The distinction between physisorption and chemisorption is the most heavily tested concept in PC-10 Surface Chemistry. Here is the comprehensive comparison organized for maximum retention.

Forces and Bonding Physisorption is driven by weak van der Waals forces (London dispersion forces, dipole interactions) — the same forces that cause gases to deviate from ideal behavior. No chemical bonds are formed. Chemisorption forms actual chemical bonds (covalent, coordinate, or ionic) between adsorbate molecules and specific surface atoms. This fundamental difference in bonding explains ALL the other differences.

Enthalpy Physisorption: 20–40 kJ/mol (comparable to heats of condensation/liquefaction — weak binding). Chemisorption: 80–240 kJ/mol (comparable to chemical bond energies — strong binding). The enthalpy value alone can distinguish the type of adsorption.

Reversibility Physisorption is easily reversible — reducing pressure or raising temperature releases the adsorbate. Chemisorption is effectively irreversible — chemical bonds must be broken, requiring as much or more energy than was released on adsorption.

Layers Physisorption can form multiple layers. The first layer is held to the adsorbent by van der Waals forces; subsequent layers are held to each other by the same forces. No limit in principle. Chemisorption forms only a monolayer — specific surface sites are limited; once occupied, no further chemisorption is possible.

Specificity Physisorption: non-specific — van der Waals forces act between all molecules, so any gas adsorbs on any solid (to varying extents). Chemisorption: highly specific — only certain adsorbate-adsorbent pairs can form chemical bonds (like a chemical reaction).

Activation Energy Physisorption: zero or negligible — occurs spontaneously when gas molecules contact the surface. Chemisorption: requires appreciable activation energy — analogous to the activation energy of a chemical reaction on the surface.

Temperature Dependence (Most Important for NEET) Physisorption: decreases monotonically with temperature. Proof: (1) exothermic reaction → Le Chatelier: raising T favors reverse (endothermic) desorption. (2) thermodynamic: at higher T, T|$\Delta S$| term eventually dominates, making $\Delta G$ positive. Chemisorption: first INCREASES with temperature (molecules gain energy to overcome Ea → more chemisorption), then DECREASES at high temperature (desorption dominates → bonds break). This "volcano curve" behavior with a maximum is unique to chemisorption and is the single most tested fact in NEET Surface Chemistry.

NEET Trap Summary The most common NEET trap: "Physisorption increases with temperature" — WRONG. It decreases. The second trap: "Chemisorption also decreases monotonically with T" — WRONG. Chemisorption shows a maximum.