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Evaporation is defined as the process where a liquid changes into a gas or vapour below its boiling point. Evaporation is one of the three main steps in the water cycle along with condensation and precipitation. Evaporation, especially from the ocean, accounts for almost 90% of moisture in the atmosphere. Evaporation is affected by a number of factors such as temperature, surface area, humidity, and wind speed.
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Key Terms: Evaporation, Water Cycle, Vaporization, Boiling, Condensation, Temperature, Atmosphere, Humidity
What is Evaporation?
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Evaporation is the process in which a liquid turns into a gaseous state below the temperature at which it boils. It is a slow process that occurs at all temperatures.
- Evaporation usually happens on the surface of a liquid. Evaporation depends on the surface area on which liquid is spread out, the type of liquid, its density, etc.
- It also depends on factors like temperature, wind speed, and humidity as well.
- Evaporation has a cooling effect. It is because particles absorb energy from their surroundings. For example, if you place acetone on your palm, you will experience a cooling sensation on your palm. That is because heat is absorbed from the surroundings as a result of evaporation.
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Factors Affecting Evaporation
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The main factors that affect evaporation are described in detail below:
Surface Area
- Evaporation increases with the increase in surface area.
- With an increase in surface area, the amount of liquid exposed to the air becomes larger.
- In other words, with a wider surface area, more molecules can escape.
- Example: When we hang clothes to dry, we spread them out so they dry quicker.
Temperature
- Evaporation increases with the increase in temperature.
- This happens because higher temperatures lead to more molecules getting kinetic energy to change into vapour.
- When the temperature rises, water molecules move faster. This makes them escape quicker.
- Example: Clothes get dried faster on a sunny hot day rather than on a cool shady day.
Read More: Matter in Our Surroundings Important Questions
Humidity
- Evaporation increases with a decrease in humidity.
- Humidity is defined as the amount of water vapour present in the surroundings at a certain time and certain temperature.
- If the humidity level is low, then the rate of evaporation will be more due to low content of moisture in the air or vice-versa.
- Example: Usually on humid days, it takes clothes a bit longer to dry in comparison to a sunny day with low humidity.
Speed of Wind
- Evaporation increases with the increase in the speed of the wind.
- Particles of vapor move away when the speed of wind increases. This leads to decreasing water vapor in the atmosphere.
- Example: A hand dryer drying hands and a hairdryer drying hair.
Nature of Liquid
- The lower the boiling point of a liquid, the more likely it is to evaporate.
- Example: Alcohol evaporates faster than water. The boiling point of alcohol is lower than that of water.
How Does Evaporation Cause Cooling?
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Energy extracted by liquid from surroundings decreases the temperature of the surroundings. This process is called evaporative cooling. For example, when perfume is sprayed, our body feels slightly cooler.
- Evaporation causes a natural cooling effect. For a matter to change its state, it must either gain or lose energy. In the case of transformation from liquid to gas, molecules must gain kinetic energy to overcome their potential energy. This energy is taken by liquid from the surroundings.
- When the transfer of energy occurs, there is an increase or decrease in the temperature of the substance depending on whether the energy is being transferred from the substance to the surroundings or vice versa.
- There is an observable change in temperature during evaporation until the boiling point is reached but there is no observable heat transfer during phase change.
- Molecules absorb heat continually from their surroundings until the substance reaches its boiling point. Molecules then break free from liquid and turn into vapour.
- As there is no change in temperature till evaporation is complete, the amount of energy required for this phase change is called Latent Heat of Vaporization. Latent means hidden implying that this heat will not change the temperature reading in a thermometer.
Applications of Evaporative Cooling
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Here are a few day-to-day examples of evaporative cooling:
- Perspiration is similar to evaporation. Sweating happens to cool our bodies. Water from the body vaporizes, absorbing heat from the body and cooling it down.
- Cotton clothes are worn in summer because they make us feel cooler. Cotton is an excellent absorber of water. It allows more sweat to be in contact with the atmosphere and hence increases the rate of evaporation.
- Earthen pots keep water cool. Like pores of cotton clothes, pores of earthen pots expose more surface area, increasing the rate of evaporation.
- Air coolers work on the principle of evaporative cooling. They are very effective on hot dry days when the temperature is high and humidity is low. Hence the rate of evaporation increases. The water in the cooler takes energy from the air and evaporates making the air cooler.
Difference Between Boiling and Evaporation
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Both evaporation and boiling involve liquid changing to gas. However, they are different in some aspects. Boiling is a bulk phenomenon while evaporation is a surface phenomenon. Key differences between boiling and evaporation have been tabulated below:
| Boiling | Evaporation |
|---|---|
| The liquid is heated to a temperature called the boiling point. | The evaporation of a liquid happens on its own. |
| Boiling happens at a specific temperature. | Evaporation happens at all temperatures. |
| Boiling happens on the surface as well as from below the surface of the liquid. | Evaporation happens only at the surface of the liquid. |
| Physical conditions do not affect the boiling point of a liquid. | The rate of evaporation of a liquid is affected by all physical conditions. |
| No cooling happens during boiling. | Evaporation always has a cooling effect. |
Read More: States of Matter MCQ
Condensation
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Condensation is a process opposite to evaporation. During condensation, a gas loses energy and turns into a liquid. Example: Water droplets appear on a tumbler when ice-cold water is stored in it. It happens because when the water vapour formed from evaporation comes into contact with the cold tumbler, it loses energy and turns into water droplets.
