CBSE Class 12 Chemistry (2026–27)
Chapter 4: Surface Chemistry
20 Important Questions and Answers
Surface Chemistry in the CBSE Class 12 syllabus covers adsorption, catalysis, colloids, and emulsions. These topics are specifically included in the current syllabus.
1. What is adsorption? How is it different from absorption?
Answer:
Adsorption is the phenomenon in which molecules of a substance accumulate on the surface of another substance, leading to a higher concentration at the surface than in the bulk. The substance being adsorbed is called the adsorbate, and the surface on which adsorption occurs is called the adsorbent. In absorption, the substance penetrates uniformly throughout the bulk of the absorbing material. For example, water vapour adsorbed on silica gel is adsorption, while water absorbed by a sponge is absorption. Adsorption is a surface phenomenon, whereas absorption is a bulk phenomenon. When both processes occur simultaneously, the process is known as sorption.
2. Distinguish between physisorption and chemisorption.
Answer:
Physisorption involves weak van der Waals forces between the adsorbate and adsorbent. It is generally reversible, occurs at low temperatures, and has low heat of adsorption. It may form multiple layers of adsorbate molecules. Chemisorption involves the formation of chemical bonds between the adsorbate and adsorbent. It is usually irreversible, occurs at higher temperatures, and has high heat of adsorption. Chemisorption forms only a single molecular layer. Physisorption decreases with an increase in temperature, whereas chemisorption initially increases with temperature due to activation energy requirements. These differences help identify the nature of adsorption in industrial and laboratory processes.
3. What are the factors affecting adsorption of gases on solids?
Answer:
Several factors influence the adsorption of gases on solid surfaces. The nature of the adsorbent is important; porous materials with larger surface areas show greater adsorption. Pressure also affects adsorption, as increasing pressure generally increases the amount of gas adsorbed. Temperature influences adsorption differently in physisorption and chemisorption. Since adsorption is usually exothermic, physisorption decreases with increasing temperature. The nature of the gas also matters; easily liquefiable gases are adsorbed more readily. Activation of adsorbents by heating or chemical treatment increases their surface area and adsorption capacity. These factors are widely utilized in gas masks, purification systems, and industrial adsorption processes.
4. Explain Freundlich adsorption isotherm.
Answer:
Freundlich adsorption isotherm is an empirical relationship that explains how the amount of gas adsorbed by a solid varies with pressure at constant temperature. It is represented by the equation:
\frac{x}{m}=kP^{1/n}
Here, x is the mass of gas adsorbed, m is the mass of adsorbent, P is pressure, and k and n are constants. The equation shows that adsorption increases with pressure but not proportionally. By taking logarithms, a straight-line relationship is obtained, helping determine the constants experimentally. Although useful for moderate pressures, the Freundlich isotherm fails at very high pressures because it predicts unlimited adsorption, which is not practically possible.
5. What is catalysis? Define catalyst.
Answer:
Catalysis is the process of increasing or decreasing the rate of a chemical reaction with the help of a substance called a catalyst. A catalyst alters the reaction rate without itself undergoing permanent chemical change. It provides an alternative reaction pathway with lower activation energy, making the reaction faster. Catalysts are widely used in industrial processes such as the Haber process, Contact process, and hydrogenation of oils. They increase efficiency, reduce energy consumption, and improve product yield. Catalysts may be solid, liquid, or gaseous depending on the reaction conditions. Their activity depends on surface area, temperature, and the presence of impurities.
6. Differentiate between homogeneous and heterogeneous catalysis.
Answer:
In homogeneous catalysis, the catalyst and reactants are present in the same phase. An example is the oxidation of sulphur dioxide by oxygen in the presence of nitric oxide gas. In heterogeneous catalysis, the catalyst and reactants are present in different phases. For example, hydrogenation of vegetable oils uses finely divided nickel as a solid catalyst while reactants are in liquid and gaseous phases. Homogeneous catalysts provide uniform mixing and easy interaction, whereas heterogeneous catalysts are easier to separate and reuse. Most industrial catalytic processes employ heterogeneous catalysts because of their practical advantages and economic benefits.
