<P> Azeotropes consisting of two constituents, such as the two examples above, are called binary azeotropes . Those consisting of three constituents are called ternary azeotropes . Azeotropes of more than three constituents are also known . </P> <P> Combinations of solvents that do not form an azeotrope when mixed in any proportion are said to be zeotropic . </P> <P> If two solvents can form a positive azeotrope, then distillation of any mixture of those constituents will result in the distillate being closer in composition to the azeotrope than the starting mixture . For example, if a 50 / 50 mixture of ethanol and water is distilled once, the distillate will be 80% ethanol and 20% water, which is closer to the azeotropic mixture than the original . Distilling the 80 / 20% mixture produces a distillate that is 87% ethanol and 13% water . Further repeated distillations will produce mixtures that are progressively closer to the azeotropic ratio of 95.5 / 4.5% . No numbers of distillations will ever result in a distillate that exceeds the azeotropic ratio . Likewise, when distilling a mixture of ethanol and water that is richer in ethanol than the azeotrope, the distillate (contrary to intuition) will be poorer in ethanol than the original but slightly richer than the azeotrope . This means the solution left behind will be richer in ethanol . </P> <P> If two solvents can form a negative azeotrope, then distillation of any mixture of those constituents will result in the residue being closer in composition to the azeotrope than the original mixture . For example, if a hydrochloric acid solution contains less than 20.2% hydrogen chloride, boiling the mixture will leave behind a solution that is richer in hydrogen chloride than the original . If the solution initially contains more than 20.2% hydrogen chloride, then boiling will leave behind a solution that is poorer in hydrogen chloride than the original . Boiling of any hydrochloric acid solution long enough will cause the solution left behind to approach the azeotropic ratio . </P>

Is a solution of ethanol and water heterogeneous or homogeneous
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