60 free MCQs on Surface Chemistry with worked answers and explanations. Explore the chemistry that happens at interfaces, from catalysts that speed up industrial reactions to colloids like milk, smoke, and gels that surround us every day.
Take the timed Surface Chemistry chapterwise test →Below are 60 practice questions on Surface Chemistry, sorted Easy → Hard. Tap “Show answer & explanation” under any question to check yourself. Want the full theory first? Read the Surface Chemistry notes.
A typical adsorption isotherm: x/m (mass adsorbed per gram of adsorbent) rises steeply at low pressure, then flattens into a saturation plateau as the adsorbent surface fills up.
Q1.
Adsorption is best described as:
Answer: B. Accumulation of molecules on the surface of a solid or liquid
Why: Adsorption refers to molecules of a gas or liquid accumulating on the surface of an adsorbent, unlike absorption which is a bulk phenomenon.
Q2.
Which type of adsorption is caused mainly by weak van der Waals forces?
Answer: B. Physisorption
Why: Physisorption (physical adsorption) results from weak van der Waals forces between adsorbate and adsorbent.
Q3.
Chemisorption involves:
Answer: B. Formation of chemical bonds between adsorbate and adsorbent
Why: Chemisorption involves actual chemical bond formation between the adsorbate molecules and the adsorbent surface.
Q4.
Which property is typical of physisorption?
Answer: B. Reversible with low enthalpy of adsorption
Why: Physisorption is generally reversible and has a low enthalpy of adsorption, typically 20 to 40 kJ per mole.
Q5.
A catalyst works by:
Answer: B. Providing an alternate pathway with lower activation energy
Why: A catalyst speeds up a reaction by providing a path with lower activation energy, without undergoing permanent chemical change itself.
Q6.
In homogeneous catalysis, the catalyst and reactants are:
Answer: B. In the same phase
Why: Homogeneous catalysis occurs when catalyst and reactants exist in the same phase, such as all being gases or all in solution.
Q7.
Enzymes are examples of:
Answer: B. Biological catalysts
Why: Enzymes are protein-based biological catalysts that are highly specific and effective even in very small amounts.
Q8.
The size range of particles in a colloidal solution is approximately:
Answer: B. Between 1 nm and 1000 nm
Why: Colloidal particles have diameters roughly between 1 nm and 1000 nm, larger than true solutions but smaller than suspensions.
Q9.
Milk is an example of which type of colloid?
Answer: C. Emulsion
Why: Milk is an emulsion, a colloidal dispersion of liquid fat droplets in a liquid (water-based) medium.
Q10.
Fog is a colloidal system best classified as:
Answer: B. Liquid in gas
Why: Fog consists of tiny liquid water droplets dispersed in air (a gas), making it an aerosol of liquid in gas.
Q11.
The Tyndall effect refers to:
Answer: B. The scattering of light by colloidal particles
Why: The Tyndall effect is the scattering of light by colloidal particles, making the path of a light beam visible through the colloid.
Q12.
Brownian motion in colloids is caused by:
Answer: B. Unequal bombardment of colloidal particles by molecules of the dispersion medium
Why: Brownian motion arises from continuous, random collisions of dispersion medium molecules with colloidal particles, keeping them in constant zig-zag motion.
Q13.
Electrophoresis is the movement of colloidal particles under the influence of:
Answer: B. An electric field
Why: Electrophoresis is the movement of charged colloidal particles toward an oppositely charged electrode when an electric field is applied.
Q14.
Lyophilic colloids are best described as:
Answer: B. Solvent-loving and generally stable
Why: Lyophilic colloids have strong affinity for the dispersion medium, making them stable and reversible, unlike lyophobic colloids.
Q15.
An emulsifying agent is added to an emulsion mainly to:
Answer: B. Stabilize the emulsion by forming a protective film around droplets
Why: Emulsifying agents like soaps or proteins coat dispersed droplets, preventing them from coalescing and stabilizing the emulsion.
Q16.
The accumulation of a substance at the surface of a solid rather than in its bulk is called:
Answer: B. adsorption
Why: Adsorption is a surface phenomenon; absorption is uptake throughout the bulk of the material.
Q17.
The substance that becomes concentrated on a surface during adsorption is the:
Answer: B. adsorbate
Why: The adsorbate is the substance adsorbed; the adsorbent is the surface on which it accumulates.
Q18.
