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How Does Evaporation Cause Cooling?

  • Last Updated : 05 Aug, 2021

When you apply perfume to your body, it feels a little colder. The same may be said for acetone and water. This is a result of evaporation, or the transformation of materials from their liquid to vapor state. The only difference is how quickly the coldness is perceived. In the case of acetone, the area of your body closest to the liquid will cool the fastest. This is due to the fact that acetone evaporates faster than water or scent. 

We dry our wet clothes after washing them by putting them in the sun, also we have seen that when water spills on the floor, it dries up after some time. But do you know what is the reason and why is this water? The answer behind this is the phenomenon of evaporation. In this article, we will discuss what is evaporation and what are the factors that affect evaporation, and how evaporation causes cooling in the body? So let us first understand what is evaporation?

What is Evaporation?

Evaporation is a type of vaporisation that occurs on the surface of liquids and includes the transformation of liquid particles into gaseous particles. As a result, this process is considered to entail a change in the matter state of liquids. 

Or in other words, Evaporation is a process in which a liquid changes into vapor without attaining it’s boiling point(or temperature). Evaporation only occurs from the surface the liquid. 

The molecules of liquid are always in random motion and collide with each other and attain kinetic energy from the collisions. Now, some of them become so much energized that they leave the liquid and turn into gas and escape out. Thus, we can see the process of evaporation does not need boiling point to occur. It can occur at temperature much lower than the boiling point of the liquid. 

The surrounding gas must not be saturated by the evaporating material. Depending on how the molecules of the liquid clash with one other, they transmit energy to each other. When a molecule at the surface uses enough energy to exceed the vapour pressure, the liquid particles will typically escape and enter the surrounding air as a gas. The energy taken from the vaporised liquid during evaporation lowers the temperature of the liquid, resulting in evaporative cooling.

Now, let’s understand the factors on which the rate of evaporation depends:

Factors affecting Evaporation

  1. Temperature: The rate of evaporation is directly dependent on the temperature. More the temperature, more is the rate of evaporation. As higher temperature, causes the liquid to gain more kinetic energy.
  2. Surface Area: The rate of evaporation is directly dependent on the surface area to which the liquid is exposed. More the surface area, more is the rate of evaporation.  
  3. Wind Speed: Greater the speed of wind , greater is the rate of evaporation. This is the reason why clothes dry up easily on a windy day.
  4. Intermolecular Force of attraction: The liquids having greater intermolecular force of attraction between them evaporates much slowly in comparison to the liquids having less intermolecular force of attraction. This is because the molecules of the liquid are loosely bound and they can easily change to it’s vapor  by gaining a lesser kinetic energy .
  5. Humidity of the Air: If the air outside is already containing higher amount of water vapor then the rate of evaporation will decrease.

How Does Evaporation Cause Cooling?

Natural cooling is caused by evaporation. The basic concept is that in order for matter to change state, it must either receive or lose energy. When matter molecules shift phases from liquid to gas, they require energy to overcome their potential energy through kinetic energy. As a result, the liquid absorbs energy from its surroundings. 

When energy is transmitted, the temperature of the material rises or falls depending on whether the energy is transferred from the substance to the surroundings or vice versa. There are, however, exceptions to this rule. Although the temperature of the material rises until the boiling point is reached during evaporation, there is no visible heat transfer.

The molecules of the material constantly absorb heat energy from their environment, cooling them until they approach the boiling point, at which point they begin to break away from the liquid and transform into vapour. Because there is no temperature difference until the evaporation process has been completed, i.e. the complete liquid is changed into vapour, the energy required for this phase change is referred to as the latent heat of vaporisation, implying that this heat will not affect the temperature interpreting on a thermometer.

