We all do it: sweat, that is! But why do we sweat? The answer lies in the chemistry and physics of the human body. 

Materials

• Cotton balls or other absorbent material (tissues), 2
• Dropper
• Isopropyl or rubbing alcohol
• Plate
• Water

Be Safe: If you get rubbing alcohol on a cut or open sore, it will sting. Adult supervision is recommended. 

Experiment

On your left wrist, swipe a water-wet cotton ball across it and note how it feels on your skin. Once you have taken note of this, dry the water from the skin and then do the same thing on the right wrist using an alcohol-wet cotton ball.  Is there a difference in how the two solutions feel on the wrist? Now take a drop of water and put it on a plate and do the same with a drop of alcohol. Check on these drops over the course of a couple of hours and notice what happens to them. 

What’s happening?

In all cases above the liquid is evaporating. This means that the substance starts out as a liquid and turns into a gas. Which liquid droplet disappeared first? The reason that a liquid disappears* into the gas phase is because heat from the room causes the liquid to warm. This, in turn, causes the molecules in the liquid state to vibrate more and more. 

As molecules vibrate more and more, some of them will break away from other molecules at the surface of the liquid and enter the surrounding air. This is evaporation. The hotter, on average, the liquid molecules get, the faster they will “disappear” into the gas phase. What’s more, when the molecules turn to gas, the remaining liquid gets cooler! That’s why your skin feels cool when water or alcohol evaporates from it. (Alcohol evaporates faster than water, making you feel even cooler!) Think of what happens when you are physically working hard. When you start to get hot, your body tells you to sweat. The sweat takes the heat that you are producing and puts that heat into the liquid molecules. Liquid sweat molecules turn into a gas, thereby cooling you off as they take away the heat. 

Extension: Dogs cool differently than we do. Their sweat glands are on the paws of their feet so you may see them leave wet prints on the ground on a hot day.  Dogs also cool themselves when they pant and their warm blood can circulate close to the surface of their skin to help cool them as well. How do other animals cool themselves? 

References

https://www.ducksters.com/science/experiment_skin_temperature.php

https://www.akc.org/expert-advice/health/do-dogs-sweat/

To view past “ChemShorts for Kids” activities, go to:
https://chicagoacs.org/ChemShorts.

- PAUL BRANDT

* Molecules don’t really “disappear.” They enter the gas phase, however, so we no longer see them. But they are still present!