Activated charcoal is used in gas masks and water filters mainly as an:
Answer: A. adsorbent
Why: Its enormous surface area lets activated charcoal adsorb gases and impurities effectively.
Q19.
The scattering of a beam of light by the particles of a colloid is called the:
Answer: B. Tyndall effect
Why: The Tyndall effect makes the path of light visible through a colloid because the particles scatter it.
Q20.
A colloid consisting of one liquid dispersed in another liquid is called an:
Answer: B. emulsion
Why: An emulsion is a liquid-in-liquid colloid, such as milk (fat in water).
Q21.
According to the Freundlich adsorption isotherm, x/m = k.P<sup>1/n</sup>, what happens to x/m at high pressure?
Answer: B. It becomes independent of pressure, reaching saturation
Why: At high pressure, the Freundlich isotherm predicts x/m approaches a constant saturation value, independent of further pressure increase.
Q22.
Which observation supports the idea that chemisorption, unlike physisorption, requires activation energy?
Answer: B. Chemisorption rate often increases with temperature up to an optimum, similar to chemical reactions
Why: Because chemisorption involves bond formation with an activation energy barrier, its rate often increases with temperature up to an optimum, unlike physisorption.
Q23.
In the Haber process for ammonia synthesis, finely divided iron acts as a catalyst in what type of catalysis?
Answer: B. Heterogeneous catalysis
Why: Iron catalyst is a solid while the reactant gases N<sub>2</sub> and H<sub>2</sub> are gaseous, making this a heterogeneous catalysis process.
Q24.
Why does activated charcoal adsorb gases more effectively than ordinary charcoal?
Answer: B. It has a much larger effective surface area due to its porous structure
Why: Activation increases the porosity of charcoal, dramatically increasing its surface area and therefore its adsorption capacity.
Q25.
A negatively charged colloidal sol is coagulated by adding electrolytes containing Na+, Ba<sup>2+</sup>, and Al<sup>3+</sup> ions separately. According to the Hardy-Schulze rule, which ion will coagulate the sol most effectively?
Answer: C. Al<sup>3+</sup>
Why: The Hardy-Schulze rule states that ions with higher valency are more effective coagulants for oppositely charged sols, so Al<sup>3+</sup> (3+) is most effective.
Q26.
Why are lyophobic colloids generally less stable than lyophilic colloids?
Answer: B. Lyophobic colloids have weak interaction with the dispersion medium and need stabilizers
Why: Lyophobic colloids have little affinity for the solvent, making them inherently unstable and often requiring stabilizing agents to prevent coagulation.
Q27.
Why is a multimolecular colloid like a gold sol different from a macromolecular colloid like a protein solution?
Answer: B. A multimolecular colloid is formed by aggregation of many small atoms or molecules, while a macromolecular colloid consists of single large polymer molecules
Why: Multimolecular colloids form from aggregates of smaller particles (like gold atoms clustering), while macromolecular colloids are individual large molecules such as proteins or starch.
Q28.
What is the role of alum in the purification of turbid drinking water?
Answer: B. It introduces Al<sup>3+</sup> ions that coagulate negatively charged clay colloidal particles
Why: Alum provides Al<sup>3+</sup> ions, which neutralize the charge on suspended colloidal clay particles, causing them to coagulate and settle.
Q29.
Why does soap form micelles only above a certain concentration, called the critical micelle concentration?
Answer: B. Above this concentration, soap molecules aggregate with hydrophobic tails inward to minimize contact with water
Why: Above the critical micelle concentration, surfactant molecules cluster together so their hydrophobic tails face inward, away from water, forming a stable micelle.
Q30.
Which of these best explains why enzyme catalysis is highly specific compared to ordinary chemical catalysts?
Answer: B. Enzymes have a specific active site shape that fits only particular substrate molecules
Why: The lock-and-key fit between an enzyme active site and its specific substrate explains why enzyme catalysis is far more specific than typical catalysts.
Q31.
In the electrostatic (Cottrell) precipitator used in factory chimneys, smoke particles are removed by:
Answer: B. Charging the smoke particles and attracting them to an oppositely charged electrode
Why: The Cottrell precipitator charges colloidal smoke particles, which are then attracted to and collected on an oppositely charged electrode, similar to electrophoresis.
Q32.
Physisorption arises from weak forces of the type:
Answer: B. van der Waals forces
Why: Physical adsorption (physisorption) is due to weak van der Waals attractions between adsorbate and adsorbent.
