Evaporation: Cooling Effects and Factors Affecting

We learned that a liquid turns to a gaseous state by increasing the temperature or decreasing the pressure during the study of change of states. Evaporation is a type of vaporisation that occurs on the surface of liquids and involves the transformation of liquid particles into a gaseous state. As a result, a change in the matter of liquids is considered involved in this process.

The energy absorbed from the vaporised liquid as it evaporates causes the temperature of the liquid to drop, resulting in evaporative cooling. Let us read the article to learn more about the evaporation process with examples.

Evaporation Definition

The process of a liquid changing into vapour or gas even below its boiling point is known as evaporation. Evaporation of liquid can occur even at room temperature, though it is faster at higher temperatures. Whenever a liquid transforms into vapour, the latent heat of vaporisation must be supplied, regardless of the temperature at which it occurs.

For example,

1. Evaporation causes the water spilt on the floor to dry up and disappear.
2. When we wipe a wet cloth across a blackboard, it quickly dries.
3. Similarly, water from wet roads and puddles near our homes and rooftops evaporates when the rain stops, forming water vapour.
4. We can see steam escaping from wet clothes when we iron (press) them. As a result, the steam produced by wet clothes while ironing is also a water evaporation process.

Process of Evaporation

In a liquid, certain particles always have greater kinetic energy than others. When a liquid is well below its boiling point, some of its particles have enough energy to overcome the forces of attraction between them and escape from the surface of the liquid as vapour (or gas). As a result, a liquid’s fast-moving particles (or molecules) continually escape from the liquid to form vapour (or gas).

Factors Affecting Evaporation

The following factors can speed up the evaporation of liquid:

1. Surface area available for evaporation: Evaporation is a surface phenomenon, meaning that only the liquid particles or molecules on the surface are converted into vapours. As a result, the higher the surface area of the liquid, the greater the pace or extent of Evaporation.

For example,

We frequently spread wet clothes to dry in the air.  Because the amount of surface area available for water evaporation has increased. Therefore, the clothes were quickly dry

2. Increase in temperature: The rate of evaporation increases by increasing the temperature of the liquid. In other words, as a liquid is heated, the rate of Evaporation raises. When the temperature of a liquid is raised by heating it, more of the particles of the liquid gain enough kinetic energy to enter the vapour stage. This accelerates the evaporation process. By heating a liquid, the rate of Evaporation can be accelerated. At the boiling point, the evaporation rate of a liquid reaches its maximum.

3. Humidity in the air: The number of water vapours is represented by the term humidity. When the water vapours in the air are low, the air appears to be ‘dry’ and has low humidity. When the amount of water vapours in the air is high, the air appears ‘damp,’ and we say that the humidity is high. As a result, the humidity of the air tells us the degree of ‘dampness’ of air.

When the humidity of air is low, the rate of Evaporation is high, and water evaporates more rapidly. When the humidity of air is high, then the Evaporation rate is low, and water evaporates slowly.

For example, in hot and humid conditions, we sweat a lot. The air surrounding us already has a high percentage of water vapours due to the conditions. This means that the sweat that escapes our skin has fewer chances to evaporate, and it sticks to our bodies. As a result, we are more likely to sweat in these conditions.

4. Increase in the wind speed: The evaporation rate of liquid increases with increasing wind speed. When the wind speed increases, the particles of water vapour move away with the wind, reducing the amount of vapour in the surrounding area. This increases the rate of evaporation of water.

For example, the washed wet clothes dry more quickly on a windy day because evaporation is faster due to the high speed of the wind.

5. Nature of the Liquid: We have discussed the external factors that influence the extent of Evaporation. Apart from this, another factor that is of great importance is the nature of the evaporating liquid. We frequently observe that alcohol evaporates faster than water. In reality, the boiling point of alcohol \(\left({350\,{\text{K}}} \right)\) is lower than water \(\left({373\,{\text{K}}} \right).\) This means that the interparticle attraction forces in alcohol are lower than those in water. As a result, the alcohol evaporates more quickly than water. Thus, we conclude that the lower the boiling point of the liquid, the greater its tendency to change into vapours or evaporate.

Difference Between Boiling and Evaporation

We discovered that both evaporation and boiling represent a change in state from liquid to gas or vapour. They are, however, distinct in some ways. Please keep in mind that evaporation is a surface phenomenon while boiling is a bulk phenomenon.

