Key concepts
Water
Heat
Temperature
Water cycle

Introduction
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?

Background
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.

Materials
•    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

Preparation
•    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.

Procedure
•    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?

4 Comments

1. 1. biodan23 12:04 PM 10/27/11

   "Discard" the candy? I have a better idea. A very good experiment.

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2. 2. Budman100 12:20 PM 10/27/11

   The experiment naturally raises the question "Just what is it about evaporation that cools the candy (or your skin, etc.) in the first place?" That is, why does it carry heat away?

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3. 3. mhamm in reply to Budman100 12:41 PM 10/27/11

   When the water molecules absorb heat energy, they disperse and transfer their excess energy to the surrounding air. But since the dry candy wrapper doesn't evaporate when it heats up, it transfers most of its heat energy to the candy instead.

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4. 4. Trafalgar 04:55 PM 10/27/11

   I would tend to think that putting a chocolate bar in a wet paper towel would be a bad way to preserve it regardless of how well it cools it...

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