Occurrence
Zeotropy occurs when the boiling difference is large or when the chemicals in a blend are similar, e. g. two ketones or two amines. Azeotropy occurs if the boiling point difference is smaller and the chemicals contain different functional groups (e. g. chloroform and methanol).
A mixture can be both zeotropic and azeotropic because the azeotropic composition changes with temperature and pressure. A good example is the mixture of ethanol and water which is azeotropic above approximately 305 K (32 °C; 89 °F) and 12 kilopascals (1.7 psi). Below that temperature and pressure the mixture is zeotropic. The separation factor at the limiting temperature is still very small, and thus the differences between liquid and vapor composition is too small to be of use for distillation.
Read more about this topic: Zeotropic Mixture
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