CBSE Important Questions Class 12 Chemistry Chapter 14 – Surface Chemistry

Class 12 Chemistry Chapter 14 Surface Chemistry is part of the CBSE Class 12 Chemistry syllabus. It studies what happens at the surface where two phases meet. Students learn about adsorption and its types, adsorption isotherms, catalysis and its kinds, enzyme catalysis, colloids and their classification, the properties of colloidal solutions such as the Tyndall effect and Brownian movement, coagulation, and emulsions.

These CBSE important questions for Class 12 Chemistry Chapter 14 cover the difference between adsorption and absorption, physisorption and chemisorption, Brownian movement and why it matters, the classification of colloids where water is the dispersion medium, aerosols and emulsions, the Tyndall effect, enzyme catalysis, and the Hardy-Schulze rule. Every question has a clear step-by-step solution in simple English, so students can practise on their own, parents can check the working, and teachers can use them for class tests.

Q1. What is the difference between adsorption and absorption? Give one example of each.

Answer: Adsorption is a surface phenomenon. The substance collects only on the surface of the other substance, and its concentration is different at the surface from in the bulk.
Example: water vapour collecting on the surface of silica gel.
Absorption is a bulk phenomenon. The substance goes uniformly right through the body of the other substance.
Example: water being taken up by a sponge, or chalk absorbing water.
Sorption is the word used when both happen together, as when a dye is taken up by cotton.

Q2. Distinguish between physisorption and chemisorption.

Physisorption Chemisorption
Caused by weak van der Waals forces Caused by strong chemical bonds
Reversible Usually irreversible
Low enthalpy, about 20-40 kJ/mol High enthalpy, about 80-240 kJ/mol
Not specific — any gas on any surface Highly specific — only if a bond can form
Favoured by low temperature; decreases on heating Favoured by high temperature at first
Forms multimolecular layers Forms a unimolecular layer only

Q3. What is Brownian movement? What causes it and why is it important?

Answer: Brownian movement is the continuous zig-zag motion of colloidal particles in a colloidal solution, seen under an ultramicroscope.
Cause: The molecules of the dispersion medium are in constant motion because of their kinetic energy. They strike the colloidal particles from all sides with unequal force. This unbalanced bombardment pushes the particle first one way and then another, giving the zig-zag path.
Why it is important: Brownian movement opposes the force of gravity and keeps the colloidal particles from settling down at the bottom. This is what makes a colloidal solution stable and keeps it from separating out like a suspension.

Q4. Classify the colloids in which the dispersion medium is water. State their characteristics and give an example of each.

Answer: When the dispersion medium is water (a liquid), there are three classes, depending on what the dispersed phase is.
1. Sol: a solid dispersed in a liquid. When the liquid is specifically water, it is called a hydrosol.
Examples: paints, cell fluids, muddy water, starch sol.
2. Emulsion: a liquid dispersed in a liquid.
Examples: milk (oil in water), hair cream, butter (water in oil).
3. Foam: a gas dispersed in a liquid.
Examples: soap lather, froth, whipped cream.

Q5. A colloid in which the dispersed phase is a gas and the dispersion medium is a solid is
(a) foam
(b) gel
(c) aerosol
(d) solid sol

Answer: (a) Foam
When a gas is dispersed in a solid, the colloid is called a solid foam (often just "foam"). Examples are pumice stone, foam rubber and styrofoam.
For comparison: an aerosol is a solid or liquid dispersed in a gas (smoke, fog), a gel is a liquid dispersed in a solid (cheese, butter, jelly), and a solid sol is a solid dispersed in a solid (coloured glass, gemstones).

Q6. How are aerosol, emulsion and hydrosol different in terms of their dispersed phase and dispersion medium? Give an example of each.

Colloid Dispersed phase Dispersion medium Example
Aerosol Solid or liquid Gas Smoke, fog, clouds
Emulsion Liquid Liquid Milk, hair cream
Hydrosol Solid Water Muddy water, starch sol

Q7. What is the Tyndall effect? Why is it shown by colloids but not by true solutions?

