To say that evaporation is a cooling process means that when evaporation occurs,

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Updated April 10, 2018

By Donald Miller

Liquid evaporating from a surface has a cooling effect. And different liquids have this effect to different degrees. For example, rubbing alcohol has more of an evaporative cooling effect than water. Alcohol evaporates comparatively more quickly than water, so scientists class it as a "volatile" liquid. But regardless of the liquid, they all follow the same principle of evaporative cooling. In its liquid state, the substance—whether water or alcohol—has a certain heat content, which is central to the process. Also critical to this are two of the three basic phases of matter: liquid and vapor. (The solid phase is, of course, the third.)

TL;DR

Evaporation causes cooling because the process requires heat energy. The energy is taken away by the molecules when they convert from liquid into gas, and this causes cooling on the original surface.

When a liquid evaporates, its molecules convert from the liquid phase to the vapor phase and escape from the surface. Heat drives this process. In order for the molecule to leave the liquid surface and escape as a vapor, it must take heat energy with it. The heat that it takes with it comes from the surface from which it evaporated. Since the molecule is taking heat with it as it’s leaving, this has a cooling effect on the surface left behind. This makes it easy to understand evaporative cooling.

An example of evaporated cooling is that of human perspiration. We have pores in our skin from which liquid water internal to our skin is escaping and converting to water vapor in the air. As this happens, it cools down our skin surface. This happens almost constantly to one degree or another. When we are exposed to an environment that is hotter than what is comfortable for us, the degree of perspiration or evaporation increases. And it follows that the cooling effect increases. The more water molecules that are escaping from the liquid phase from our skin surface and from our pores, the more cooling effect there is. Again, this is because the liquid molecules, as they escape and become vapor, require heat and they take it with them.

Plants do something similar, through a process called transpiration. Plant roots "drink" water from the soil and transport it up through the stem to the leaves. Plant leaves have structures called stomata. These are essentially pores that you can think of as comparable to the pores in our skin.

One of the main functions of this process in plants is to transport water needed by plant tissues in other parts of the plant besides the roots. But this evaporative cooling effect also benefits the plant. This keeps the plant—which might very well be exposed to direct, intense sunlight—from overheating. And this also explains why, on a hot day, if we enter a forested area, we feel considerably cooler. Part of that is due to the shade, but part is also due to the evaporative cooling effect from the trees through this process of transpiration.

Wind increases the effect of evaporative cooling, and this is a familiar concept. Anyone who's ever been swimming and has come out of the water into a calm environment, versus one that's windy, can attest to it feeling colder in the wind. The wind increases the evaporation rate of the liquid water from our skin surface and accelerates the amount that's being converted to vapor.

Incidentally, this process also causes so-called wind chill. Even in colder conditions, when we're outside and our skin is exposed to the elements, a certain amount of perspiration occurs. When it's windy, more evaporative cooling takes place from exposed skin. This explains the basics behind the so-called wind-chill factor.

Evaporation Cause Cooling because the particles (atoms and molecules) present at the surface of the liquid surface absorb energy from its surroundings and transform it into vapor, which then causes the cooling effect. Evaporation is a type of vaporization 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 be responsible for the change in the matter state of liquids. Thus, when the heat of evaporation is positive cooling is then caused by the evaporation.

Definition of Evaporation

The process through which a liquid changes into vapour before achieving its boiling point is called Evaporation. Evaporation only occurs from the surface the liquid.

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

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 vapor. Because there is no temperature difference until the evaporation process has been completed, i.e. the complete liquid is changed into vapor, the energy required for this phase change is referred to as the latent heat of vaporization, 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 the spirit requires latent vaporization heat to transition from a liquid to a vaporized form. The spirit pulls this dormant vaporizing 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 outside. This water evaporates constantly, absorbing the latent heat necessary for vaporization 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.

FAQs on Evaporation Cause Cooling

Question 1: We wear cotton clothes in summer. Explain this fact with 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.

Question 2: How does water keep in the 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.

Question 3: We often sprinkle water on the ground or on the roofs during summer. 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.  

Question 4: We are able to sip hot tea or 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.

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

Answer:

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.

Question 6: How do air coolers work on the principle of Evaporation? 

Answer:

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.

Question 7: We feel cool when we apply hand sanitizers to our hands. Give a reason Why?

Answer:

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.

Question 8: In summer our clothes dry up much quicker compared to winter and monsoons. Why?

Answer:

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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