1. The problem asks: What is a pure substance? Provide an example. 2. A pure substance is a type of matter that has a fixed composition and distinct properties. It cannot be separated into other kinds of matter by physical means. 3. Pure substances include elements and compounds. Elements consist of only one type of atom, while compounds consist of two or more elements chemically combined in fixed ratios. 4. An example of a pure substance is water (H_2O), which is a compound made of hydrogen and oxygen atoms chemically bonded. 5. In contrast, mixtures contain two or more substances physically combined and can be separated by physical methods. 6. Therefore, pure substances are uniform and consistent throughout, unlike mixtures which vary in composition.