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Methods of Separation - Fractional Distillation Method

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Fractional distillation:

Fractional distillation is often used to separate mixtures of liquids that have similar boiling points. 

It is the same as simple distillation except for the fact that it requires a fractionating column additionally.

Application:

  1. Purification of air

  2. Purification and separation of many organic compounds.

experiment

The experiment of Separation of two miscible liquids by fractional distillation

Aim: To separate a mixture of two or more miscible liquids for which the difference in boiling points is less than 25 K

Requirements: Fractionating column, distillation flask, a mixture of acetone and water, thermometer, condenser, etc.

Principle: A simple fractionating column is a tube packed with glass beads. The beads provide a surface for the vapours to cool and condense repeatedly. Basically, fractional distillation involves repeated distillations and condensations and hence many cycles of this ensure a purer substance.

Procedure:

  • Take the mixture in a distillation flask.

  • Fit it with a thermometer and arrange the apparatus as shown

  • Heat the mixture slowly keeping a close watch at the thermometer.

Observation: Pure liquid was obtained.

Inference/ Result: Pure component could be extracted from a mixture of components even with a little difference in their boiling points.

experiment

Experiment of obtaining a different type of gases from the air by fractional distillation.

Aim: Air is a homogeneous mixture and can be separated into its components by fractional distillation.

Principle: This liquid air is allowed to warm-up slowly in a fractional distillation column, where gases get separated at different heights depending upon their boiling points.

Steps:

  • Under high pressure, the air is compressed and then cooled by reducing the temperature. This liquefies the air.
  • The liquid air is then passed through the fractional distillation column.
  • Inside the column, warmer air is present at the bottom and cool air at the top.
  • Each gas starts to separate at different temperatures because of their different boiling points.
  • Considering the table above, oxygen has more boiling point so it will flow out of the flask through a tube in the bottom while nitrogen will move out from the top.

  • Hence by this method, we can easily separate various components of air.

Example

Arrange the gases present in air in increasing order of their boiling points.
Increasing order of boiling points gases present in air:
Nitrogen < Argon < Oxygen
77K  87K  90K

Example

Which gas forms the liquid first as the air is cooled?

Oxygen is well known to have a higher boiling point than other gases. If you cool it, oxygen forms a liquid. Thus, it is oxygen that forms liquid first as the air is cooled.

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