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Miscible liquids are ones that can mix together – like water and ethanol. Immiscible liquids are ones that can’t – like oil and water.

Fractional distillation separates miscible liquids that have different boiling points. It is useful for separating ethanol from a mixture of ethanol and water, and for separating crude oil into different products such as petrol, diesel and kerosene.

When the mixture is heated:

• the liquids boil at their boiling point.
• their vapours rise through a column which is hot at the bottom and cooler at the top.
• the vapour condense when they reach a part of the column that is below the temperature of their boiling point.
• the liquids which condense in the column drip back into the flask.
• any gas that makes it to the top of the fractioning column enters the condenser where it is changed to a liquid.
• the liquid runs into a collecting vessel.
• distillates collected over a narrow temperature range are known as fractions.

Fractional distillation process to separate ethanol from water