Simple Distillation of a Toluene-Cyclohexane Mixture

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There are four basic distillation techniques for separating and purifying the components of a liquid mixture: simple distillation, fractional distillation, vacuum distillation and steam distillation. The chosen distillation method and extent of purification will depend on the nature of the mixture, specifically the difference in the boiling points of miscible liquids. In distillation, the mixture is heated, vaporizing a substance. Under boiling reflux, the vapor phase becomes richer in the lower boiling component as vapors continue to condense and move up the distillation head, purifying the mixture.

Simple distillation is most effective when applied to mixtures where the liquid components differ in their boiling points by at least 50°C. As the first component distills, the temperature is measured from vapor it condensing on the bulb of a thermometer positioned just below the sidearm of the distilling head. With simple distillation, the rate of change of temperature is a slow as the composition of the boiling liquid changes as distillation progress. Thus, the range over which liquid is purified is not sharp. The temperature of the distilling liquid is observed to plateau and then drop before rising again, as the process of distilling the second component begins. Here, the temperature will plateau near the boiling point of the second lowest boiling liquid in the mixture, thus distilling the second fraction. The process continues for each subsequent component, leaving the highest boiling liquid in the distilling flask. By carefully controlling the rate of distillation, it is possible to affect reasonably good separation. If distillation is rapid, then separation of the components of the mixture is poorer than if the mixture is distilled slowly.