Thursday, January 31, 2008

IIT JEE Revision Ch. 11. SURFACE CHEMISTRY Core Points

JEE syllabus

Surface chemistry:
Elementary concepts of adsorption (excluding adsorption isotherms);
Colloids: types, methods of preparation and general properties;
Elementary ideas of emulsions, surfactants and micelles (only definitions and examples).

The term adsorption implies the presence of excess concentration of any particular component in one of the three phases of matter (known as adsorbate) at the surface of liquid or solid phase (known as adsorbent) as compared to that present in the bulk of the material.

On the basis of the forces of attraction between adsorbent and adsorbate, two types of adsorption, namely, physisorption (i.e. physical adsorption) and chemisorption, may be identified.

Colloids or sols are the substances whose sizes lie in between the solutes present in a true solution (e.g., salt, sugar) and the solutes present in suspension (e.g., sand).

The diameters of colloidal particles may range from 1 to 100 nm. The particles in colloidal state do not settle down on standing, are not visible and they can pass through a filter paper. However, they do not pass through a perchment paper or animal membrane.

Emulsion is a liquid dispersed in a liquid.

Any substance which can decrease the surface tension of water to a large extent is known as surfactant. Examples of soap and detergents. Such substances have larger concentrations at the surface of water as compared to the bulk of the solution.

Surfactants in solution are often association colloids, that is, they tend to form aggregates of colloidal dimensions, which exist in equilibrium with the molecules or ions from which they are formed. Such aggregates are termed micelles.

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