenomenon of evaporation be explained in terms of the kinetic molecular theory?
According to the kinetic molecular theory, all molecules in a liquid do not have the same amount of energy. Some molecules have higher energy, while others may have low energy. The molecules keep changing their energies due to molecular collisions. Due to this some molecules may acquire sufficient energy to escape from the free surface of the liquid. The escape of some high energy molecules from the surface of a liquid is called evaporation.
Why do wet clothes dry quickly in the sun than in the shade?
Rate of evaporation is faster at higher temperatures. The temperature in sunny areas is higher than that in the shade. As a result, wet clothes dry quicker in the sun than in the shade.
Why should we wear cotton clothes during summer?
During summers, we perspire more due to the body mechanism to keep it cool. The perspiration comes out through the pores in the skin. It evaporates absorbing heat from our body leaving it cool. Cotton is a good absorber of water. It absorbs the sweat and exposes it to the atmosphere. This speeds up the evaporation of the sweat and makes us comfortable.
Why does water get cooled in an earthen pot?
An earthen pot has small pores in its walls. When water is poured into it, some of it seeps through these pores to its outer surface, and then it evaporate. The heat required for evaporation is taken from the earthen pot and from the water in it. As a result, the water in an earthen pot gets cooled down.
Why do we feel relief under a fan when we perspire?
The air from the fan causes rapid evaporation of sweat. During evaporation, the sweat takes away heat from the body. As a result,we feel relief under a fan.
Why do trees acquire more leaves during summer?
Evaporation of water from the leaves is called transpiration. Transpiration helps plants/trees to keep cool. During summer, when the temperature is high, a tree must transpire more to keep itself cool. More transpiration requires more leaves. So, trees acquire more leaves during summers.
Why does the air passing through a room cooler become cooler?
In a room cooler, hot and dry passes through wet pads of wood-shaving. Water takes heat from the hot air and evaporates. The evaporation of water cools the pads, and the circulating water too. As a result, the incoming air also gets cooled down.
Read More: Question and Answer of Is Matter Around Us Pure?
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