When you spray perfume on your body, your body feels slightly cooler. The same goes for acetone and water. This is an effect of evaporation or the change of matter from its liquid state to its vapor state. The only difference is the rate at which the coolness is felt. In the case of acetone, the part of your body that is in contact with the liquid will cool the fastest. This happens because the evaporation rate of acetone is higher than that of water or perfume.
Evaporation causes cooling naturally. The underlying principle behind this is, in order to change its state, the matter must either gain or lose energy. In the case of change of phase from liquid to gas, molecules of matter require energy to overcome their potential energy by their kinetic energy. So, the liquid takes this energy from its surroundings.
Generally, when energy transfer occurs, it results in an increase or decrease in temperature of the substance, depending on whether the energy is being transferred from the substance to the surroundings or vice versa. However, there are exceptions to this rule.
Although there is an increase in temperature of the substance till the boiling point is attained during evaporation, phase change results in no observable heat transfer.
The molecules of the substance absorb heat energy continuously from the surroundings and thus cool the surroundings till they reach the boiling point, after which they start to break free from the liquid and turn into vapor. Since there is no change in temperature till the evaporation process is complete i.e. the entire liquid gets converted into vapor, the amount of energy required for this phase change is called the latent heat of vaporization, where the word ‘latent’ means ‘hidden’, meaning this heat will not change the temperature reading on a thermometer.
We sweat in order to cool our bodies. Perspiration is essentially evaporation. Water from our body evaporates, taking energy from our body in the process and thus results in the lowering of our body temperature.
During summer, we wear cotton clothes. Cotton, being a good absorber of water allows more sweat to be in contact with the atmosphere, consequently helping in more evaporation. It is for this reason that we feel cooler when we wear cotton clothes.
Water is stored in earthen pots so as to make it cool. The pores of the earthen pot, just like the pores of cotton cloth provide a larger surface area for more evaporation.
An air cooler is more effective on hot, dry days. The basic principle behind the working of an air cooler is evaporative cooling. As on a hot, dry day, the temperature is high and humidity is low, the evaporation rate is higher. The water takes energy from the air and gets converted to vapor. This makes the air cooler.
This is the opposite of evaporation: a gas turns into a liquid and heat energy is lost in this process. An example of this can be the water droplets on the surface of a glass full of ice-cold water. When water vapor formed from evaporation comes in contact with the cold tumbler, it loses energy and gets converted to water.