Question

How will you separate a mixture of benzene and toluene?

Answer 1

Separation of a mixture of Benzene $\left({\mathrm{C}}_6{\mathrm{H}}_6\right)$ and Toluene $\left({\mathrm{C}}_7{\mathrm{H}}_9\right)$:

The suitable method for separating a mixture containing benzene and toluene is fractional distillation.

Fractional distillation: It is a technique for separation of a mixture of two miscible liquids which have comparable boiling points i.e.; the difference between their boiling points is less than 25 K or not very large difference in the boiling points.

Boiling point of benzene is $80.1°\mathrm{C}$ and boiling point of toluene is $110.1°\mathrm{C}$.

Principle involved in the separation of mixture:

• The component with lower boiling point evaporates first and leaves the mixture due to the difference in the boiling points.
• The vapours of other liquid cool and condense back to the flask due to the increased surface area for cooling available through the glass beads of the fractional distillation column.
• Miscible liquids: A mixture of two or more liquids which mix with each other to form a homogeneous solution. Example: mixture of benzene and water

Set-up for the fractional distillation:

• The apparatus used for fractional distillation is called fractionating column.
• Mixture is poured into a round bottom flask held by a clamp and placed on the burner.
• The mouth of the round bottom flask is fitted with a fractionating column and thermometer is placed in it.
• This is glass tube with glass beads that pose hindrance to the vapours of the liquid coming from the flask.
• A water condenser called Liebig's condenser is fitted into the side of the column. Its inner tube is connected to the flask for the liquid to pass through and cool water is circulated through its outer tube to condense the vapours coming from the round bottom flask.
• An empty flask is placed at the other end of the condenser to collect the liquid.

Process of separation is as follows:

• Mixture of benzene and toluene is heated in the round bottom flask on water bath.
• As the mixture begins to heat, vapours of the benzene will be formed as it has low boiling point.
• Along with benzene, some of the vapours of toluene will also be formed as they have less difference in their boiling points.
• Both the vapours will try to rise up through the fractionating column but the vapours of toluene will condense while passing through the fractionating column and condense back to the round bottom flask and vapours of benzene will rise to the top of the column and will pass through the condenser and received in the flask.
• Benzene is collected in the beaker first and toluene is left behind and the mixture is separated.