How will you cool something ?

No, not by keeping it in a fridge. Think about how can we cool something below room temperature? Almost all cooling appliances (AC, refrigerator) work on the same simple principle – evaporation. Put some nail polish remover or alcohol or ammonia on your hand. As we watch it evaporate, our hand feels cold. Because volatile substances take away heat from our hand and evaporate, consequently cooling the air around them. That’s what happens in these devices. Hot air is made to flow across a pipe filled with coolant. Air loses heat to coolant, coolant evaporates, air is returned to the room and coolant is made to condense again through a pump.

Evaporation is also the reason why we feel cold when we are wet. A thin layer of water is present over the body. Large surface area favours evaporation so water takes heat from the body and evaporates. It is interesting to note that the coolant (alcohol, nailpolish remover etc) are also at room temperature and yet they are able to absorb heat. This is because when a substance is absorbing heat for evaporation, its temperature doesn’t increase. The energy is used by the molecules to overcome the intermolecular forces of attraction and become a gas. This amount of heat which a substance can absorb without an increase in its temperature, is called the Latent heat of that substance.

Evaporation is by itself an interesting phenomena. It doesn’t just happen at the boiling point. In fact all fluids are evaporating at all temperatures. Here is a glimpse at evaporation.