What's Happening? Have you ever seen a chemical reaction? Even if you didn't know it, you see it hundreds of times. You saw one when you made your slime. A chemical reaction changes 2 or more substances into a new substance. Have you ever baked a cake? You start off with some eggs, flour, and sugar. Then you bake it and you have fluffy cake! That's an example of a chemical reaction. Chemical reactions can be either make or take in heat. Reactions that produce heat are called exothermic. Burning is an exothermic reaction. The other type of reaction absorbs or takes in heat from its surroundings and is called endothermic. Can you guess which type of reaction produces the slime you made? How did the slime feel when you made it? It should have felt cold. As you knead the slime and handle it, it absorbs heat from your hands. Do you know what kind of reaction it is yet? You guessed it! It's endothermic. We already talked about polymers. Remember that a polymer is a large molecule made up of many repeating subunits. You might be wondering if polymers have chemical formulas like water - H2O. They don't have definite chemical formulas because the chains that are formed all have different lengths.