Surface Chemistry & Colloids: JEE Main Complete Guide

Surface chemistry is a chapter that many students underestimate. It contributes two to three questions every JEE Main session, is almost entirely fact-based (no heavy calculation), and its question types have been virtually unchanged for the past five years. A focused two-day revision can lock in full marks here, making it one of the best effort-to-marks chapters in the entire Chemistry syllabus.

Adsorption: Physisorption vs Chemisorption

Adsorption is the accumulation of a substance (adsorbate) on the surface of another (adsorbent). Physisorption (physical adsorption) is due to van der Waals forces: it is reversible, has low enthalpy of adsorption (20–40 kJ/mol), is favoured at low temperatures, and multi-layer adsorption is possible. Chemisorption is due to chemical bond formation: it is irreversible, has high enthalpy (40–400 kJ/mol), is favoured at high temperatures for activation, and is restricted to a monolayer. JEE frequently asks which type is associated with a given characteristic — memorise the contrast table.

The Freundlich adsorption isotherm: x/m = kP^(1/n) where x is mass adsorbed, m is mass of adsorbent, and 1/n is between 0 and 1. The Langmuir isotherm gives monolayer adsorption. Adsorption increases with surface area (reason powdered adsorbents are preferred) and with pressure for gases. Industrial applications: activated charcoal removes toxic gases, silica gel is used as a desiccant, and zeolites are selective adsorbents in petroleum refining. Test your recall with a free chemistry mock.

Catalysis

Heterogeneous catalysis occurs when catalyst and reactants are in different phases — the reaction proceeds on the catalyst surface. The intermediate compound theory explains how the catalyst forms a temporary intermediate with lower activation energy. Promoters increase catalyst activity; poisons reduce it. Enzyme catalysis is a special case of homogeneous biological catalysis, characterised by high specificity, mild conditions, and the lock-and-key mechanism. Zeolites function as shape-selective catalysts by admitting only molecules of certain sizes into their pores.

Colloids: Types and Properties

A colloid has particle sizes between 1 nm and 1000 nm. The dispersed phase is the substance in small particles; the dispersion medium is the surrounding substance. Eight colloid types arise from three combinations of solid, liquid, and gas phases — memorise the most tested: sols (solid in liquid), emulsions (liquid in liquid), aerosols (liquid in gas), and gels (liquid in solid). Colloids in which the dispersed phase has an affinity for the medium are lyophilic (proteins, starch — stable, reversible); those without affinity are lyophobic (gold sol, silver sol — unstable, charge-stabilised).

The Tyndall effect — scattering of light by colloidal particles making the beam visible — distinguishes colloids from true solutions. Brownian motion (random zigzag movement of colloidal particles) is caused by the unsymmetrical bombardment by dispersion medium molecules and keeps particles suspended. Coagulation is the aggregation and precipitation of colloidal particles, achieved by adding electrolytes. Hardy-Schulze rule: higher the valency of the coagulating ion, greater its coagulating power — so Al³⁺ is more effective than Na⁺ in coagulating a negatively charged sol. Emulsions are stabilised by emulsifiers (soap is an emulsifier for oil-in-water emulsions). For the broader context, see our solid state guide and chemistry strategy 2026.