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Distillation: Meaning, Principle & Types - Raoult’s law and Dalton’s law

Have you heard of distilled water filling up in batteries? Is it the same drinking water or different? 

Do you like traveling?

Travel is an important aspect of our lives. All of it, be it day-to-day travel or trips between cities or countries, is made possible by automobiles, aircraft, etc., and all of them require fuel like petrol, diesel, etc.

These fuels are manufactured from crude oil by a process called fractional distillation. Crude oil is heated and, after putting it in the distillation column, it is separated into different liquids and gases. 

By the fractional distillation of crude oil, various products like butane, propane, petrol, kerosene, diesel, fuel oil are produced.

Table of content:


Distillation is a simple physical separation method based on the difference in boiling temperatures of the components of a solution. The solution is heated at a selective boiling temperature that forces one of the components to enter the gaseous phase and is thus separated from the solution. This vapour-phased component is further condensed and collected separately. 

By this process, non-volatile impurities and also liquids having a sufficient difference in boiling points are separated from volatile liquids.

Principle of Distillation

On heating, a liquid starts converting into its vapour phase at its boiling point. It involves heating a liquid to generate vapour and then cooling it to return it to its liquid state.

Dependence of Raoult’s law and Dalton’s law

On heating, at a particular temperature, when the vapour pressure of a liquid becomes equal to the external (atmospheric) pressure, the liquid starts boiling, and this temperature is known as the boiling point of a liquid.

The boiling point of a liquid varies with the external (atmospheric) pressure. The boiling point is different for different liquids and is characteristic of the chemical nature of the substance.

The distillation of a mixture of liquids depends on Raoult’s law and Dalton's law. 

According to Dalton’s law of partial pressures, the total pressure exerted by a mixture of gases is equal to the sum of the partial pressures of all the constituent gases.

As per Raoult’s law, the partial pressure of a liquid component in an ideal liquid mixture equals the product of the vapour pressure of the pure component and its mole fraction in the liquid mixture. 

Simple distillation

  • Simple distillation entails heating a liquid mixture to the boiling point and then condensing the vapours instantly.
  • It is used for liquids having a considerable difference in boiling points. (minimum ).
  • Raoult's law governs the purity of liquid. 

Fractional distillation

Fractional distillation is widely used to separate mixtures of liquids having similar or close boiling points. Several steps of vaporization-condensation are involved (which take place in a fractionating column). The fractional distillation equipment is quite complex and consists of the following components:

  • Round bottom flask.
  • Heat source
  • Fractionating column
  • Collection flask to collect condensed vapours
  • Thermometer
  • Condenser.

Steam distillation

  • This method is used to separate compounds that are steam volatile but water-immiscible.
  • When the sum of the vapour pressure of liquid mixtures becomes equal to the atmospheric mixture, the liquid starts boiling.
  • Aniline is separated from the aniline-water mixture by steam distillation.
  • Essential oils are obtained from aromatic plants using steam distillation.

Vacuum distillation

  • This approach is used to separate liquid combinations that have extremely high boiling temperatures or disintegrate at their boiling point.
  • Pressure is decreased in a closed container to reduce the external pressure and allow liquids to boil at temperatures below their boiling points.
  • Glycerol is extracted from spent-lye in the soap industry via vacuum distillation.

Air-Sensitive Vacuum distillation

The vacuum distillation technique is employed for chemicals that are sensitive to air and react quickly with it, but the vacuum must be replaced with an inert gas after the procedure is completed. This type of procedure is known as air-sensitive vacuum distillation.

Short path distillation

At high temperatures, short-path distillation is used to purify a small amount of a volatile chemical. This is done at lower pressures, and the distillate is usually recovered after a short journey (thus the name "short path"). As a result, the distillate travels a shorter distance, resulting in less loss along the apparatus's walls.

Zone distillation

The technique of partially melting a substance and then condensing the resulting vapours to obtain a pure distillate is known as zone distillation. This is done in a long container with the help of a zone heater.

Cryogenic distillation

  • Cryogenic distillation is a method for separating components that requires extremely low boiling temperatures.
  • The distillation equipment includes a refrigeration system as well as a condenser.
  • This method is often used to separate propylene and propane mixtures.

Application of distillation

  • Many water purification procedures rely heavily on distillation. This approach is used by several desalination plants to get drinking water from seawater.
  • Distilled water is used in lead-acid batteries and low-volume humidifiers, among other things.
  • Distillation is used to make several perfumes and food flavourings from herbs and plants.
  • Oil stabilization is a sort of distillation that lowers the vapour pressure of crude oil, allowing it to be safely stored and transported.
  • On a larger scale, distillation is used to purify liquid products derived through chemical synthesis.
  • The method of cryogenic distillation can be used to separate air into nitrogen, oxygen, and argon.
  • This process is used to purify a variety of fermented items, including alcoholic beverages.

Practice Problems

Q 1. Which two processes are involved in distillation? 

a. Freezing and condensing
b. Distilling and condensing
c. Distilling
d. Only condensation

Answer: B)

Distillation involves the two processes of distilling and condensing, which are carried out in a distillation column.

Q 2.  Distillation is discovered by which scientist? 

a. Guldberg and Waage
b. Charles Chamberland
c. Clarence Birdseye
d. Jabie ibn hayyan

Answer: A)

Distillation was discovered by Jabie ibn hayyan.

Q 3.  For which process double distillation is used? 

a. Coffee for commercial uses
b. Wine for commercial uses
c. Distilled water for laboratory purposes
d. Distilled water for commercial use.

Answer: C)

Flaks with heating elements are used in double distillation. In addition to providing for reflux distillation, it is employed to carry out reactions under stirring conditions. It's used to make distilled water for lab experiments.

Q 4.  Which factors affect distillation? 

a. Relative volatility
b. Surface area
c. Solubility
d. All of these

Answer: B)

There are various factors that affect the distillation process, which are solubility, relative volatility, and activity coefficient, and it also depends on the surface area.

Q 5.  MaCabe-Thiele plot is explained for which process? 

a. Distillation
b. Crystallization
c. Molecular separation
d. Evaporation

Answer: A)

The distillation process is used to explain the distillation process. This plot is used for binary mixtures to calculate the number of theoretical trays required for the distillation process.

Frequently asked questions

Q 1. Why are plastic beads used in fractional distillation?

Answer: Plastic beads or steel wool are used to increase the surface area, which improves the separation between liquids being distilled.

Q 2. Give an example for which the triple distillation method is used? 

Answer: Triple distillation method is used for the manufacturing of smooth whiskies.

Q 3. Give one application of steam distillation?

Answer: To obtain essential oils from aromatic flowers

Q 4. How does the boiling point of a liquid vary with pressure?

Answer: The boiling point of a liquid is a temperature at which the vapour pressure of a liquid becomes equal to atmospheric pressure, so when atmospheric pressure changes, the boiling point of a liquid also changes.

Q 5. Which distillation techniques are used for the molecules having very little difference in boiling points?

Answer: Fractional distillation is used to separate those liquids which have fewer differences in boiling points.

Related Topics 

Adsorption Chromatography Sublimation
Crystallization Chromatography
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