Chemical energy is the energy that's stored in the bonds and atoms that make up molecules. If different chemicals are allowed to react, these bonds can rearrange themselves. Sometimes they need extra energy to do this, so they soak up some from their surroundings. These reactions are called 'endothermic'. But sometimes they'll release some extra energy into the environment, heating it up. These are called 'exothermic' reactions.
Probably the most common dramatic example of an exothermic reaction is when something burns. When something like wood burns, it is actually a chemical reaction between the molecules in the wood and the oxygen in the air. When these things react, a lot of chemical energy gets changed into heat energy and light energy, which is why you get a hot, bright flame.
Another common example involving chemical energy is a battery. Chemicals in different chambers of the battery are allowed to interact, reducing chemical energy and producing electrical energy. (Same thing happens for lemon batteries and potato batteries.) Similarly, the instant heat packs that you can find in first-aid kits use a chemical reaction that converts chemical energy into heat energy.
Any reaction that is exothermic if it starts with an excess of material on one side of the reaction formula and proceeds from those reactants to the products on the other side will be endothermic if it proceeds the opposite way. That will happen if at the start the excess is on the other side of the reaction formula.
Although most dramatic reactions are exothermic, not all are. Wikipedia lists the following dramatic endothermic reaction: "The mixing of barium hydroxide
and ammonium thiocyanate causes a powerful endothermic reaction that causes the products to become so cold that the moisture from the air forms a layer of frost on the outer surface of the beaker."
An exothermic example that you may be able to try at home involves cement. If you get some dry powdered cement and mix it with water, you will probably notice that it also gets quite warm. This is because of a reaction between the water and the lime in the cement, which release some of their chemical energy as heat energy. Like I said, there's a lot of examples out there... This is just a few to get you started. Good luck with your project!
-Tamara (w. small changes by Mike W.)
(republished on 07/25/06)