what is Chromatography

CHROMATOGRAPHY

Definition: It is a technique used for the separation and identification of dissolved constituents of a mixture by adsorbing them over an appropriate material.

Principle: This process is based on the difference in adsorption of constituents by a surface of an appropriate adsorbent material or solid medium (stationary Phase) and its affinity for the adsorbing material.

Example: Separation of coloured constituent in a mixture of ink by paper chromatography. 

Procedure: A spot of ink is placed at the centre of a base line on a strip of filter paper.

The spot is dried and the paper is hung in a glass jar with its lower end immersed in solvent.

The Solvent flows over the spot and carries the components to distances along the paper indicated by colour spots, which is characteristic of each constituent in mixture.

Chromatography finds applications in research laboratories for the identification of new compounds.

Characteristics of chemical reaction

•  In a chemical reaction, the substances known as reactants are converted into new substances called products. 
• The conversion of reactants into products in a chemical reaction is often accompanied by some features which can be observed easily.
• The easily observable features (or changes) which take place as a result of chemical reactions are known as characteristics of chemical reactions. 
• The important characteristics of chemical reactions are: 
• (i)   Evolution of a gas
• (ii)  Formation of a precipitate
• (iii) Change in colour
• (iv) Change in temperature and
• (v) Change in state
• Any one of these general characteristics can tell us whether a chemical reaction has taken place or not. 
• For example, if on mixing two substances a gas is evolved, then we can say that a chemical reaction has taken place.
• Evolution of a Gas:

Some chemical reactions are characterised by the evolution of a gas. For example, when zinc granules react with dilute sulphuric acid then, bubbles of hydrogen gas are produced. So,the chemical reaction between zinc and dilute sulphuric acid is characterised by the evolution of hydrogen gas. (Please note that we can also use dilute hydrochloric acid in place of dilute sulphuric acid in this reaction). We can perform this chemical reaction in the laboratory as follows :

• Take some zinc granules in a conical flask (or a test-tube).
• Add dilute sulphuric acid over zinc granules.
• We will see the bubbles of hydrogen gas being formed around zinc granules.
• If we touch the conical flask with our hand, we will find that it is somewhat hot. So, a change in temperature (rise in temperature) also occurs in this chemical reaction.
• Let us take another example of a chemical reaction in which a gas is evolved. When dilute hydrochloric acid is poured over sodium carbonate in a test-tube, then carbon dioxide gas is evolved. So, the chemical reaction between sodium carbonate and dilute hydrochloric acid is characterised by the evolution of carbon dioxide gas.

Formation of a Precipitate:

• A precipitate is a ‘solid product’ which separates out from the solution during a chemical reaction. A precipitate can be formed by mixing aqueous solutions (water solutions) of reactants when one of the products is insoluble in water. A precipitate can also be formed by passing a gas into an aqueous solution of a substance (likepassing carbon dioxide gas into lime water).
• Some chemical reactions are characterised by the formation of a precipitate. For example, when potassium iodide solution is added to a solution of lead nitrate, then a yellow precipitate of lead iodide is formed. Thus, the chemical reaction between potassium iodide and lead nitrate is characterised by the formation of a yellow precipitate of lead iodide. We can carry out this chemical reaction as follows :
• Take some lead nitrate solution in a test-tube.
• Add potassium iodide solution to it.
• A yellow precipitate of lead iodide is formed at once.
• A change in colour (from colourless to yellow) also takes place in this chemical reaction.
• Let us take another example of a chemical reaction in which a precipitate is formed. When dilute sulphuric acid is added to barium chloride solution taken in a test-tube, then a white precipitate of barium sulphate is formed. 
• Thus, the chemical reaction between sulphuric acid and barium chloride solution is characterised by the formation of a white precipitate of barium sulphate.