15 extra multiple-choice questions for Surface Chemistry (12th Standard Chemistry, Samacheer Kalvi), beyond the ones printed in the textbook — each with the correct option highlighted and a clear, worked explanation. Free to read in English and Tamil.
Q1
Which of the following conditions defines positive adsorption at an interface?
- A. Concentration of the substance at the interface is higher than in the bulkCorrect
- B. Concentration of the substance at the interface is lower than in the bulk
- C. The substance is distributed uniformly throughout the entire system
- D. The substance is completely removed from the surface phase
Explanation. Adsorption involves the attraction of molecules to a surface, leading to an increase in concentration at the interface. Positive adsorption specifically refers to the condition where the concentration is higher at the surface than in the bulk phase.
Q2
Which type of force is primarily responsible for physical adsorption (physisorption)?
- A. Strong covalent chemical bonds
- B. Weak Van der Waals forcesCorrect
- C. Electrostatic forces between ions
- D. Metallic bonding interactions
Explanation. Physical adsorption is caused by weak intermolecular attractions known as Van der Waals forces. Unlike chemisorption, it does not involve the formation of specific chemical bonds or the transfer of electrons, making it non-specific and reversible.
Q3
Based on the nature of the adsorbate, which of the following gases will be adsorbed most readily on the surface of charcoal?
- A. Hydrogen (\(H_2\))
- B. Nitrogen (\(N_2\))
- C. Oxygen (\(O_2\))
- D. Sulphur dioxide (\(SO_2\))Correct
Explanation. Easily liquefiable gases like sulphur dioxide have stronger Van der Waals forces and higher critical temperatures. Consequently, they are more readily adsorbed on surfaces compared to permanent gases like hydrogen or oxygen which have low critical temperatures.
Q4
In the Freundlich adsorption isotherm, what does the slope represent in a plot of \(\log(x/m)\) against \(\log p\)?
- A. \(\log K\)
- B. \(K\)
- C. \(1/n\)Correct
- D. \(n\)
Explanation. In the logarithmic form of the Freundlich equation, the plot of \(\log(x/m)\) against \(\log p\) yields a straight line. The slope of this line corresponds to the constant \(1/n\), while the y-intercept provides the value of \(\log K\).
Q5
In the industrial synthesis of ammonia by Haber's process, what is the role of Molybdenum (Mo)?
- A. Catalyst
- B. PromoterCorrect
- C. Catalytic poison
- D. Adsorbate
Explanation. Promoters are substances that do not act as catalysts themselves but increase the catalytic activity of another substance. In the Haber process, molybdenum is added to the iron catalyst to enhance the production of ammonia.
Q6
Which theory of catalysis explains that the reaction rate increases due to the formation of active centres on a rough surface?
- A. Intermediate compound formation theory
- B. Adsorption theoryCorrect
- C. Collision theory
- D. Electronic theory
Explanation. The adsorption theory explains that reactant molecules are adsorbed onto the catalyst's active centers. This brings them closer together and provides sufficient energy for the formation of an activated complex, which subsequently decomposes into the products.
Q7
In the mechanism of enzyme action, which part is represented by the 'Lock' according to the lock and key model?
- A. Substrate
- B. Co-enzyme
- C. EnzymeCorrect
- D. Product
Explanation. The lock and key model describes the high specificity of enzyme action. The enzyme acts as the lock with a uniquely shaped active site, into which only a specific substrate, the key, can fit to undergo a reaction.
Q8
How did Thomas Graham distinguish colloids from crystalloids during his research on diffusion?
- A. Based on their solubility in organic solvents
- B. Based on their ability to diffuse through a membraneCorrect
- C. Based on their boiling point elevations
- D. Based on their magnetic properties
Explanation. Thomas Graham classified substances based on their diffusion rates through a semi-permeable membrane. He defined colloids as substances like glue or gelatin that diffuse very slowly or not at all, unlike crystalloids which pass through easily.
Q9
Which of the following common substances is an example of a 'Sol' (solid dispersed in liquid)?
- A. Smoke
- B. InkCorrect
- C. Cloud
- D. Cheese
Explanation. An ink is a colloidal system where solid particles are dispersed in a liquid medium, specifically known as a sol. In contrast, smoke is a solid-in-gas aerosol, and clouds are liquid-in-gas aerosols.
Q10
Which preparation method involves striking an electric arc between metal electrodes to create a colloidal solution of metals like gold?
- A. Peptization
- B. Bredig's arc methodCorrect
- C. Ultrasonic dispersion
- D. Dialysis
Explanation. Bredig's arc method involves striking an electric arc between metal electrodes submerged in a dispersion medium cooled by ice. The intense heat vaporizes the metal, which then condenses into particles of colloidal dimensions within the liquid.
Q11
What is the process of removing ionic impurities from a colloidal solution using a semi-permeable membrane called?
- A. Condensation
- B. DialysisCorrect
- C. Coagulation
- D. Peptization
Explanation. Dialysis is a purification technique that utilizes a semi-permeable membrane to separate colloidal particles from smaller ionic or molecular impurities. The impurities diffuse out into the surrounding water while the larger colloidal particles are retained.
Q12
Which optical property of colloidal solutions explains why the sky appears blue?
- A. Tyndall effectCorrect
- B. Brownian movement
- C. Electrophoresis
- D. Sedimentation
Explanation. The Tyndall effect is the scattering of light by colloidal particles when a beam passes through a sol. This phenomenon is why the sky appears blue, as small dust and water particles scatter the shorter blue wavelengths of sunlight.
Q13
According to the Hardy-Schulze rule, which of the following ions has the highest coagulating power for a negatively charged \(As_2S_3\) sol?
- A. \(Na^+\)
- B. \(Ba^{2+}\)
- C. \(Al^{3+}\)Correct
- D. \(Cl^-\)
Explanation. According to the Hardy-Schulze rule, the coagulating power of an ion increases with its valency. For a negatively charged sol like arsenic sulphide, cations with higher charges, such as aluminum ions (\(Al^{3+}\)), are most effective at causing coagulation.
Q14
What is meant by the 'inversion of phase' in the study of emulsions?
- A. The complete separation of oil and water layers
- B. The change of a water-in-oil (W/O) emulsion into an oil-in-water (O/W) emulsionCorrect
- C. The conversion of an emulsion into a solid gel
- D. The process where the dispersed phase evaporates
Explanation. Inversion of phase occurs when the type of emulsion changes, such as a water-in-oil emulsion becoming an oil-in-water emulsion. This can be induced by adding specific electrolytes that change the nature of the emulsifying agent.
Q15
In the leather industry, what is the specific role of adding tannin to animal hides?
- A. To dissolve the hair present on the skin
- B. To coagulate the positively charged protein particles in the skinCorrect
- C. To act as a coloring agent for the final leather product
- D. To increase the water content of the hides
Explanation. Skin and hides contain positively charged protein particles. Tannin, which contains negatively charged particles, is used to coagulate these proteins. This process, known as tanning, hardens the skin and produces leather suitable for various industrial applications.
