A mixture is a combination of at least two substances that can be separated from each other. This distinguishes them from compounds, which are made up of two or more substances that cannot be separated because they are chemically united. Mixtures have several different characteristics that distinguish them from compounds.
One of the most notable characteristics of a mixture is that it does not have a fixed composition. Water, a compound, always has 11.2 percent hydrogen and 88.8 percent oxygen by weight. Brass is a mixture because it has varying amounts of copper and zinc. One type of brass may contain as much as 45 percent zinc, while another type may contain around 10 percent zinc. Mixtures may also have changing properties depending on their composition. The boiling point of a mixture of alcohol and water varies based on how much alcohol is in the mixture.
Mixtures are classified as homogeneous or heterogeneous. Homogeneous mixtures have a uniform appearance. An example of a homogeneous mixture is air, which is made up of nitrogen, oxygen and other gases. Heterogeneous mixtures are not uniform and can consist of solids and liquids mixed together. An example of a heterogeneous mixture would be a salad dressing made of oil and vinegar.