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Separation by Fractional Distillation

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  • Last Updated : 21 Jul, 2021
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Distillation is a separation technique that is used to extract a mixture solid in a liquid. It is basically the process of heating the liquid to form vapours, and then condensing the vapours in order to get back the liquid. The liquid that is obtained by performing the condensation of vapour is called the distillate. Miscible liquids mix together to form a solution, for instance, a mixture of ethanol and water. Immiscible liquids don’t mix well together. For example, oil and water. The two liquids that are miscible with each other in all proportions can be termed as a binary mixture of liquid. This technique is used for the segregation of components of a mixture of two miscible liquids with sufficient difference in boiling points (B.P), that can be boiled without undergoing decomposition. The volatile liquid evaporates on heating which can be recovered by cooling its vapours by the process of condensation.

For instance, distillation is used for the separation of non-volatile substances from volatile ones or removing salt from water to prepare drinking water and purification of drugs manufactured using the chemical process.

Distillation is preferable where both solid and liquid both have to be extracted from the solution. Also known as simple distillation, it is based on the differences noticed in the volatilities and corresponding vapour pressures observed in the components of a mixture. 

Fractional Distillation

Fractional Distillation is used to separate miscible liquids that are volatile in nature. The boiling points of these liquids are close enough. The fractionating column apparatus is used to simulate the separation. Also known as rectification, since the vapour is condensed partially and returned as a liquid. It is basically a process where the vaporization of a liquid mixture gives rise to a mixture of constituents, followed by the extraction of the required component in its pure form.

However, this technique can be only used to separate miscible liquids, which form PURE azeotropic mixtures.

Fractionating Column

The apparatus used in this process is similar to that for simple distillation except for a fractionating column which is fitted in between the distillation flask and the condenser. A simple fractionating column is a tube packed with glass beads. The beads provide a surface for the vapours to cool and condense repeatedly.


On distillation of the liquid mixture, the partial condensation of the vapour occurs in a fractionating column.

In the column, advancing vapour from the still comes in touch with the condensing vapour that returns back to the still. This causes the enrichment of vapour with the more volatile component.

By constant condensation and heating of the vapour, an equilibrium is achieved between liquid and vapour. This results in the extraction of a more volatile component from the mixture.

Applications of Fractional Distillation

Fractional distillation can be used to separate the mixtures like:

  • Acetone and water
  • Chloroform and benzene
  • Separation of gases of air

Separating Funnel

The technique used for the separation of two immiscible liquids is done using an apparatus called a separating funnel. It is a special type of funnel which has a stop-cock in its stem which permits or stops the flow of a liquid from it. The separation of two immiscible liquids by a separating funnel depends on the differences in the densities of the composing liquids. The heavier liquid settles at the bottom and the lighter liquid remains as an upper layer.

Applications of Separating Funnel

  • Separating a mixture of water and kerosene
  • Separating a mixture of petrol and water
  • Groundnut oil or mustard oil from water
  • Mercury, carbon disulphide, chloroform, benzene from water

Principal of Separating Funnel

A separating funnel is used for the separation of components of a mixture between two immiscible liquid phases. One phase is the aqueous phase and the other phase is an organic solvent. This separation is based on the differences in the densities of the liquids. The liquid having more density forms the lower layer and the liquid having less density forms the upper layer.

How Separating Funnel Works

When we pour the mixture in a separating funnel Then liquid with the lower density floats on top and When the tap is opened, the liquid with a higher density starts to flow through the separating funnel into the container. The tap is then closed just before the liquid with the lower density starts to flow through. The liquid with the lower density remaining in the separating funnel can then be drained into a different container to separate the two immiscible liquids

Sample Problems

Question 1. How can you separate two liquids that have less than 25 K difference of boiling points?


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

Question 2. How will you separate a mixture of oil and water?


To separate a mixture of oil and water, we need a separating funnel as both are immiscible liquids. Pour the mixture in separating funnel and let the funnel stand undisturbed for sometime. So that separate layer of oil and water are formed. Open the stopcock of the separating funnel and pour out the lower layer of water carefully.

Question 3. Can we separate alcohol dissolved in water by using a separating funnel? If yes, then describe the procedure. Of not, explain.


No, being miscible they cannot be separated by using a separating funnel.

Question 4. Difference between Simple Distillation and Fractional Distillation?


Simple Distillation Fractional Distillation
Simple distillation is used to separate mixture of miscible liquids with sufficiently large difference in their boiling points. Fractional distillation is used when the difference in boiling point is quite less.
It consists of simple apparatus with two flasks and a condenser. It consists of more complex apparatus with fractionating column.
Example: To purify seawater Example: Crude oil refining

Question 5. Difference between Separating Funnel and Fractional Distillation?


Separating Funnel Fractional Distillation
Separating funnel is used to separate two immiscible liquids. Fractional distillation is used to separate two miscible liquids.
Fractional distillation separates two miscible liquids based on their boiling point.  Separating funnel separates two liquids of different density.
Example: Water and alcohol. Example: Oil and water.

Question 6. Differentiate between miscible and immiscible liquids. Give an example of each.


Miscible Liquids Immiscible Liquids
Miscible liquids are liquid substances that can mix in all proportions to form a homogeneous solution Immiscible liquids are liquids that are incapable of mixing an attaining homogeneity
Mix with each other in all possible proportions Immiscible liquids do not mix with each other in all proportions
Gives a clear solution after mixing Gives a cloudy solution after mixing
Liquids with the same polarity can be miscible Liquids with different polarity are immiscible
Example: Water and ethanol are miscible Example: Oil and water are immiscible

Question 7. How separating funnel is useful in extraction of iron from its ore?


The principle behind separating funnel is that immiscible liquids separate out in layers depending on their densities. In the extraction of iron ore, the lighter stag is removed from the top by this method to leave the molten iron at the bottom in a furnace.

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