An azeotrope is a mixture of two or more liquids that has a constant boiling point because the vapor has the same composition as the liquid mixture. This means that the proportions of the components in the mixture cannot be altered or changed by simple distillation. Azeotropes can either have a higher boiling point or a lower boiling point than either of the components. They occur when a fraction of the liquids cannot be altered by distillation, which means that simple distillation cannot alter their proportions. Azeotropes are useful in separating zeotropic mixtures. Combinations of solvents that do not form an azeotrope when mixed in any proportion are said to be zeotropic. Azeotropes consisting of two constituents are called binary azeotropes, while those consisting of three constituents are called ternary azeotropes/Physical_Properties_of_Matter/Solutions_and_Mixtures/Nonideal_Solutions/Azeotropes). Azeotropes of more than three constituents are also known.