Applications of Evaporative Cooling

  1. When we put some spirit or petrol on the back of our hands and wave them about, the spirit soon evaporates and our hands get extremely chilly. This is due to the fact that spirit requires latent vaporisation heat to transition from a liquid to a vaporised form. The spirit pulls this dormant vaporising energy from our grasp. The hand loses heat and cools down.
  2. During hot summer days, water is frequently kept cold in a clay pot called a picture or a Matka. For cooling during hot summer days, water is generally kept in an earthen pot known as a pitcher or Matka. The earthen pot contains a huge number of extremely small pores, or holes, in its walls. Some of the water is constantly passing through these pores to the pot’s outside. This water evaporates constantly, absorbing the latent heat necessary for vaporisation from the clay pot and the remaining water. As a result, the remaining water loses heat and becomes chilly. This is an example of evaporation-induced cooling as well. It should be mentioned that owing to the large value of the latent heat of water vaporisation, all of the water on the planet does not evaporate.
  3. During the hot summer evenings, many people, especially in villages, sprinkle water on the ground in front of their homes. This water evaporates by eliminating the high latent heat of vaporisation from the ground and surrounding air. After the heat is removed from the area, it becomes chilly and comfortable. The water that evaporates from the leaves of trees cools the surrounding air in the same manner.
  4. Perspiration, often known as sweating, is the mechanism through which our bodies maintain a constant temperature. When our body temperature rises too high on a hot day or after engaging in strenuous exercise, our sweat glands release moisture or perspiration on our bodies. When perspiration evaporates, our bodies absorb the vaporising latent heat. This keeps our bodies cool.
  5. A desert cooler cools better on a hot and dry day since it operates on the evaporation principle. The greater the temperature on a hot day, the slower the rate of water evaporation, and the higher the rate of water evaporation, the faster the rate of water evaporation. As a result of the higher rate of water evaporation, a desert cooler cools better on a hot and dry day.
  6. It’s a well-known fact that humans can drink hot tea from a saucer faster than from a cup. This is explained by the phenomenon of evaporation-induced cooling. The saucer has a large surface area. Because of the huge surface area of the saucer, hot tea evaporates more quickly. This rapid evaporation cools the hot tea faster, making it easier to drink.

Sample Problems

Problem 1. We wear cotton clothes in summer. Explain this fact proper reasons.

Solution:

During summer, we perspire (sweat) a lot due to our body’s mechanism of cooling by evaporation of sweat which takes heat from the body and makes the body cool. Cotton, being a good absorber of sweat, absorbs the sweat and helps in increasing the speed of the evaporation by increasing the surface area and exposing it to the atmosphere.

Problem 2. How does water kept on earthen pot (matka) keep cool?

Solution:

When we kept water on the earthen pot (matka), the water molecules evaporate from the several tiny pores of the pot. During this process, it takes the heat from the water and thus the water cools down due to the loss of heat. This is the reason why water kept in earthen pot keep cool.

Problem 3. We often sprinkle water on the ground or on the roofs during summers. Why?

Solution:

This is because when we sprinkle water over the ground or roof top, the water from there evaporates taking the heat from the ground or roof top and in the process making it cool.  

Problem 4. We are able to sip hot tea of milk on a saucer rather than a cup. Give reasons.

Solution: 

We know that the rate of evaporation increases as the surface area of the liquid increases. Thus, when we put the hot milk or tea on a saucer, the surface of the liquid increases, thus the rate of evaporation of the milk or tea increases and due to increase in the rate of evaporation, more heat is taken from the hot milk or tea and it cools down more rapidly as compared to when it was in the cup and we are easily able to sip the hot milk or tea on saucer rather than a cup.

Problem 5. Why does our palm feel cold when we put some acetone or perfume on it?

Solution:

When we pour some acetone or petrol on our palm, the acetone or perfume starts evaporating. In this process, it takes the necessary heat required from our palm and due to loss of heat we feel cold. This is the reason why our palm feels cold when we put some acetone or perfume on it.

Problem 6. How air coolers work on the principle of Evaporation? Explain using appropriate diagram.

Solution:

This is because of the working mechanism of the air cooler. The air coolers has an exhaust fan which sucks in the dry air of the room inside. In the cooler there is a pad that has been wet by water. The water evaporates from the pad by taking the heat from the dry air thus, in the process making the air cool. This cool air released from the air coolers which circulates inside the room. Thus, air coolers work on the principle of cooling effect caused by Evaporation.

Fig 1.1: Model of Air Cooler

Problem 7. We feel cool when we apply hand sanitizers on our hand. Give reason why?

Solution:

The sanitizers that we apply over our hand contains alcohol as one of it’s primary constituent and the alcohol evaporates much faster than water. Thus, when we apply hand sanitizers over our hand, the alcohol starts evaporating by taking the heat from our own body and thus, due to the loss of heat from our body, we feel cool.

Problem 8. In summer our clothes dry up much quickly as compared to winter and monsoons. Why?

Solution:

During summer, the temperature is higher than winter and monsoons and we know that the rate of evaporation is directly dependent on the temperature, so in summer clothes dry up much faster in summer in comparison to winter and monsoon. Another important factor is humidity, in summer humidity in air is much less than that in monsoon, thus clothes dry up slowly in monsoons.  


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