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What is fractional distillation?

Fractional distillation is the process used to separate two miscible liquids based upon differences in boiling point.


Ethanol has a boiling point of 172 ºF or 78 °C. Water has a boiling point of 212 ºF or 100 °C. Both water and ethanol are infinitely miscible in one another. This means the resulting mixture looks like a single liquid no matter how much ethanol or water is added.

Water and ethanol can be separated by the process of fractional distillation.*


How does fractional distillation work?

When a mixture of soluble liquids is heated all liquids are evaporated. The liquid with the lowest boiling point however, forms the greatest percentage of vapor.

A fractionating column is needed to further separate the mixture.

As the vapor moves up the fractionating column it becomes progressively richer with the component that has the lowest boiling point.

This is due to the vapor mixture continually condensing and evaporating as it moves up the column.

A thermometer is used to measure temperature of the gaseous fractions before they condense.

The liquid with the lowest boiling point will be the first 'fraction' or portion of the distillate.

* In reality an azeotropic mixture is produced when the mixture reaches 95% ethanol and 5% water. This prevents further purification by fractional distillation. An azeotrope is a mixture of two miscible liquids that has a constant boiling point and composition.



1. Fractional distillation is used to separate

i) what type of mixtures?
ii) on the basis of what physical property?

2. What additional piece of glassware is needed for fractional distillation compared to distillation?

3. What is the maximum purity of ethanol that obtained by the fractional distillation of ethanol and water?

4. Why can't 100% ethanol be obtained by the fractional distillation of a mixture of ethanol and water?

5. Research: How is fractional distillation used in the petroleum industry?