Suppose we have a substance either in the liquid or solid form. Let us consider a molecule in the bulk of the substance. It experiences forces of attraction uniformly from all sides by neighbouring molecules. There is no net force acting on it. If we consider a molecule near its surface we find that such a molecule is not bonded to other molecules from all sides but has certain unutilised valencies, as there are no molecules above the surface. In order to satisfy the unutilised valencies, the molecules at the surface attract and retain other molecular species when the substance is brought in contact with them. The substance, which has different concentration at the surface than in the bulk of the adjoining phases, is said to the adsorbed. The phenomenon of existence of a substance in different concentrations at the surface of another substance than in the bulk phases is called adsorption.

The substance whose surface adsorbs molecular species of another substance is called adsorbent and the substance, which gets adsorbed, is called adsorbate .

If the concentration of a substance at the surface of another substance is more than in the bulk of the adjoining phases, it is called positive adsorption . If the concentration of a substance at the surface of another substance is less than in the bulk of the adjoining phases, it is called negative adsorption . This type of adsorption takes place only in case of solutions. Adsorption can take place at a solid - gas surfaces; for example charcoal adsorbs colouring matter in solutions and is used to decolourise brown cane juice and colloidal aluminium hydroxide is used in the purification of water because of its adsorbing properties. Hence, powdered charcoal and aluminium hydroxide are good adsorbents. Gases and liquids adsorbed on the surface of a solid are called adsorbates. It should be clearly understood that though adsorption is a surface phenomenon, the extent and firmness of adsorption are dependent to a great extent on the nature of the adsorbent and the adsorbate.

Adsorption is different from absorption. When a substance is uniformly distributed throughout the
bulk of another substance, this shows that absorption has taken place. It occurs at the uniform rate throughout the body of the material. On the contrary, adsorption involves unequal distribution of the molecular species in the bulk and the surface. It is a surface phenomenon. It is rapid in the beginning and gradually slows down at the equilibrium. The forces involved in adsorption are either weak Vander Waal's forces or strong chemical bond forces.

In certain cases, both the adsorption and the absorption take place simultaneously and it is not easy to distinguish between them. Such substance is said to be sorbed and the phenomenon is known as sorption .

The adsorption of one substance on the surface of another leads to the existence of new types of forces between them. Therefore, it is an exothermic process and is accompanied by the release of energy. The enthalpy or heat of adsorption is defined as the heat energy evolved when one mole of adsorbate is adsorbed on the surface of adsorbent. Since physical adsorption involves weak forces of attraction between the molecules of the adsorbent and the adsorbate, the heat of physisorption is generally low, of the order of `20 - 40` `kJ mol ^(- 1)`. Chemical adsorption, on the other hand involves strong chemical bond formation and the heat of chemisorption is quite high, of the order of `80 - 400` `kJ mol^(-1)` .

There is always a competition between different adsorbates to adsorb on the adsorbent. A strongly absorbable substance can easily displace a weakly adsorbed substance. For example, on the surface of activated charcoal, gases such as `O_2, N_2` etc. are already adsorbed. But when charcoal comes in contact with poisonous gases such as `CH_4, CO` which are strongly adsorbable, `O_2 ` and `N_2` get displaced. If a mix of gases is allowed to adsorb on a particular adsorbent, the strongly adsorbable adsorbate adsorbs to a greater extent than its partial pressure indicates. For example, moisture though present in small proportion in air is strongly adsorbed by silica gel. Charcoal adsorbs poisonous gases even though they are present in small concentration in air.