7. What is enzyme catalysis? Mention its characteristics.
Answer:
Enzyme catalysis is a biochemical process in which enzymes act as catalysts to speed up reactions occurring in living organisms. Enzymes are highly specific and usually catalyse only one type of reaction. They are extremely efficient and function under mild conditions of temperature and pH. Enzyme activity depends strongly on environmental conditions; excessive heat or unsuitable pH can denature the enzyme. Enzymes form temporary enzyme-substrate complexes that lower activation energy. Examples include the conversion of starch into glucose by amylase and decomposition of urea by urease. Due to their specificity and efficiency, enzymes are essential for metabolism and industrial biotechnology.
8. What are colloids? How do they differ from true solutions?
Answer:
A colloid is a heterogeneous system in which particles of one substance are dispersed in another medium. The particle size generally lies between 1 nm and 1000 nm. Unlike true solutions, colloidal particles do not settle under gravity and cannot be seen with the naked eye. True solutions are homogeneous, have particle sizes less than 1 nm, and pass through semipermeable membranes. Colloids scatter light due to the Tyndall effect and show Brownian movement. Examples include milk, fog, and blood. Because of their unique properties, colloids have applications in medicine, food technology, paints, and cosmetics.
9. What is the Tyndall effect?
Answer:
The Tyndall effect is the scattering of light by colloidal particles when a beam of light passes through a colloidal solution. Because colloidal particles are larger than molecules in true solutions, they scatter light and make the path of the beam visible. This phenomenon helps distinguish colloids from true solutions. Examples include sunlight passing through a dusty room or headlights visible in fog. The Tyndall effect depends on the size and concentration of colloidal particles as well as the difference in refractive indices between the dispersed phase and the dispersion medium. It has important applications in analytical chemistry and atmospheric studies.
10. Explain Brownian movement.
Answer:
Brownian movement is the continuous random zigzag motion of colloidal particles suspended in a dispersion medium. It occurs due to unequal bombardment of the colloidal particles by molecules of the dispersion medium. This motion prevents colloidal particles from settling under gravity and contributes to the stability of colloidal systems. The intensity of Brownian movement increases with temperature and decreases with increasing particle size. The phenomenon was first observed by Robert Brown while studying pollen grains in water. Brownian movement provides evidence for the kinetic theory of matter and is one of the characteristic properties of colloidal solutions.
11. What is electrophoresis?
Answer:
Electrophoresis is the movement of colloidal particles towards oppositely charged electrodes when an electric field is applied. Colloidal particles usually carry an electric charge due to selective adsorption of ions from the surrounding medium. Positively charged particles move towards the cathode, while negatively charged particles move towards the anode. Electrophoresis helps determine the nature and magnitude of charge on colloidal particles. It is widely used in laboratories for the separation and analysis of proteins, DNA fragments, and other biomolecules. This property also contributes to understanding the stability and behaviour of colloidal systems.
12. What is coagulation of colloids?
Answer:
Coagulation is the process of precipitating colloidal particles by removing their charge and reducing their stability. When electrolytes are added, oppositely charged ions neutralize the charge on colloidal particles, causing them to aggregate and settle. Coagulation can also be achieved by mixing oppositely charged sols or by prolonged electrophoresis. The effectiveness of an electrolyte in causing coagulation depends on the charge of the ion. Coagulation is important in water purification, rubber processing, and treatment of industrial waste. Understanding coagulation helps control the stability of colloidal systems in various industrial and environmental applications.
13. State the Hardy–Schulze rule.
Answer:
The Hardy–Schulze rule states that the coagulating power of an electrolyte depends on the valency of the ion carrying the charge opposite to that of the colloidal particles. The greater the valency of the oppositely charged ion, the greater is its ability to cause coagulation. For example, for negatively charged sols, the coagulating power of cations follows the order:
Al³⁺ > Ba²⁺ > Na⁺.
This rule explains why highly charged ions are more effective in destabilizing colloidal systems. It is widely used in water treatment, purification processes, and industrial operations involving colloidal materials.