Q33.
Chemisorption is characterised by the formation of:
Answer: B. chemical bonds
Why: Chemisorption involves genuine chemical bonds, so it is stronger and more specific than physisorption.
Q34.
Physical adsorption of a gas on a solid is generally favoured by:
Answer: B. low temperature
Why: Being exothermic, physisorption increases as temperature falls.
Q35.
The Freundlich adsorption isotherm is written as x/m equal to:
Answer: B. k·p<sup>1/n</sup>
Why: The Freundlich isotherm is x/m = k·p<sup>1/n</sup>, where 1/n lies between 0 and 1.
Q36.
A catalyst speeds up a reaction by:
Answer: B. lowering the activation energy
Why: A catalyst provides an alternative pathway with a lower activation energy, so more molecules can react.
Q37.
Enzymes are biological catalysts that are notably:
Answer: B. specific
Why: Enzymes are highly specific, each catalysing essentially one reaction via a lock-and-key fit.
Q38.
The continuous random zig-zag movement of colloidal particles is termed:
Answer: B. Brownian motion
Why: Brownian motion results from unbalanced bombardment of colloidal particles by the molecules of the medium.
Q39.
Colloidal particles have a size range of about:
Answer: B. 1 to 1000 nm
Why: Colloidal particle sizes lie between 1 and 1000 nm, larger than in true solutions but smaller than in suspensions.
Q40.
Removing dissolved ionic impurities from a colloid by diffusion through a membrane is called:
Answer: B. dialysis
Why: In dialysis, small ions pass through a semipermeable membrane while the larger colloidal particles are retained.
Q41.
A given mass of activated charcoal adsorbs a gas according to the Freundlich isotherm with n = 2. If the pressure of the gas is quadrupled, by what factor does x/m increase, all else being constant?
Answer: A. 2 times
Why: x/m = k.P<sup>1/n</sup> = k.P<sup>1/2</sup>. If P becomes 4P, x/m becomes k(4P)<sup>0.5</sup> = k.P<sup>0.5</sup> x 2, so x/m increases by a factor of 2.
Q42.
Why does the enthalpy of chemisorption tend to be much higher than that of physisorption, typically 80 to 240 kJ/mol versus 20 to 40 kJ/mol?
Answer: B. Chemisorption involves actual bond formation, which releases more energy than weak van der Waals attraction
Why: Because chemisorption forms genuine chemical bonds between adsorbate and adsorbent, it releases substantially more energy than the weak van der Waals interactions of physisorption.
Q43.
A colloidal sol of As<sub>2</sub>S<sub>3</sub> is negatively charged. Which combination correctly ranks the coagulating power of NaCl, MgCl<sub>2</sub>, and AlCl<sub>3</sub> from least to most effective, based on the Hardy-Schulze rule?
Answer: B. NaCl less than MgCl<sub>2</sub> less than AlCl<sub>3</sub>
Why: Coagulating power increases with the valency of the oppositely charged ion. Since the sol is negative, cations matter: Na+ (1+) is weakest, Mg<sup>2+</sup> (2+) is intermediate, and Al<sup>3+</sup> (3+) is the strongest coagulant.
Q44.
Why can a catalyst not alter the position of equilibrium of a reversible reaction, even though it increases reaction rate?
Answer: B. A catalyst lowers the activation energy of both forward and reverse reactions equally, so both rates increase proportionally and equilibrium is reached faster but at the same position
Why: Since a catalyst lowers the activation energy barrier equally for forward and reverse reactions, both rates speed up by the same factor, so equilibrium is reached sooner without shifting its position or changing K.
Q45.
Gold sol prepared by reduction of gold salts is stabilized by adsorbed negative ions on its surface. If a small amount of a strongly hydrated lyophilic colloid such as gelatin is added before adding an electrolyte, the gold sol becomes more resistant to coagulation. This protective effect is best explained by:
Answer: B. Gelatin forms a protective hydrophilic coating around the lyophobic gold particles, reducing their tendency to aggregate
Why: This is known as protective colloid action: a lyophilic colloid like gelatin coats lyophobic particles such as gold, providing a hydrated protective layer that resists coagulation, the basis of the gold number concept.
Q46.
Which statement correctly distinguishes a gel from a sol and an emulsion?
