Subtitles section Play video Print subtitles We also talked about Chemistry not only being the study of matter, but the changes that matter undergoes. We're going to say that this is broken down into two types of category. We're going to have our physical changes and our chemical changes. Now, in the first blank, we have their changes in the form of the substance, but not its chemical composition. Basically, we're going to say that these are physical changes. What do I mean by not a change in its chemical composition? For example, I have ice—which is solid water and then I just leave it out and it melts. It goes from solid water to liquid water. At the end of the reaction, it's still water. It began as water. It's still water at the end. I didn't change its chemical makeup. Once I change its chemical makeup or chemical composition, it should stop being water. It should change into something entirely different. Now the second type of change creates new substances with different properties and different chemical compositions, so we say that this is chemical. For example, I have that liquid water and I super heat it. That little triangle means heat, so I super heat the water, that ice, heat it so much so, then now it becomes H2 gas and O2 gas. It becomes hydrogen gas and oxygen gas. It started off as water, but now it's no longer water. It's hydrogen gas and oxygen gas which are very different from water. As a result, because they're so different from water, they're going to have different properties, different melting points, different boiling points, different densities. That's what we mean by a chemical change.