Evaporation

Ever felt curious about why a simple mud pot can cool water even on a scorching summer day? Or how an air cooler makes a warm room a breezy haven without using a compressor like an AC? The answer lies in the process of evaporation, a fascinating natural phenomenon powering several everyday activities like the ones mentioned earlier. In this lesson, we will get an insight into this simple yet crucial concept of science. 

1.0Evaporation Definition

Evaporation is defined as the slow and steady surface-level process in which a liquid, mostly water, transforms into its vapour form at a temperature below its “boiling point”. Here, the surface-level means that the process occurs on the surface layer of the liquid only, unlike boiling, which occurs throughout the liquid, and that too at a certain temperature. However, evaporation is a gradual and continuous process, happening only at the surface. Some real-life examples of evaporation include drying clothes, sweating and cooling, cooling in Earthen Pots (Matkas), summer air coolers, acetone, etc. 

2.0Evaporation Process – How It Happens

The science behind the evaporation process includes molecular movement, derived from heat energy, often from the sun or ambient heat. It is well known that all liquids are made up of mobile molecules. Some of these molecules at the surface move faster than others due to the kinetic energy, in the form of heat, gained from the sun. When these surface molecules gain enough kinetic energy, they break the attractive force that exists between them, which ultimately helps them set free in the atmosphere as vapour. 

Based on the explanation above, it is clear that this is based on energy transfer, molecular movement, and the conditions around it. When high-energy particles exit the liquid, the average energy (and temperature) of the leftover molecules reduces, which is the reason why evaporation causes cooling.

3.0Factors Affecting Evaporation

Evaporation doesn’t happen at a constant rate; there are a number of factors affecting evaporation that determine how quickly or slowly it will happen. These include:  

1. Temperature: A higher temperature results in an increase in the rate of evaporation due to the increase in molecular movement. Warm weather dries wet surfaces faster due to increased heating.
2. Surface Area: The rate of evaporation on an exposed or large surface is higher in comparison to a narrow one. This is due to the more space available for molecules to be set free. 
3. Humidity of the Surrounding Air: Humidity is the amount of water droplets present in the air, and the point at which there is no space for more water droplets to escape into the atmosphere is known as the saturation point. This is why, at the saturation point, the rate of evaporation slows down, while it increases when there is less humidity. 
4. Wind or Air Movement: Wind is an excellent source for carrying away water droplets, eventually making room for more molecules to escape from the surface. This is the reason why clothes dry faster on a windy day. 
5. Nature of the Liquid: The nature of the liquid also determines the rate of evaporation; that is, a liquid with weak intermolecular forces tends to evaporate more easily than its counterpart. For example, alcohol evaporates faster than water due to its lower boiling point and weaker molecular bonds. 

4.0Applications of Evaporation

The applications of evaporation can be found in a wide range of industries, some of which are: 

• Cooling Systems: Cooling systems, like air coolers and earthen pots, rely largely on the concept of evaporation. In earthen pots, water escapes through the tiny pores in the clay and evaporates from the outer surface, absorbing heat and ultimately cooling the water inside. On the other hand, in air coolers, warm air dries the water-soaked cooling pads, absorbing heat from the air and making the room feel cooler. 
• Salt Production: When the water in the ocean evaporates, salt is left behind and collected. This is a traditional and the most cost-effective method for salt extraction across the globe. 
• Food Processing and Preservation: Evaporation is also used to concentrate certain liquid products, such as milk or fruit juices. This removes the water content without heat, with the nutrients intact in the liquid that can be affected due to high heat. The process ultimately increases the shelf life of the product. 
• Textile and Paper Industries: In textile industries, evaporation is widely used to dry the dyed or washed fabrics, while during paper manufacturing, the process is used to evaporate water from pulp to form paper sheets. 
• Medicinal and Chemical Use: Liquid medicines, like tinctures, and hand sanitisers work on the concept of evaporation; that is, they evaporate so quickly that they leave the active ingredients in the medicine behind. In laboratories, evaporation is a useful method for separating mixtures of liquids.  

5.0Why Does Evaporation Cause Cooling?

Cooling is the most basic identification as well as the effect of the evaporation process. When molecules from a liquid escape, they also carry energy (in the form of heat) with them. The remaining liquid has fewer high-energy molecules, hence its overall temperature decreases. That is why:

• Sweat evaporates to cool the body.
• Water in earthen pots remains cool.
• Alcohol feels cold on the skin.