Question

Question 12.18 Give a brief description of the principles of the following techniques taking an example in each case.

(a) Crystallisation

(b) Distillation

(c) Chromatography

Class XI Organic Chemistry : Some Basic Principles and Techniques Page 363

Answer 1

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CRYSTALLISATION

Crystallisation is the common way of purifying organic solids. It is based on the difference in the solubilities of the compound and the impurities in a suitable solvent. The impure compound is dissolved in a solvent in which it is sparingly soluble at room temperature but appreciably soluble st higher temperature. The solution is concentrated to get a nearly saturated solution. On cooling the solution, pure compound crystallises out and is removed by filtration.


It the compound is highly soluble in one solvent and very little in another solvent, crystallisation can be satisfactorily carried out in a mixture of these solvents. Coloured impurities are removedby adsorbing them over activated charcoal. The crystals are separated by filtering unnder reduced pressure using Buchner funnel. The crystals are finally dried over sulphuric acid or calcium chloride in vaccum desiccator.

Repeated crystallisation becomes necessary for the purification of compounds containing impurities of comparable solubilities.


DISTILLATION 

Distillation is one of the effective method for separation of liquid type mixture and it work on the principle of difference in boiling point. When two liquids of high boiling point difference are mixed they can be separated out by using this method.

Ex:- Ether presents as impurity in alcohols because of weak inter molecular forces ether are volatile in nature where as alcohols have inter molecular hydrogen bonding and this mixture can be separated by simple heating above 30 C where ether is evaporated first and the vapour's are condensed where alcohols is left over in the flask.

Fractional distillation:- It is the one of the effective method to separate liquid in liquid solutions of similar boiling point which are having less boiling point difference.

Ex:- Fractional distillation of petroleum. Petroleum is a crude oil is a mixture of different hydrocarbons and this can be separated out different columns of fractional distillations. In the fractional distillation plant the hydro- carbon having less boiling point is separated at the top of the container and the hydro-carbon with high boiling point is separated as the bottom of the container.

CHROMATOGRAPHY

Chromatography is one of the effective separation for liquids and pigments and it work under principle of Adsorption.

Adsorption it is a surface phenomenon where one of the substance get accumulated on the surface of another surface.

Adsorption in separation of a mixture it is taken on a stationary face kept in a mobile phase (solvent) different substance present in a mixture will have different rate of absorption one of the effective method.

Column chromatography:- Column chromatography involves separation of a mixture over a column of adsorbent ( stationary phase) packed in a glass tube. The column is fitted with a stopcock as it lower end. The mixture adsorbed is placed on the top of the adsorbent column packed in a glass tube. A suitable eluant, either a single solvent or a mixture of solvents is allowed to flow down the column slowly. Depending upon the degree to which the compounds are adsorbed, complete separation takes place. The most readily adsorbed substances are retained near the top and others come down accordingly to various distances in the column.

Thin layer chromatography is another type of adsorption chromatography, which involves separation of the substances of a mixture over a thin layer of an adsorbent coated on glass plate. A thin layer (about 0.2mm thick) of an adsorbent is spread over a glass plate of suitable size. The plate is known as thin layer chromatography plate or chromaplate. The solution of the mixture to be separated is applied as a small spot about 2 cm above the end of the thin layer chromatography plate.

PARTITION CHROMATOGRAPHY

Partition chromatography is based on continuous differential partitioning of components of a mixture between stationary and mobile phases. Paper chromatography is an example. In paper chromatography a special quality paper known as chromatography paper is used. This paper contains water trapped in it, which acts as the stationary phase.

A strip of chromatography paper spotted at the base with the solution of the mixture in suspended in a suitable solvent or a mixture of solvents. This solvent acts as the mobile phase. The solvent rises up the paper by capillary action and flows over the spot. 

The paper selectively retains different components according to their differing partition in the two phases. The paper strip so developed is known as chromatogram. The spots of the separated coloured compounds are visible at the different heights from the position of initial spot on the chromatogram. 


Hope it helps you !

Answer 2

(a) Crystallisation :- crystals are the purest form of a substance having definite geometrical shapes . the process by which an impure compound is converted into its crystal is known as Crystallisation.
it is based on the difference in solubilities of organic compound and the impurities in a suitable solvent .
The main steps of this process are :
(i) choice of the solvent
(ii) preparation of solution
(iii) filtration of the solution
(iv) Crystallisation and separation of crystal
(v) Drying of crystal
(vi) removal of color


(b) Distillation :- The method is used to separate volatile liquids from non-volatile impurities and the liquid having large difference in their boiling points {e.g., chloroform ,pb = 334K and aniline ,pb = 457K} are separated by this method .

(c) Chromatography :- This technique is used to separate and purify a mixture of a compound . it is also used to test the percentage purify of compounds . it is of the following types :
(i) column Chromatography
(ii) Thin layer chromatography
(iii) partition chromatography or paper Chromatography.