The differences between Boiling and Evaporation are as follows;

Cooling Caused by Evaporation

We’ve already established that the latent heat of vaporisation must be supplied whenever a liquid evaporates. The cooling effect of evaporation is based on the fact that when a liquid evaporates, it draws the latent heat of vaporisation from ‘anything’ it comes into contact with. This ‘anything’ is cooled by losing heat.

Following is some examples of cooling caused by evaporation:

1. If we put a little spirit (ether or petrol) on the back of our hand and wave it around, the spirit evaporates quickly, and our hand feels very cold. This spirit requires latent heat of vaporisation to change from a liquid to a vapour state. The spirit absorbs the heat of vaporisation from our hands. The hand loses heat and cools.
2. During the summer, we usually prefer cotton clothes. We perspire a lot when it is hot and humid outside. Because evaporation causes cooling, the body temperature drops. Cotton, being porous, is a good absorber of water that comes out of the pores like sweat. Synthetic clothing is less porous and does not absorb sweat as quickly. As the sweat evaporates, it absorbs some energy from the body since the clothes are contact skin. The temperature of the body drops, making us feel more comfortable. During the summer, we are less comfortable wearing nylon and terylene clothing.
3. Water is usually kept cool during hot summer days in an earthen pot (called a pitcher or Matka). The walls of the earthen pot have a large number of extremely small pores. Some of the water is constantly seeping through these pores to the outside of the pot. This water constantly evaporates, absorbing the latent heat required for vaporisation from the earthen pot and the remaining water. The remaining water loses heat and cools as a result of this process.
4. It is a well-known fact that we can drink hot tea (or milk) faster from a saucer than from a cup. This is due to the large surface area of the saucer. The evaporation of hot tea from the saucer is accelerated due to the large surface area of the saucer. This faster evaporation cools the hot tea faster, making it more convenient for us to drink.
5. In the summer, a desert cooler is very effective. Hot and dry air passes through the wet pads of wood shavings. The water in the cooler evaporates, lowering the temperature inside the cooler. As a result, the air escaping from it is extremely cool. The fan inside the desert cooler accelerates the evaporation of water.

Evaporator

An evaporator is a device used to convert a chemical substance, such as water, from its liquid form to its gaseous/vapour form. In this process, the liquid is evaporated or vaporised into a gas form of the targeted substance.

Working of Evaporator

In the evaporator, the desired product solution is fed into the evaporator and passes across a heat source. The heat is utilised to vaporise the water in the solution. The vapour is separated from the rest of the solution and condensed, while the concentrated solution is either pumped into a second evaporator or discarded.

Types of Evaporators

a. Natural/ forced circulation evaporators
b. Falling film evaporators
c. Rising film evaporators
d. Climbing and falling-film plate evaporators
e. Multiple-effect evaporators
f. Agitated thin film evaporators

Summary

The evaporation process occurs when a liquid transforms into vapour or gas below boiling point. Evaporation of liquid can occur even at room temperature, though it is faster at higher temperatures. Evaporation causes the water that was spilt on the floor to dry up and disappear. Temperature, surface area, wind speed, humidity and nature of the liquids are the factors on which evaporation depends. We have also learned in this article that evaporation always results in cooling.

FAQ’s

Q.1. What is evaporation explained?
Ans: The process of a liquid changing into vapour or gas even below its boiling point is known as evaporation. Evaporation of liquid can occur even at room temperature, though it is faster at higher temperatures. Whenever a liquid transforms into vapour, the latent heat of vaporisation must be supplied, regardless of the temperature at which it occurs.

Q.2. What is evaporation in simple words?
Ans: The changing of water into water vapour is called evaporation.

Q.3. What is evaporation in the matter?
Ans: Every molecule in a liquid is in motion. Some move faster than others, though. As the molecules on the surface of a liquid absorb heat, they begin to move faster. This provides them with the energy needed to break the bonds that bind them to other water molecules. When the molecules move quickly enough, they can “escape.” They leave the surface of the liquid as gas molecules.

Q.4. Why is evaporation so important?
Ans: Evaporation is an important part of the water cycle because it allows large amounts of water to move from bodies of water on Earth into the atmosphere. Water vapour in the atmosphere can then condense into clouds, which move around the globe and produce rain.

Q.5. What are the types of Evaporators?
Ans: The types of evaporators are as follows:
a. Natural/ forced circulation evaporators
b. Falling film evaporators
c. Rising film evaporators
d. Climbing and falling-film plate evaporators
e. Multiple-effect evaporators
f. Agitated thin film evaporators

We hope this article on Evaporation has helped you. If you have any queries, drop a comment below, and we will get back to you.