A zeotropic mixture is a chemical mixture that never has the same vapor phase and liquid phase composition at the vapor–liquid equilibrium state. The dew point and bubble point curves in the graph shown do not touch each other over the entire composition range, with the exception of the pure components (the curve end points). Azeotropic describes mixtures in which the dew point and bubble point curves are touching each other at at least one point – indicating the same composition in the vapour and liquid phase.