Things to Remember
- Evaporation is the process in which liquid turns into gas. It usually happens on the surface of a liquid.
- The main factors affecting evaporation are surface area, temperature, humidity, speed of the wind, and nature of the liquid.
- Evaporation has a cooling effect. It is because particles absorb energy from their surroundings. The energy extracted by the liquid from the surroundings when vaporization happens will decrease the temperature of the surroundings. The process is called evaporative cooling.
- Both evaporation and boiling involve liquid changing to gas. Boiling is a bulk phenomenon while evaporation is a surface phenomenon.
- Condensation is the opposite process of evaporation. During condensation, a gas loses energy and turns into a liquid.
Sample Questions
Ques. What is evaporation? Give examples. (3 Marks)
Ans. Evaporation occurs when a liquid turns into vapour. It is a crucial part of the water cycle that creates rain. It is a slow process that occurs at all temperatures. Evaporation usually happens on the surface of a liquid. For example, Rain puddles disappear quickly on an especially hot day or wet clothes dry when left out in the sun.
Ques. Why are cotton clothes often worn in the summer? (3 Marks)
Ans. Cotton clothes are worn during summer as cotton absorbs more sweat from the body than any other fabric, thus, exposing the sweat to the atmosphere for easy and fast evaporation. People sweat more during summers and cotton fabric being an excellent absorber of water, absorbs sweat and helps the body to cool down.
Ques. Explain what is meant by the latent heat of vaporization. (3 Marks)
Ans. The latent heat of vapourisation is defined as a physical property of a substance. When a liquid changes to vapour without a change in temperature, the energy absorbed in the process is called the latent heat of vaporization. The word latent means hidden which means that this heat will not change the temperature reading in a thermometer.
Ques. How does evaporation cause cooling? Explain with examples. (3 Marks)
Ans. During the process of evaporation, the particles of the liquid absorb energy from the surroundings in order to overcome the inter-particle forces of attraction and change into gaseous form. This absorption of heat from the surroundings makes the surroundings cool. For instance, when sweat is exposed to air, we feel cool.
Ques. What is meant by condensation? Give examples. (3 Marks)
Ans. Condensation is the opposite process of evaporation. During condensation, a gas loses energy and turns into a liquid. Example: Water droplets appear on a tumbler when ice-cold water is stored in it. It happens because when the water vapour formed from evaporation comes into contact with the cold tumbler, it loses energy and turns into water droplets.
Ques. Is boiling the same as evaporation? If not, what are the differences? (4 Marks)
Ans. No, boiling is not the same as evaporation. Even though both evaporation and boiling involve liquid changing to gas, there are some differences in both phenomena.
- When a liquid is heated to its boiling point, it is called boiling whereas evaporation of a liquid happens on its own.
- Boiling happens only at a specific temperature, whereas evaporation happens at all temperatures.
- Boiling can happen on the surface as well as from below the surface of the liquid. Evaporation, on the other hand, happens only from the surface of the liquid.
- Boiling Point is not affected by physical conditions whereas the rate of evaporation of a liquid is affected by all physical conditions.
Ques. What factors affect evaporation? (5 Marks)
Ans. The main factors that affect evaporation are:
- Surface Area: An increase in the surface area will lead to a higher rate of evaporation as the amount of liquid exposed to the air becomes larger.
- Temperature: The higher the temperature would be, the higher would be the rate of evaporation as higher temperatures lead to more molecules getting kinetic energy to change into vapor.
- Humidity: A decrease in humidity will lead to a high rate of evaporation as the moisture content would be low in the air and would allow more moisture to be observed.
- Speed of Wind: The faster the wind would be, the more the rate of evaporation would be. Particles of vapour move away when the speed of the wind increases. This leads to decreasing water vapour in the atmosphere.
- Nature of Liquid: A liquid with a low boiling point will evaporate faster or vice-versa.
Ques. Why are we able to sip tea easily from a saucer rather than a cup? (2 Marks)
Ans. We are able to sip tea faster from a saucer than a cup as the surface area of the saucer is larger and evaporation occurs faster in this case in comparison to a cup, thus, making the tea cool faster and making it easy for us to sip tea from it.
Ques. Why do clothes dry faster on a hot sunny day? (2 Marks)
Ans. Clothes dry faster on a hot sunny day as the temperature is more as well as the humidity level is also low, thus, leading to an increase in the rate of evaporation, and allowing the clothes to dry faster.
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