 A chemist needs to understand the rate of reaction in order to control the experiment and make it safe and also to maximize the amount of product with the least cost. For example, if you eat a piece of candy, your body digests that piece of candy, which is mostly sucrose and breaks it down into carbon dioxide and water vapor. Let's do that in the lab at a faster rate than happens in your body. We're going to heat the potassium chlorate. You can see that the potassium chlorate is beginning to melt. After it melts, it will begin to decompose. You can see gas bubbles coming off, so we're going to drop our gummy bear into the solution. We define rate as the change over time. And so when we look at a chemical reaction, we often measure the change in concentration over time or the amount of product over time.