Separation by Distillation

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. 

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

Process of Distillation

• A homogeneous mixture containing a solid and liquid are heated together in a closed distillation flask.
• The liquid escapes forming vapours since they are volatile.
• These vapours are cooled by passing them through the condenser.
• Pure liquid is obtained on condensation in the vessel taken.
• The non-volatile solid is left as a residue.

Equipment needed for Distillation

The process of distillation basically uses the following apparatus: 

• Still: A vaporising chamber is used to place the material which is to be distilled. It is heated in order to provide vaporisation of the volatile constituents. It is also attached to a condenser and a trap is fixed in between.
• Condenser: It is used to condense the vapour. It is kept cold by circulating water/air through the jacket. Condensers are available as single surface condensers, multi-surface condensers.
• Receiver: It is used to collect the distillate.

Principle of Distillation 

• Distillation is carried out at the solvent’s boiling points. Boiling takes place when the vapour pressure is equivalent to the atmospheric pressure.
• Separation of the components of the mixture takes place better in case of the higher relative volatility of a liquid.
• On supplying heat to the liquid, the vapour boils and then condensation takes place.

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 vaporisation 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.

Principle:

• 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:

• Water and kerosene
• Petroleum and water

Applications of distillation: 

• Purification of organic solvents-absolute alcohol (100%).
• Separation of non-volatile substances from volatile ones.
• Purification of drugs manufactured using the chemical process.
• Refining of petroleum products- Petroleum ether 60,80.
• Recovery of solvents- synthesis.
• Remove salt from water to prepare drinking water.
• Methanol or ethanol from water
• The concentration of oils and beverages in the food industry.

Sample Questions

Question 1. Differentiate between simple distillation and fractional distillation?

Answer:

Simple Distillation	Fractional Distillation
Vapour is directly passed through the condenser.	Vapour must pass through a fractionating column in which partial condensation of vapour occurs.
Condensate is collected directly onto the receiver.	Condensation occurs in the fractionating column, which results in a part of the condensing vapour returning to the still.

Question 2. Is it possible to purify a mixture completely on the basis of distillation?

Answer:

Any component in the mixture cannot have zero partial pressure. As a result of this, a completely pure sample of a component from a mixture cannot be obtained. However, samples of high purity can be obtained if any of the components of the mixture has a partial pressure whose value is close to zero.

Question 3. Experimentally show that seawater contains salt. 

Solution:

It can be proved with the help of distillation that seawater contains salt. When the seawater is boiled, the water molecules contained within it evaporate from the surface in the form of steam. They are then condensed at the cooler part of the beaker. As a result of this, all the water evaporation takes place, leaving only salt behind as residue. 

Question 4. How do the obstructions in a fractionating column help in fractional distillation?

Answer:

In fractional distillation, separation of the components occur based on their boiling point. These obstructions help in arrangement of the substances in the order such that the substances having a lower boiling point will be on the top whereas the substances with higher boiling point will be at the bottom.

Question 5. What happens on increasing pressure in a distillation column?

Answer:

• Distillation efficiency will decrease.
• Relative volatility will decrease.
• Reflux ratio will be higher.
• The equilibrium curve shrinks to a larger extent towards diagonal line.

Question 6. What are the constraints under which the process of simple distillation be performed?

Answer:

Simple distillation can be carried out for: 

• A pure or “almost pure” substance. Any substances with lesser than 10% impurities is considered to be pure.
• A mixture where the boiling points of the impurities are very high,
• A Mixture with a large difference between the boiling points of the constituents.

Question 7. Explain the nature of the residue at the bottom of the flask.

Answer:

The process of distillation is performed for the mixture of liquids that have a sufficient difference in their corresponding boiling points. Therefore, the residue at the bottom of the flash should be non-volatile in nature.