Key concepts
Water cycle

Have you ever wondered how sweating helps keep you cool on a hot summer day? Sweat, which is mostly water, cools us down when it evaporates. When water evaporates, it changes from a liquid to a gas. The gas carries away heat with it, helping to remove heat from your skin.

Not only can evaporation cool down your body, it can also cool down other things, such as chocolate. Heat on a hot summer day can turn your solid candy bar into a gooey, melted mess, but how well can evaporation keep your chocolate from melting?

When water is heated enough, it turns into a gas. You see this when water is boiled. The gas phase of water is called water vapor, or steam, and it is hotter than liquid water. Consequently, as water evaporates, the vapor carries heat away from the water.

•    Paper towel
•    Scissors
•    Small bowl of room-temperature water
•    Chocolate candies (two of the same kind, in a plastic or foil wrapper)
•    Ruler
•    Desk lamp with adjustable height and a 60-watt (W) lightbulb. If you don't have an adjustable-height lamp, a normal desk lamp and a pile of books can be used.
•    Timer

•    Cut the paper towel sheet into strips that are about one and a half inches wide. You will need to cut two strips of paper.
•    A 60-watt lamp can be very hot after it has been on—and even for awhile after it has been turned off. Always be careful when handling something hot.

•    Take one paper towel strip and wet it in the bowl of water. Carefully wring it out.
•    Keeping the chocolate candies in their wrappers, wrap one candy in the wet paper towel strip and the other candy in a dry paper towel strip.
•    Put the two candies side by side underneath the lamp. Be careful not to get the dry paper towel strip wet. Adjust the lamp (or place the candies on books if the lamp is not adjustable) so that the lightbulb is only one to two inches away from the candy, and evenly distanced from both candies.
•    Turn on the lamp light and leave it lit for 10 minutes.
•    After 10 minutes turn the lamp off, remove the paper towel strips from the candies and carefully unwrap them.
•    For each candy, try to flatten the wrappers around the candy, if possible, to more easily inspect the entire candy and the wrapper. How melted does the candy that was in the wet paper towel strip look compared with the one in the dry strip? Are there larger areas melted on one candy compared with the other? Is there more melted chocolate on one wrapper than the other?
•    Extra: You can repeat this procedure using thermometers to determine the starting temperature of each candy before putting them underneath the lamp, and the final temperature of each candy after it has been under the lamp for 10 minutes.
•    Extra: A gust of wind or a fan can feel very refreshing when you are sweating on a hot day. Is it refreshing for the chocolate candies as well? You can repeat this procedure to see how circulating air affects the candies by having a fan blow over them while they are under the lamp for 10 minutes—or fanning them yourself with a sturdy piece of paper. Does a fan affect how the candies melt? Does it affect their final temperatures?

Observations and results
Were you able to see the effect of evaporation on the chocolate candies? Did the candy wrapped in the wet paper towel show fewer signs of melting than the one wrapped in the dry towel?

Just as sweating cools you on a hot day, when water is evaporated from the wet paper towel strip, it keeps the chocolate candy cooler compared with the candy wrapped in a dry towel. When the lightbulb heats the water on the wet strip, the liquid evaporates and turns into a gas, called water vapor. This vapor carries heat away, removing it from the area surrounding the candy, and keeps the candy cooler than it would be if there were no water to evaporate. This will probably not completely prevent the candy from melting but, overall, it should be visibly obvious that the candy in the wet paper towel strip melted less than the candy in the dry strip.

If you tried fanning the chocolate candies while they were under the lamp, the breeze should have helped to increase the rate of evaporation. Because water vapor is less dense than dry air, it tends to stay around the object it evaporated from, making the surrounding air more and more humid and thereby difficult for additional evaporation to continue over time, because it has nowhere to go. The wind disperses the existing water vapor, making room for more water vapor to be made, which allows more evaporation—and thus cooling—to take place. Putting additional water on your skin or an object can also help increase the evaporation rate, which is why some people put wet bandanas around their necks when exercising.

Because chocolate can stain, be careful not get any melted chocolate on your clothes or furniture before discarding it and the candy wrappers.

More to explore
"Digit's B-Day Surprise," CYBERCHASE from Thirteen/WNET, Educational Broadcasting Corporation, New York
The Water Cycle: Evaporation, from USGS
"What's Sweat?" from KidsHealth
"Keep Your Candy Cool with the Power of Evaporation!" from Science Buddies

This activity brought to you in partnership with Science Buddies and Cyberchase