14. What are lyophilic colloids?
Answer:
Lyophilic colloids are colloidal systems in which the dispersed phase has a strong affinity for the dispersion medium. These colloids are easily formed by direct mixing and are highly stable. They are reversible in nature, meaning that if the solvent is removed, the colloid can be reformed by adding the solvent again. Examples include starch, gelatin, gum, and proteins in water. Lyophilic colloids are less sensitive to electrolytes and act as protective colloids for lyophobic sols. Their stability and ease of preparation make them useful in pharmaceuticals, food products, and industrial formulations.
15. What are lyophobic colloids?
Answer:
Lyophobic colloids are colloidal systems in which the dispersed phase has little or no affinity for the dispersion medium. These colloids are difficult to prepare and require special methods. They are relatively unstable and can be easily coagulated by adding small amounts of electrolytes. Examples include sols of gold, sulphur, silver iodide, and arsenic sulphide. Unlike lyophilic colloids, they are irreversible in nature. Their stability depends mainly on the electric charge present on the colloidal particles. Lyophobic colloids are important in research, nanotechnology, and industrial applications where controlled colloidal behaviour is required.
16. Differentiate between lyophilic and lyophobic colloids.
Answer:
Lyophilic colloids have a strong attraction for the dispersion medium, whereas lyophobic colloids have little attraction. Lyophilic colloids are easy to prepare, highly stable, reversible, and less sensitive to electrolytes. In contrast, lyophobic colloids require special preparation methods, are comparatively unstable, irreversible, and easily coagulated by electrolytes. Examples of lyophilic colloids include starch and gelatin, while examples of lyophobic colloids include gold and sulphur sols. Lyophilic colloids often act as protective colloids, stabilizing lyophobic colloids against coagulation. These differences are important in understanding the behaviour and applications of colloidal systems.
17. What are multimolecular colloids?
Answer:
Multimolecular colloids are colloidal systems in which a large number of small molecules or atoms aggregate together to form particles of colloidal dimensions. These aggregates are held together by weak intermolecular forces such as van der Waals forces. The individual molecules are not large enough to form colloids on their own, but their combined aggregates behave as colloidal particles. Examples include sols of gold and sulphur. Multimolecular colloids are generally lyophobic in nature and require special methods for preparation. Their study helps explain the structure and behaviour of many naturally occurring and synthetic colloidal systems.
18. What are macromolecular colloids?
Answer:
Macromolecular colloids consist of very large molecules whose sizes themselves fall within the colloidal range. When dissolved in a suitable solvent, these molecules form stable colloidal solutions. Examples include proteins, starch, cellulose, nylon, and synthetic polymers. These colloids are generally lyophilic because of their strong interaction with the solvent. Macromolecular colloids possess high molecular masses and exhibit many properties of true solutions while still behaving as colloidal systems. They play a significant role in biological systems, plastics, textile industries, and pharmaceutical products due to their stability and unique physical properties.
19. What are emulsions? Give examples.
Answer:
Emulsions are colloidal systems in which one liquid is dispersed in another immiscible liquid. They are stabilized by substances called emulsifying agents. Emulsions are mainly of two types: oil-in-water (O/W) emulsions and water-in-oil (W/O) emulsions. Milk is an example of an oil-in-water emulsion, while butter and cold cream are examples of water-in-oil emulsions. Emulsifying agents such as soap and detergents prevent the dispersed droplets from coalescing. Emulsions are widely used in food products, medicines, cosmetics, paints, and agricultural formulations. Their stability is essential for maintaining product quality and effectiveness.
20. Mention important applications of colloids.
Answer:
Colloids have numerous applications in daily life and industry. In medicine, colloidal solutions are used for drug delivery and blood substitutes. In water purification, coagulation removes suspended impurities. Smoke precipitation in factories is achieved using electrical methods based on colloidal principles. Colloids are widely used in paints, inks, cosmetics, food products, and rubber industries. Milk, butter, creams, and many medicines are colloidal systems. Soil fertility is also influenced by colloidal particles present in the soil. The unique properties of colloids, such as stability, adsorption, and large surface area, make them valuable in scientific, industrial, and biological applications.