Evaporation

Evaporation is a process in which a liquid changes into vapours when it is below its boiling point. We can say that evaporation involves the change of state of a substance from liquid to vapour or gaseous state without boiling it. So, in evaporation, a liquid substance changes into a gas. For example, when water is heated, the molecules of water vibrate so fast that they escape into the atmosphere in the form of water vapours.

Evaporation is a natural phenomenon that uses the surrounding heat or heat of the liquid itself. Although it happens mostly with warmer liquids, it can happen even when the liquids are cold or at room temperature. In simple words, during higher temperature, the molecules of a substance have higher energy and when the energy of molecules reaches a certain level, their phase tends to change. Evaporation is linked with the energy of molecules, not with the average energy of the system. The average energy of the system may be very low and evaporation still happens if the energy of molecules is high.

Evaporation is also a surface phenomenon as it mainly occurs at the surface of the liquid. For example, in a glass of water placed in sunlight the water from the surface starts evaporating that reduces the water level slowly that shows evaporation is a surface phenomenon.

Let us understand how it happens;

The constituting particles of matter always remain in motion as they vibrate or move continuously. This movement is less in solid-state but more in the liquid state this is because in solid state the force of attraction between the particles is more than in the liquid state. So, in liquid state particles move more randomly and thus they possess kinetic energy. Similarly, the liquid particles in the glass are also moving continuously and possess kinetic energy due to their motion. Due to random motion, these particles also collide with each other and may have different kinetic energy as some may be moving faster than others. Besides this, there is also a force of attraction between the surface particles and the particles below them. Now, when the surface particles get energy from surrounding such as sunlight they can overcome the force of attraction with the below particles and thus get separated from the liquid's surface and the level of the liquid surface goes down that justifies that evaporation is a surface phenomenon.

Factors affecting evaporation

As evaporation is a natural phenomenon, there are lots of factors that affect evaporation which means may enhance or reduce its rate. Let us some common factors that affect evaporation.

Surface area:

It is the most common factor that affects evaporation and is directly proportional to the rate of evaporation which means more is the surface area, more will be the evaporation. So, a liquid with a large surface area will evaporation faster than a liquid with less surface area. For example, the same amount of liquid in a glass and spread on a floor, then liquid spread over the floor will evaporate or dry up faster than the liquid contained in a glass of water.

Increase in temperature:

Evaporation increases with an increase in temperature. When temperature increases the heat energy or kinetic energy of particles or molecules of a substance also increases. The particles with more kinetic energy can easily overcome the force of attraction with other molecules and start escaping from the surface at a higher rate when the temperature increases.

Decreases in humidity:

The moisture content or the amount of water vapours present in the air is known as humidity. In this case, the evaporation tends to decrease with an increase in humidity. This is because the water content in the humid air is high so it tends to take less water from wet surfaces or liquids' surfaces. Whereas, when the air is less humid and is dry or moisture is less, the capacity of air to take water from wet or liquids' surfaces increases, which increases the rate of evaporation.

Wind Speed:

When the wind is blowing at high speed the rate of evaporation is more. For example, wet clothes dry up faster on a windy day. The reason for this is that wet air or moisture around the wet surface is carried away with the wind faster when the speed of wind is more, so, the air around the wet surface becomes dry again which can take moisture from the wet surface more easily and quickly that enhances the rate of evaporation.

Nature of liquid:

The nature of liquid also affects the rate of evaporation. Liquids can be of different types with different properties for example alcohol has a boiling point of 78 degree centigrade, whereas, the boiling point of water is 100 degree centigrade. So, if the temperature of both the liquids is increased, alcohol will start boiling earlier than water. So, in the case of a liquid with a low boiling point rate of evaporation would be high than a liquid with a high boiling point.

How does evaporation cause cooling?

The water when evaporates takes the heat or energy from the surface that tends to reduce the surface temperature and thus causes cooling. For example, when water is sprinkled on the roof in summers, the hot surface cools down due to the cooling effect caused by the evaporation of water. Besides this, the cooling effect will be more if the rate of evaporation increases.

The principle that is responsible for producing the cooling effect is that a substance loses or gains energy when its state is changed, for example, liquid to gaseous state or gaseous to the liquid state. In case of evaporation, the water changes its state from liquid to gaseous state. The water molecules in a liquid state require energy to increase their kinetic energy and thus to overcome the force of attraction with other water molecules to escape into the atmosphere. The liquid molecules get this energy from the surroundings including the surface.

A substance not only takes energy from surrounding but it may lose energy into the surrounding. Its temperature may increase or decrease depending on whether the energy is transferred from the surroundings to the substance or from a substance to the surroundings. In the case of evaporation of water, the energy is transferred from the surrounding into the water, that lowers the temperature of the surface and surrounding and thus causes cooling.

Applications of Evaporation:

• Sweating cools down the body in summers or on a hot day through evaporation. The sweat produced from the body evaporates by absorbing heat from our body and thus lowers the body temperature.
• People generally wear cotton clothes in summer. This is because cotton is a good absorber of water that is sweat in this case. Cotton absorbs sweat and allows it to come in contact with air and thus sweat is evaporated through cotton quickly that produces the cooling effect and makes us feel less hot.
• In summers, people prefer to store water in earthen pots as it cools down water through evaporation. There are pores in the earthen pot that absorb water like cotton and also increases surface area to enhance the rate of evaporation and thus tends to keep the pot's surface cool that keeps the water cool.
• The air cooler also produces a cooling effect due to evaporation. The moisture in the air that comes from cooler when evaporates by taking heat from the surroundings produces a cooling effect. It evaporates more quickly on a hot and dry day as compared to a humid day.

Furthermore, boiling also changes liquids into vapours, but it is considered different from evaporation. Let us see the major differences between boiling and evaporation.