A substance that is added to a homogenous azeotrope to convert it to a heterogenous azeotrope that can then be readily separated by distillation. A high concentration is usually needed and is only justified when the improvement in the relative volatility offsets the extra investment, replacement, and recycle costs involved in the purchase, heating, and pumping of the entrainer. Azeotropic distillation is useful for separations where the overhead component is present in small amounts. Entrainers should ideally be cheap, readily obtainable, non-corrosive, non-toxic, unreactive, thermally stable, and easily recoverable. As well as being able to form an azeotrope, it is required to have a low latent heat of vaporization. See extractive distillation.