Adsorption:- The phenomenon of attracting and retaining the molecules of a substance on the surface of a material (solid or liquid), resulting into a higher concentration of the molecules on the surface than in the bulk is called adsorption. The substance thus adsorbed is called the adsorbate and the material on which it is adsorbed is called the adsorbent.

The reverse process i.e. removal of the adsorbate substance from the surface of adsorbent is called Desorption. The adsorption of gases on the surface of metals is specifically called Occlusion.

Difference between Absorption and Adsorption:-

Adsorption	Absorption
It is the surface phenomenon i.e. it occurs only on the surface of adsorbent.	It is a bulk phenomenon i.e. it occurs throughout the body of material.
In this phenomenon, the concentration on the surface of adsorbent is different form that in bulk.	In this phenomenon the concentration is same throughout the material.
Its rate is high in the beginning and then decreases till equilibrium is attained.	Its rate remains same throughout the process.

  If there is some doubt whether adsorption takes place or the absorption taken place or both adsorption and absorption process occurs it is termed as Sorption. e.g. dyes get adsorbed as well as absorbed in the cotton fibers i.e. sorption takes place.

Types of Adsorption:-

An experimental study of the adsorption shows that there are two main types of adsorption:

1. Physical adsorption or Vander Waal’s adsorption or Physicosorption
2. Chemical absorption or Longmuir or chemisorptions

Difference between physical and chemical adsorption:-

Physical adsorption	Chemical adsorption
The force operating in these cases are weak Vander Waal’s forces	The force operating in these cases are similar to chemical bonds.
The heats of adsorption are low about 20-40 KJ mole-1.	The heat of adsorption are high about 80-240 mole-1.
No. compound formation takes place.	Surface compounds are formed.
The process is reversible i.e. desorption can occur (on increasing temperature).	The process is irreversible i.e. desorption is not possible.
It does not require any activation energy.	It requires activation energy.
This type of adsorption decreases with increase in temperature.	This type of adsorption first increase and than decreases with temperature.
It is not specific  i.e. all the gases are adsorbed on solid to some extant.	It is specific in nature i.e. only the specified gas adsorbed on the solids.
It forms multimolecular layer	It forms unimolecular layer.

Factors Affecting the adsorption of gases by solids:-

The following factors affect the adsorption of gases on solids:-

1. Nature of surface area of adsorbent:- the amount of the adsorbed on the solid adsorbent depends upon its nature e.g. charcoal and silica is the excellent adsorbents. Greater the site available for adsorption hence the extent of adsorption is also more.
2. Nature of gas being adsorbed:- The adsorbent of gases upon the surface of some material depends upon the nature of gas. It is commonly observed that easier is the liquefaction of gases greater is its adsorption on the material.
3. Activation of solid adsorbent:- A solid adsorbent can be made activated for adsorption. It may be done by making the surface of solid rough, by subdividing the adsorbent into pieces or by treating chemically e.g. charcoal is boiled in conc. sulphuric acid to give activated charcoal.
4. Pressure:- pressure plays a crucial role in the adsorption of gases on solids. Generally the increase in pressure of the gas increases the amount of gas adsorbed “A study of the variation on the amount of gas adsorbed on the adsorbent with pressure of gas at constant temperature is called Adsorption Isotherms.

The adsorption isotherm property was studied by Freundlich as discussed bellow:\

Freundlich Adsorption isotherm:- A study of the adsorbed of gases on the surface of solid adsorbent shows that he amount of gas adsorbed per unit mass of adsorbent (x/m) first increases on increase in pressure, them the increase becomes less and finally no increase is observed. It is to be mentioned here that temperature remains constant. The curve shown in the graph can be represented be the equation. The curve shown in the graph can be represented by the equation.

This relationship was put foreword by Freundlich and hence is known as Freundlich adsorption isotherm. The validity of this equation is tested by expanding it by logarithm. Taking log of (1)

(⬄ y = mx + c)

Which is the equation of straight line with slope 1/m and intercept log k. experimentally also it is found to have same nature for solution. The concentration  (c) used for pressure (p).

5.Temperature:- “A study of the variation in the amount of gas adsorbed on the adsorbent with temperature of gas at constant pressure is called Adsorption Isobars.”

The  adsorption isobar of physical adsorption and chemical adsorption comes out to be different. The physical adsorption shows a decrease in  throughout with rise in temperature, while the chemical adsorption shown an initial increase with temperature and then expected decrease. The decrease in extent of physical adsorbent exists in equilibrium i.e. SOLID + GAS    SOLID/GAS + HEAT.

Thus on increase in temp. the equilibrium shifts in backward direction and adsorption decreases. The initial increase in chemical adsorption is because of the fact that the heat supplied acts as activation energy requited in chemisorptions and the particles of adsorbate which were not chemically adsorbed can get adsorbed.