Answer: B. A gel has a liquid dispersed in a solid medium forming a semi-solid network, while a sol is a solid dispersed in a liquid, and an emulsion is liquid dispersed in liquid
Why: A gel (like jelly) is a liquid trapped in a solid colloidal network, a sol (like paint) is solid particles dispersed in liquid, and an emulsion (like milk) is liquid droplets dispersed in another liquid.
Q47.
In delta formation at a river mouth, fine clay and silt particles carried by river water as a negatively charged colloidal sol settle out. What is the most likely chemical reason for this?
Answer: B. Sea water contains a high concentration of electrolytes whose cations coagulate the negatively charged clay sol
Why: Sea water is rich in dissolved electrolytes (Na+, Mg<sup>2+</sup>, Ca<sup>2+</sup> ions). These cations neutralize the charge on negatively charged clay colloids from river water, causing coagulation and the buildup of a delta.
Q48.
Why does increasing pressure favor physisorption more strongly at lower temperatures than at higher temperatures?
Answer: B. At lower temperatures, gas molecules have less kinetic energy to escape the weak attractive forces holding them to the surface, so increased pressure packs more molecules onto the surface effectively
Why: Physisorption relies on weak van der Waals forces that are easily overcome by thermal motion. At lower temperature, molecules adsorb more readily, and added pressure further increases surface coverage; at higher temperatures, increased kinetic energy makes desorption more likely despite higher pressure.
Q49.
A protective colloid is added to a lyophobic sol before adding an electrolyte to prevent coagulation. The effectiveness of a protective colloid is expressed using gold number. What does a lower gold number indicate about a protective colloid?
Answer: D. It has greater protective power, since a smaller amount is needed to prevent coagulation of the gold sol
Why: Gold number is the minimum mass in milligrams of a protective colloid needed to prevent coagulation of a standard gold sol on adding electrolyte; a smaller gold number means greater protective power.
Q50.
Why does a finely divided catalyst, such as platinum black or Raney nickel, generally show higher catalytic activity than the same mass of catalyst in a bulk lump form?
Answer: C. Greater subdivision increases the total surface area available for adsorption of reactant molecules
Why: Catalytic activity in heterogeneous catalysis depends on the surface area available for adsorption of reactants; finely dividing the catalyst greatly increases this surface area, raising activity.
Q51.
By the Hardy–Schulze rule, the coagulating power of an ion increases with its:
Answer: B. charge
Why: The higher the charge on the oppositely-charged (coagulating) ion, the greater its coagulating power.
Q52.
Which ion is the most effective at coagulating a negatively charged sol?
Answer: C. Al³⁺
Why: A negative sol is coagulated by cations, and by Hardy–Schulze the highest positive charge (Al³⁺) is most effective.
Q53.
In the Freundlich adsorption isotherm x/m = k·p<sup>1/n</sup>, the value of 1/n normally lies:
Answer: A. between 0 and 1
Why: The exponent 1/n typically ranges from 0 to 1, showing that adsorption rises with pressure but less than proportionally.
Q54.
The catalyst used in the Haber process for manufacturing ammonia is:
Answer: B. iron with promoters
Why: The Haber process uses finely divided iron with molybdenum or K₂O/Al₂O₃ promoters.
Q55.
The catalyst used in the Contact process for sulfuric acid is:
Answer: B. vanadium pentoxide
Why: V₂O₅ catalyses the oxidation of SO₂ to SO₃ in the Contact process.
Q56.
Adsorption is generally exothermic, so the enthalpy change of adsorption is:
Answer: B. negative
Why: Because bonds/attractions form as the adsorbate settles on the surface, heat is released and ΔH is negative.
Q57.
A lyophobic sol is one in which the dispersed phase has ___ affinity for the dispersion medium:
Answer: B. little or no
Why: Lyophobic (solvent-hating) sols have little affinity for the medium and need special methods to prepare and stabilise.
Q58.
The Tyndall effect is observed when the diameter of the dispersed particles is:
Answer: B. comparable to the wavelength of light
Why: Effective scattering (Tyndall effect) occurs when particle size is comparable to the wavelength of the light used.
Q59.
A gold sol prepared by the Bredig’s arc method is an example of a:
Answer: B. lyophobic sol
Why: Metal sols such as gold are lyophobic; Bredig’s arc strikes an electric arc between metal electrodes under water.
Q60.
The settling of colloidal particles into a precipitate on adding an electrolyte is called:
Answer: B. coagulation
Why: Adding a suitable electrolyte neutralises the particle charge, causing coagulation (flocculation).