A substance whether it is an element or a compound must have homogenous properties whereas a mixture is a combination of pure substances that have no definite composition. It may be either a homogenous or heterogeneous. Examples of substances are liquid water and carbon dioxide. If you take a drop of water and put under the microscope, you will see that it the molecules of water are the same in every curve. For mixtures, air and sand. Air has a lot of components in it, it has oxygen, nitrogen, argon, etc.