Distillation

Simple distillation

Simple distillation is used to separate a solvent from a solution. It is useful for producing water from salt solution.

Simple distillation works because the dissolved solute has a much higher boiling point than the solvent. When the solution is heated, solvent vapour evaporates from the solution. The gas moves away and is cooled and condensed. The remaining solution becomes more concentrated in solute as the amount of solvent in it decreases.

Salty water is heated.

1. Salty water is heated

Simple distillation can also be used to separate two liquids with a large difference in boiling points, such as ethanol and water. The mixture must be carefully heated and the temperature controlled, so that only the liquid with the lower boiling point evaporates. This can then be collected using a condenser.

In the first stage of ethanol distillation, water and ethanol solution is heated.

1. Water and ethanol solution is heated

Fractional distillation

Fractional distillation is used to separate different liquids from a mixture of multiple liquids. Fractional distillation of a mixture of several liquids works because the liquids have different boiling points.

Fractional distillation can be used to separate different fractions from crude oil. This uses a fractionating column that is hotter at the bottom, and cooler at the top. The different liquids are collected from different parts of the column.

In the laboratory a glass fractionating column is used. This is connected to a condenser. It is often filled with glass beads to increase the surface area available for condensing.

When the mixture is heated:

  • vapours rise through a fractionating column which is hot at the bottom, and cold at the top
  • vapours condense when they reach a part of the column that is below the temperature of their boiling point
  • the liquid flows back down the column
  • if the temperature at the top of the column is above the boiling point of a substance, its vapour can pass into the condenser and be collected
  • as the mixture is heated, eventually all the substances (fractions) will be collected

Which type of distillation is best to separate a mixture of two liquids?

A mixture of two liquids can be separated by either distillation method. However, simple distillation requires very careful control of the temperature. Fractional distillation is better if the two liquids have very similar boiling points. If simple distillation is used instead, the distillate is likely to be a mixture of the two liquids as both will evaporate when heated.

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