Answer: The Tyndall effect is the scattering of a beam of light by the particles of a colloidal solution, which makes the path of the beam visible as a bright cone.
Why colloids show it: The size of colloidal particles (1 to 1000 nm) is comparable to the wavelength of visible light. Particles of this size scatter the light in all directions, so the beam becomes visible from the side.
Why true solutions do not: The particles in a true solution are far too small (less than 1 nm) to scatter light. The beam simply passes straight through and cannot be seen from the side.
Everyday examples: a beam of sunlight entering a dark room through a small hole, and the headlights of a car visible in fog.

Q8. What are enzymes? Describe in brief the mechanism of enzyme catalysis.

Answer: Enzymes are complex nitrogenous organic compounds — essentially proteins of high molecular mass — produced by living plants and animals. They form colloidal solutions in water and are often called biochemical catalysts.
Mechanism (lock and key model):
1. The enzyme surface has cavities of a particular shape carrying active groups such as −NH₂ and −COOH.
2. The substrate molecule, having a complementary shape, fits into the cavity like a key into a lock, forming an enzyme-substrate complex.
3. Inside the complex the substrate bonds are weakened, the reaction takes place, and the product is formed.
4. The product leaves the cavity, and the enzyme is free to take up another substrate molecule.
This shape-matching is why enzymes are so highly specific — one enzyme usually catalyses only one reaction.

Q9. What is coagulation of a colloid? State the Hardy-Schulze rule.

Answer: Coagulation (or flocculation) is the settling down of colloidal particles as a precipitate, when the charge that was keeping them apart is neutralised. It is usually brought about by adding an electrolyte.
Hardy-Schulze rule: It has two parts.
1. The ion that causes coagulation is the one carrying a charge opposite to that of the colloidal particles.
2. The greater the charge on that ion, the more effective it is at coagulating.
For a negatively charged sol (such as As₂S₃): the effectiveness is Al³⁺ > Ba²⁺ > Na⁺
For a positively charged sol (such as Fe(OH)₃): the effectiveness is [Fe(CN)₆]⁴⁻ > PO₄³⁻ > SO₄²⁻ > Cl⁻

Q10. What are emulsions? Name the two types and give one example of each. What is an emulsifying agent?

Answer: An emulsion is a colloid in which both the dispersed phase and the dispersion medium are liquids.
Two types:
1. Oil in water (O/W): oil is the dispersed phase and water the medium. Examples: milk, vanishing cream.
2. Water in oil (W/O): water is dispersed in oil. Examples: butter, cold cream.
Emulsifying agent: Emulsions are unstable by themselves and separate out. An emulsifying agent (emulsifier) is added to make them stable. It forms a protective film around the droplets and stops them from joining together.
Examples: soaps and detergents, proteins and gum. In milk, the natural emulsifier is the protein casein.

Q11. What are the factors that affect the adsorption of a gas on a solid?

Answer: 1. Nature of the gas: gases that are easily liquefied (with higher critical temperature), such as SO₂ and NH₃, are adsorbed more readily than permanent gases like H₂ and N₂.
2. Surface area of the adsorbent: the larger the surface area, the more the adsorption. This is why finely powdered or porous materials such as charcoal and silica gel are used.
3. Temperature: adsorption is an exothermic process, so by Le Chatelier's principle it decreases with a rise in temperature.
4. Pressure: adsorption increases with pressure, but only up to a point. Once the surface is saturated, further increase in pressure has no effect.
5. Activation of the adsorbent: increasing the roughness or making the surface porous increases adsorption.

Q12. Write the Freundlich adsorption isotherm. What does it tell us at low and high pressure?

Answer: The Freundlich adsorption isotherm gives the relation between the amount of gas adsorbed and the pressure at constant temperature:
x/m = k p1/n  (where n > 1)
Here x is the mass of gas adsorbed, m is the mass of the adsorbent, p is the pressure, and k and n are constants.
Taking logarithms gives the straight line form:
log(x/m) = log k + (1/n) log p
At low pressure: 1/n approaches 1, so x/m is directly proportional to p.
At high pressure: 1/n approaches 0, so x/m becomes independent of pressure — the surface is saturated and no more gas can be adsorbed.
Limitation: The equation is purely experimental and holds only over a limited range of pressure.

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