Evaporation is the process by which a liquid substance, such as water, changes into a gas or vapour state when heated. There are some examples of evaporation process:
Boiling | Evaporation |
Occurs at a specific temperature | Occurs any temperature |
Requires sufficient heat to convert liquid to gas | Occurs without adding heat, but can be sped up with heat |
Occurs throughout the liquid | Occurs at the surface of the liquid |
Results in complete conversion of liquid to gas | Results in partial conversion of liquid to gas |
Can be used for distillation and separation of mixtures | Can be used for drying and cooling |
Takes longer than evaporation | Occurs faster than boiling point |
Example: Water boils at 100°C | Example: Clothes drying on a clothesline |
The table below shows the freezing and boiling of some important solvents:
Solvent | Freezing Point(oC) | Boiling Point( oC) |
Water | 0.0 | 100 |
Acetic acid | 17.0 | 118.1 |
Benzene | 5.5 | 80.2 |
Chloroform | – 63.5 | 61.2 |
Ethanol | -114.7 | 78.4 |
The table below shows the freezing and boiling of some important solvents:
When evaporation occurs, the molecules of water with greater Kinetic energy from its surface.This is because when a liquid evaporates, it takes energy from the surrounding environment to overcome the intermolecular forces holding the liquid together. This energy is usually in the form of heat, which is absorbed by the evaporating molecules, leading to a decrease in the temperature of the remaining liquid and the surrounding environment. That’s why we feel cool when we sweat, because the sweat evaporates, taking heat away from our bodies.
Some liquids have low boiling points due to which they change from liquid to vapour easily at ordinary temperature, these liquids are called “volatile liquids”.
An example of evaporation causing cooling is the process of sweating in humans. When we sweat, the moisture on our skin evaporates into the air, taking heat with it and causing a cooling effect on our skin. This is because the heat energy required for evaporation is taken from the surrounding environment, including our skin, causing the surface to cool down. This is why sweating is an effective way for our bodies to regulate our internal temperature, particularly when we’re hot and need to cool down.
Another example is when a wet cloth is left in the sun. As the water on the cloth evaporates, it takes heat energy from the cloth, causing it to cool down. This is why wet clothes or towels can be used to provide a quick, temporary cooling effect on the body, particularly in hot weather or after exercise.