Evaporative Cooling

What is evaporation? Oh, that’s easy:-

"Evaporation occurs when any liquid changes to gas under the sun."

This is the reply you might be receiving from a fifth grade student. Ask him why, and he will go numb.

Clearly, it's not as simple as it may sound.

So pickup your reading glasses and let's delve deeper!

i.EXPERIMENT

Let's take a container (a container with maximum surface area is ideal). Fill it with water and place it in an

area where the setup may receive the maximum  sunlight. Also mark the water level on the container.

After a while notice that the water level decreased from the mark. Such a decrease in water level is caused as the water gets evaporated into gas. Measure the temperature of the surface of water.

We observe that the temperature stays way below than 100(degree) Celsius. By general rule water must attain the temperature of 100d Celsius to change its phase from liquid to gas. But here we see that water changes its phase before 100d Celsius/boiling point. This is evaporation.

Thus we can say that "Evaporation is the phenomenon where liquid changes to gas without attaining its boiling point."

Now with this understanding let's see how it takes place.

ii.EXPERIMENT

Perform the same experiment as done in (i), this time let's try it with different containers. {Note that the containers must be different regarding their part of the surface area exposed} Pour same amount of water in all of them and keep them under the sun. Measure the time taken for the water to evaporate completely.

We observe that the water in containers with more surface area exposed towards the sun takes less time to evaporate. "Thus, we can conclude that the more the surface area of water exposed to the sun the more rapidly it evaporates."

fig.1

With this we also come to know that evaporation occurs on the surface of the liquid taken. This is because the surface of the liquid- say water, contains water molecules with less intermolecular  attraction with each other compared to that of under the surface of the water. These molecules(present on the surface of liquid) absorb heat from the sun gain enough energy to overcome their bond/attractive force and escape to form gas. (See fig.1) The water molecules present beneath the water surface are attracted by molecules from all directions and cannot escape.

Why Does Evaporation Cause Cooling?

We already know that evaporation is caused due to the low inter molecular force of attraction of the liquid particles on its surface. When these molecules gain enough energy , they escape their bonds and turn into gas. But these phenomenon occurs only in that molecules which have gained enough kinetic energy aka temperature. This might be same among the molecules in a stable liquid surface, but is mostly different. For instance water sprinkled on your hand is unevenly spread and thus its molecules gain different temperature which results in different energy levels. When the kinetic energy is uneven the molecule with highest kinetic energy  escapes the bonds and turn to gas.

[Note : Kinetic energy is the measure of the motion among the molecules. The temperature of a substance is directly proportionate to its average kinetic energy. 0 Kelvin indicates 0 kinetic energy i.e. no movement among the molecules.]

After sprinkling a few drops of water in your hand bring it outside beneath the sun.  You are likely to experience a feeling of coolness in your hand. This is due to evaporation. Just before evaporation the sprinkled water  contains a specific temperature i.e. an average kinetic energy. During evaporation when one of its molecule with highest kinetic energy escapes the bonds , the average kinetic energy of the water drops down meaning its temperature is reduced. Now, when the temperature drops down it starts absorbing our body's heat. When it absorbs our body, a cooling sensation is felt by us.

As the water molecules gain heat from our body and sun in this case ,they gain kinetic energy and again the molecules  containing highest kinetic energy evaporates and this cycle goes on until all the molecules evaporate or you feel boring and walk away from the sun.

Such a phenomenon is observed and used in many cases such as earthen pots to keep water cool, sweat causing coolness, drop  of acetone (nail polish) on palm etc.

Cooling of water in earthen pot is an interesting case. Here the water present inside the pot seeps out through the pores of the pot and stays there. This is the reason why we find the walls of the pot wet all time. This water evaporates due to the external temperature or other reasons. This evaporation causes cooling which cools down the water inside it. Water again seeps out, evaporates  and the process repeats.

Conclusion: The molecules of liquid absorb energy from the surrounding  to regain its lost energy  during evaporation. This absorption of energy makes the surroundings  around it cool.

Feel free comment in the comment section below. Thanks for stopping by and of course stay tuned.