Have you ever seen a metallic sheep??

What happened to the temperature in the last experiment? And how did the steel wool look at the end? You should have seen that the temperature went up by several degrees, and that the steel wool looked rusty after the experiment was done. What happened?

First of all, what exactly is steel wool? Is it the fleece from metallic sheep? Of course not. Steel wool is just very thin wires made of iron that is sold in clumps that looks something like sheep's wool. It is often used instead of sandpaper for removing paint or smoothing surfaces.

So, where did the heat come from? When iron comes in contact with the oxygen (O₂) and water (H₂O) in air, a chemical reaction called oxidation occurs. Rust is actually a mixture of several compounds of iron, oxygen, and hydrogen, with names like iron oxide and iron hydroxide. Do you remember learning about thermal energy in "Where Does Heat Come From?"? We said that a wood fire feels hot because the thermal energy of the wood was higher than that of the ashes and other material left over from the fire. The same thing applies to the iron. The thermal energy of rust is lower than that of the original iron, and you feel the energy lost as heat.

But why didn't the steel wool rust before you put it into the jar? Normally, steel wool is coated with oil or some other material that protects it from air. The vinegar you rinsed the steel wool with took the coating away, and the bare iron was exposed. So, the vinegar wasn't actually involved in the reaction, but it did allow the reaction to happen.

If you go to the drugstore, you can actually find a product that uses the oxidation of iron to generate heat! ThermaCare heat wraps contain iron (and some other chemicals) that react with air and produce warmth that helps relieve the pain of arthritis and sore muscles.

Chemical Heat

Here's an experiment that shows how a common chemical reaction creates heat. See if you can figure out what the reaction is.

You'll need a good-sized jar with a lid, a small thermometer that will fit in the jar, some steel wool (plain steel wool, not the scrubbing pads that contain soap), and vinegar.

First, take a handful of steel wool, and place it in the jar. Put the thermometer in, making sure that the bulb (temperature-sensing part) in surrounded by steel wool. Record the temperature. Wait a few minutes, then check again. Boring, huh? The temperature shouldn't change very much.

Now for some chemistry! Take the thermometer and steel wool out of the jar. Soak the steel wool in vinegar for a few minutes, then remove it and squeeze the vinegar out (wear a rubber glove when doing this). Quickly put the steel wool and thermometer back in the jar the way it was before. Watch the thermometer for several minutes. Does anything happen? Check back a few times over the next hour or so. What happens to the temperature? Does the steel wool look the same? What do you think happened to it? Here's a hint: don't get distracted by the vinegar - it's not involved in the reaction.

Mechanical Hands

In the last post, you were promised some experiments involving mechanical energy that you can do with your own hands. Wait a minute, you might be saying, my hands aren't mechanical (I assume that none of you are robots!). Here's another case where scientists sometimes use words a little differently than most people do. Mechanical energy is simply the energy of moving objects. Your hands are objects, and when you move them, you are using mechanical energy!

The first experiment is very simple. Place your palms together and rub quickly. What do you feel? Why do your hands get warm? Your muscles create mechanical energy which turns into heat because of friction, which makes it hard to rub two things together. OK, that wasn't much of an experiment. Let's try another one...

Find a simple wire clothes hanger like the ones that come from the dry cleaner. If it is covered in paper, remove the paper. Grab the straight bottom of the hanger with one hand in each corner. Bend the straight part of the wire about eight to ten times. It may be hard at first, but will get easier. Touch the bent part of the hanger. It's hot! Again, your muscles supplied the mechanical energy, and the friction among the atoms in the metal created heat.

Can you think of any other examples of how mechanical energy can be turned into heat?