Adsorption = accumulation of substance ON a surface (surface phenomenon). Absorption = penetration of substance INTO the bulk (bulk phenomenon). Sorption = both occurring together. Adsorbent = surface on which adsorption occurs (charcoal, silica gel, alumina). Adsorbate = substance being adsorbed (gas or solute). Desorption = release of adsorbate from surface (reverse of adsorption). Adsorption is always exothermic (delta_H < 0) because surface energy decreases. Since adsorption decreases randomness (gas to surface), delta_S < 0. For spontaneous adsorption: delta_G < 0, meaning |T*delta_S| < |delta_H|.