Why is adsorption always exothermic?

Hint: Accumulation of molecular species on the surface rather than in the bulk of solid or liquid is termed as adsorption. i.e. When different molecular species get accumulated on the surface rather than in bulk that is inside a solid or liquid is termed as adsorption.

Complete step-by-step answer:

Adsorption is always exothermic. This statement can be explained in two ways;
(i) Adsorption leads to a reduction in the residual forces on the outside of the adsorbent. Adsorption is an exothermic process since surface particles of the adsorbent are unstable and when the adsorbate is adsorbed on the surface, the energy of adsorbent decreases, and this results in the evolution of heat. Therefore, adsorption is always exothermic.
(ii) ${ \Delta H }$ of adsorption is always negative. At a point, when gas is adsorbed on a solid surface, its movement is confined leading to a reduction in the entropy of the gas i.e., ${ \Delta S }$ is negative. Now for a process to be spontaneous, ${ \Delta G }$ should be negative.
${ \Delta G=\Delta H-T\Delta S }$.
Since ${ \Delta S }$ is negative, ${ \Delta H }$ has to be negative to make ${ \Delta G }$ negative. Hence, adsorption is always exothermic.

Additional Information:
In adsorption, the substance gets accumulated at the surface only. It does not enter through the surface to the bulk of the solid or liquid.
In most applications activated carbon removes impurities from fluids, vapours, or gas by a process called adsorption.

Note: The possibility to make a mistake is that adsorption and absorption are two different terms. Absorption is an endothermic process while adsorption is an exothermic phenomenon.