Students can use NCERT Class 9 Advanced Science Notes and Chapter 8 Mixtures and their Separation Class 9 Notes to understand complex concepts with ease.
Mixtures and their Separation Notes Class 9 Advanced Science
Class 9 Mixtures and their Separation Notes
1. Chromatography
Chromatography was first developed by the Russian botanist Mikhail Tswett in 1906 while studying plant pigments. He used this technique to separate the different coloured constituents of chlorophyll. This method was named chromatography of chlorophyll components. The name ‘chromatography’ comes from the Greek words chroma (colour) and graphein (to write). The technique is used to separate the mixtures into components, purification of compounds and also to test the purity of compounds.
Principle: The technique is based on the difference in the rates at which the components move through a stationary medium under the influence of a moving phase.
Application: Today, chromatography is widely used in chemistry, biology, and medicine to identify and separate different substances in a mixture.
Column chromatography is a modern method for the separation of mixtures into their components. The selective removal of the components may be due to adsorption or a partition process.
When a mobile phase is allowed to move over a stationary phase, the components of the mixture move by varying distances over the stationary phase because of different
adsorption tendencies. In this case, the stationary phase can be held on a cylindrical column of solid material. Hence, it is called column chromatography.
Principle: It is based on the fact that different compounds are adsorbed on an adsorbent to different degrees.
Procedure:
- Take a long glass tube with a stopcock at the bottom, called a column.
- Place a plug of cotton or glass wool at the bottom of the column.
- Fill the column with a solid material like silica gel or alumina, which acts as the stationary phase.
4. Carefully place the mixture to be separated on top of the stationary phase.
5. Add a small layer of glass wool above the mixture to keep it undisturbed.
6. Pour a suitable liquid solvent from the top of the column. This solvent is called the eluent or mobile phase.
7. Allow the solvent to flow down through the column under gravity.
8. Collect the liquid coming out of the column in different fractions.
9. The components of the mixture get separated and are collected separately.
Column chromatography is widely used in chemistry laboratories to separate and purify different components of a mixture.
2. Fractional Distillation
This method is used for the purification of liquids that boil without decomposition and contain non-volatile impurities.
Principle: Fractional distillation is a technique to separate a mixture of two miscible liquids whose boiling points differ by less than 25 °C, as shown in Fig. 8.2(a).
In this process, the mixture of liquids is heated in a distillation flask, which is fitted with a fractionating column before the condenser, as shown in Figure 8.2(b).
Fractionating Column: The fractionating column is a long tube provided with obstructions to the passage of vapours moving upwards and liquid moving downwards. It increases the cooling surface area.
Procedure:
- Take the mixture in a distillation flask.
- Heat the flask so that the mixture starts boiling.
- The vapours of the more volatile liquid (lower boiling point) rise up into the fractionating column.
- Due to obstacles in the column, some vapours cool down and condense, falling back into the flask.
- The condensed liquid gets heated again by rising vapours and revaporises.
- This repeated process makes the vapours richer in the low-boiling component.
- These vapours move upward, pass into the condenser, and cool down to form liquid.
- The liquid is then collected in a receiver.
- The process of condensation and vaporisation continues repeatedly, improving separation.
- By carefully controlling the temperature, different liquids are separated one by one based on their increasing boiling points.
Fractional distillation separates liquids with different boiling points through repeated condensation and vaporisation, ensuring efficient separation.
How is it different from simple distillation?
- In fractional distillation, a fractionating column is placed between the distillation flask and the condenser. The column provides many surfaces where repeated condensation and vaporisation occur. This allows better separation of liquids whose boiling points are close to each other.
- Simple distillation is used to separate miscible liquids that differ in boiling point by at least 25 °C, but in fractional distillation, the boiling points differ by less than 25 °C.
Application
Crude oil is a complex mixture of many hydrocarbons with different boiling points. In a refinery, the crude oil is heated, and the vapours enter a tall fractionating column. As the vapours rise in the column, they cool and condense at different levels according to their boiling points. Different fractions are collected at different heights of the column, as shown in the figure below, along with the temperature range.
The various important fractions used in our daily life or industry are the following:
- Petroleum gas (LPG, which contains butane and propane),
- Petrol (gasoline)
- Kerosene
- Diesel
- Fuel oils
- Lubricating oil and heavy oils
