Fractional distillation

Fractional distillation

(i)What is it used for:

This is used to separate a mixture of two or more liquids which mix completely with one another (they are said to be miscible).

(ii)What happens:

The mixture of liquids is heated in a flask. The mixture boils. The liquid with the lowest boiling point comes out on top of the fractionating column. It is then cooled by a condenser, which changes it back into liquid, called the distillate. After most of the first liquid has distilled, the liquid with the next lowest boiling point is then distilled.

(iii)How it works:

The liquids must have different boiling points. When the mixture of liquids boil, the fractionating column reduces the percentage of high boiling liquids in the lower boiling point liquids to near 0% using the repeated process of boiling and condensing. This ensures that the resulting distillate is almost pure.

(iv)Experiment: Separating ethanol and water

Heat the mixture of ethanol and water in a round-bottomed flask. The thermometer shows a constant temperature at 78°C. At this point, ethanol starts to boil into vapour, and is distilled. After most of the ethanol has been distilled, the temperature rises to 100°C, where it remains constant again. At this point, water boils into vapour and is distilled.

Result: The mixture has been separated to give pure water and (almost)pure ethanol.

(v)Notes:

-The thermometer shows a constant temperature during distillation as pure liquid is being distilled. This temperature = the boiling point of the liquid being distilled.

-When a mixture of two liquids is being fractionally distilled, the percentage of the higher boiling point liquid remaining in the flask increases.

(vi)Apparatus needed:

Bunsen burner, round-bottomed flask, fractionating column, stoppers, thermometer, condenser, beaker