Last Updated: April 2026
Surface Chemistry is a high-yield chapter in JEE Main Chemistry, appearing in 2–3 questions every year. In JEE Main 2025 (January Session), there were 3 surface chemistry questions — all from adsorption and colloids. This guide provides complete notes for JEE Main 2027, covering all theory, classification tables, and 10 practice MCQs.
Adsorption
Adsorption is the accumulation of a substance (adsorbate) on the surface of another substance (adsorbent). It is a surface phenomenon (not to be confused with absorption, which is a bulk phenomenon).
Types of Adsorption
| Feature | Physisorption (Physical) | Chemisorption (Chemical) |
|---|---|---|
| Force involved | Van der Waals forces | Chemical bonding (covalent/ionic) |
| Enthalpy change | Low (20–40 kJ/mol) | High (40–400 kJ/mol) |
| Reversibility | Easily reversible | Irreversible |
| Effect of temperature | Decreases with rise | Increases initially, then decreases |
| Specificity | Non-specific | Highly specific |
| Layers formed | Multilayer | Monolayer |
| Example | N₂ on mica | H₂ on Ni |
Factors Affecting Adsorption
- Temperature: Physisorption decreases; chemisorption increases then decreases
- Pressure: Adsorption increases with pressure (for gases)
- Surface area: Greater surface area → more adsorption (finely divided adsorbents preferred)
- Nature of adsorbate: More condensable gases (higher critical temp) adsorb more
Freundlich Adsorption Isotherm
x/m = k·p^(1/n) where x = mass of adsorbate, m = mass of adsorbent, p = pressure, k and n are constants.
In log form: log(x/m) = log k + (1/n) log p — gives a straight line with slope 1/n and intercept log k.
Catalysis
A catalyst is a substance that increases the rate of a reaction without being consumed. It lowers the activation energy.
Types of Catalysis
| Type | Definition | Example |
|---|---|---|
| Homogeneous | Catalyst in same phase as reactants | SO₂ oxidation catalyzed by NO gas |
| Heterogeneous | Catalyst in different phase from reactants | Haber process: Fe catalyst, gaseous reactants |
| Enzyme (biocatalysis) | Biochemical catalysts (proteins) | Salivary amylase, pepsin |
| Autocatalysis | Product of reaction acts as catalyst | Permanganate reaction — Mn²⁺ product catalyses |
Promoters and Inhibitors (Poisons)
- Promoter: Enhances efficiency of catalyst (e.g., Mo in Haber process with Fe catalyst)
- Catalyst poison/inhibitor: Reduces catalytic activity (e.g., CO poisons Pt in H₂/O₂ reaction)
Characteristics of Catalysts
- Small amount required
- Does not alter equilibrium constant (only speeds reaching equilibrium)
- Specific in action (each catalyst works for specific reactions)
- Can become inactive over time (deactivation)
Colloids
A colloidal system consists of particles (1–1000 nm diameter) dispersed in a medium. Between true solutions (< 1 nm) and suspensions (> 1000 nm).
Classification of Colloids
| Dispersed Phase | Dispersion Medium | Type | Example |
|---|---|---|---|
| Gas | Liquid | Foam | Whipped cream, soap foam |
| Gas | Solid | Solid foam | Pumice stone, foam rubber |
| Liquid | Gas | Aerosol | Fog, clouds, insecticide spray |
| Liquid | Liquid | Emulsion | Milk, mayonnaise |
| Liquid | Solid | Gel | Cheese, butter, jelly |
| Solid | Gas | Aerosol (smoke) | Smoke, dust |
| Solid | Liquid | Sol | Blood, starch in water, paint |
| Solid | Solid | Solid sol | Ruby glass, gemstones |
Properties of Colloids
Tyndall Effect
Scattering of light by colloidal particles. Only observed in colloids (not true solutions). Used to distinguish colloids from true solutions. Example: beam of light in a dark room (dust particles scatter light).
Brownian Motion
Random, zigzag motion of colloidal particles due to unequal bombardment by dispersion medium molecules. Stabilizes the colloid against gravity settling.
Electrophoresis
Movement of colloidal particles under an electric field. All colloidal particles of a given sol carry the same type of charge. Positive particles move to cathode; negative to anode.
Coagulation (Flocculation)
The process of settling out of colloidal particles by adding an electrolyte. Hardy-Schulze Rule: Greater the valency (charge) of the coagulating ion, greater its coagulating power.
Example: For a negatively charged sol, coagulating power: Al³⁺ > Ba²⁺ > Na⁺
Gold Number
The minimum mass (in mg) of protective colloid that prevents coagulation of 10 mL of gold sol by adding 1 mL of 10% NaCl solution. Smaller gold number = greater protective power.
Protective Colloids
Lyophilic colloids (gelatin, starch, egg albumin) that protect lyophobic colloids (gold sol, ferric hydroxide sol) from coagulation by forming a protective layer.
Dialysis and Electrodialysis
Dialysis: Purification of a colloid by diffusion of smaller ions/molecules through a semipermeable membrane (colloid particles too large to pass). Used in kidney dialysis machines.
Electrodialysis: Dialysis accelerated by applying electric field — faster removal of ions.
Practice MCQs — JEE Main Surface Chemistry 2027
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Frequently Asked Questions (FAQ)
What is the difference between adsorption and absorption?
Adsorption is a surface phenomenon — the adsorbate concentrates on the surface of the adsorbent (e.g., silica gel adsorbing water vapour). Absorption is a bulk phenomenon — the absorbate uniformly distributes throughout the absorbing medium (e.g., water absorbing into a sponge). Adsorption is an exothermic process; its enthalpy is called heat of adsorption.
What is the Hardy-Schulze rule?
Hardy-Schulze rule states that the coagulating power of an electrolyte increases with the valence (charge) of the coagulating ion. For a negatively charged sol, the cation coagulates it: Al³⁺ > Ba²⁺ > Na⁺. For a positively charged sol, the anion coagulates: [Fe(CN)₆]⁴⁻ > PO₄³⁻ > SO₄²⁻ > Cl⁻. This is regularly tested in JEE Main.
How many questions come from Surface Chemistry in JEE Main?
Surface Chemistry typically contributes 2–3 questions in JEE Main. The chapter is part of Physical Chemistry in the JEE Main syllabus. Questions focus on: type of adsorption (physisorption vs chemisorption), type of colloid given dispersed/dispersion phases, Hardy-Schulze rule, gold number, and Tyndall effect. All are conceptual and fact-based — no calculation required.
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