ChemistryCHEM 15
Surface & Environmental Chemistry
Adsorption, colloids, Tyndall effect, air/water pollution, ozone depletion, green chemistry
NCERT XII Ch.5+16
NCERT Ref
Concept Core
Essential theory — everything NCERT tests on Surface & Environmental Chemistry
ADSORPTION
Adsorption: accumulation of substance on surface. Adsorbent (surface) + adsorbate (substance). Physisorption: weak van der Waals, reversible, low selectivity. Chemisorption: chemical bond, specific, irreversible.
Freundlich adsorption isotherm: x/m = kP^(1/n). Adsorption increases with decrease in temperature for physisorption; increases with T for chemisorption (activation needed).
COLLOIDS
Heterogeneous mixture. Particle size: 1–1000 nm. Dispersed phase + dispersion medium. Types: sol (solid in liquid), gel (liquid in solid), aerosol (solid/liquid in gas), foam (gas in liquid).
Tyndall effect: scattering of light by colloid particles. Brownian motion: random zigzag movement. Electrophoresis: migration of colloidal particles in electric field. Coagulation: destabilisation of colloid (Hardy-Schulze rule).
HARDY-SCHULZE RULE & GOLD NUMBER
Higher charge of added ion = greater coagulating power. For negative sol: Al³⁺ > Ca²⁺ > Na⁺. Gold number: minimum amount (mg) of protective colloid to prevent coagulation of 10 mL gold sol by 1 mL 10% NaCl. Lower gold number = better protective colloid. Gelatin < starch.
AIR POLLUTION
Ozone depletion: CFCs release Cl radicals → Cl + O₃ → ClO + O₂ (chain reaction). Greenhouse effect: CO₂, CH₄, N₂O, H₂O absorb IR → global warming. Photochemical smog: NO₂ + hydrocarbons + sunlight → ozone, PAN (peroxyacetyl nitrate). Eye irritation.
Smog: London type (SO₂, winter) vs Los Angeles type (photochemical, summer).
WATER POLLUTION & GREEN CHEMISTRY
BOD: oxygen consumed by microbes to decompose organic matter. High BOD = more pollution. Eutrophication: excess nutrients → algal bloom → O₂ depletion → fish death.
Green chemistry: 12 principles. Atom economy = (MW desired/ΣMW reactants) × 100. Solvent-free, catalytic processes preferred. Reagents safer for environment.
Fact & Formula Vault
High-yield facts, numbers, and formulas
Colloid Types
Sol: solid dispersed in liquid
Aerosol: solid/liquid in gas
Gel: liquid in solid (agar, cheese)
Foam: gas dispersed in liquid
Emulsion: liquid in liquid (milk)
Colloid Properties
Tyndall effect: light scattering
Brownian motion: random zig-zag
Electrophoresis: moves in E-field
Hardy-Schulze: higher charge = coagulates better
Pollutants
CFCs → ozone depletion
CO₂, CH₄ → greenhouse effect
PAN → photochemical smog
BOD → water pollution measure
SO₂, NO₂ → acid rain
Worked Examples
NEET-style questions solved step-by-step
EASYTyndall effect is shown by:▾
Tyndall effect is shown by:
Colloidal solutions (particle size 1–1000 nm). The colloidal particles scatter light, making the beam visible. True solutions (particle <1 nm) do NOT show Tyndall effect.
MEDIUMHardy-Schulze rule: for coagulating a negative sol, which is most effective?▾
Hardy-Schulze rule: for coagulating a negative sol, which is most effective?
Al³⁺ (trivalent) > Ca²⁺ (divalent) > Na⁺ (monovalent). Higher valence of oppositely charged ion = more coagulating power. Al³⁺ is most effective for negative sol.
HARDPAN (peroxyacetyl nitrate) is formed during:▾
PAN (peroxyacetyl nitrate) is formed during:
Photochemical smog (Los Angeles type). NO₂ + sunlight + hydrocarbons → ozone, PAN, and other oxidants. PAN causes eye irritation and damages plant leaves. Different from London smog (SO₂-based).
Mistake DNA
Common NEET traps for this chapter
⚠ Tyndall effect scope
Tyndall effect is shown by COLLOIDS, not true solutions. Milk shows Tyndall. NaCl solution does not.
✓ Fix: True solution: no Tyndall. Colloid: yes Tyndall.
⚠ London vs Photochemical smog
London smog: SO₂ + fog, winter, reducing in nature. Photochemical smog: NO₂ + HC + sunlight, summer, oxidising (PAN, O₃).
✓ Fix: L = London = SO₂. P = Photochemical = NO₂/PAN.
⚠ Adsorption vs Absorption
Adsorption: surface phenomenon (accumulation ON surface). Absorption: bulk phenomenon (throughout the substance). Silica gel ADSORBS water; sponge ABSORBS water.
✓ Fix: AD-sorption = surface. AB-sorption = bulk/throughout.
Chapter Intelligence
Exam data and last-minute strategy
NEET Frequency
1–2 Q/year. Tyndall effect, Hardy-Schulze rule, PAN/photochemical smog, ozone depletion mechanism, gold number.
High-Yield
Colloid size: 1–1000 nm. Tyndall: colloids only. Al³⁺ > Ca²⁺ > Na⁺ coagulation. CFC → Cl radicals → O₃ depletion. PAN = photochemical smog.
Strategy
Colloid type table: dispersed + medium = name. Pollution: memorise London vs LA smog. Gold number: lower = better protective colloid.
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