Crude oil is useful in a variety of ways. It is so powerful in industry because it contains hydrocarbons of varying molecular masses, forms and lengths (such as paraffins, aromatics, naphthenes (or cycloalkanes), alkenes, alkynes, and dienes. In crude oil, hydrocarbons are the most common form of molecules, which are molecules of varying lengths and complexity made of hydrogen and carbon atoms, and a small number of oxygen atoms. The differences in the structure of these molecules account for their varying physical and chemical properties. Despite of this, there are also sulfur and nitrogen atoms in crude oil, which means harmful sulfide and nitride will be formed when burning if they are not removed from the final product. So refinery is very important, adn that is what we are going to talk about next. Before we get the things we need and use them, we need to seperate them from crude oil. This is achieved by boiling the crude oil, so different material can be vaporized at their boiling points, which are different. All we need to do is to collect vaporized material on time.  Differnt from chemical reaction,this is totally a physical change, which means this process won't change the the physical property of the materials once they are cooled down.