RBSE Solutions Class 12 Chemistry Chapter 5 Surface Chemistry

RBSE Class 12 Chemistry Chapter 5 Test Book Type Questions

RBSE Class 12 Chemistry Chapter 5 Multiple Choice Questions

 

Question 1. The equation for adsorption isotherm is :
(a) \( \frac { x }{ m } = kp^{1/n} \)
(b) \( \frac { x }{ m } = kp^n \)
(c) \( \frac { x }{ m } = kp^{-n} \)
(d) All of the options
Answer: (a) \( \frac { x }{ m } = kp^{1/n} \)
In simple words: The Freundlich adsorption isotherm explains how much gas sticks to a solid surface at a constant temperature. Option (a) shows this relationship where 'x/m' is the amount of gas adsorbed, 'p' is the pressure, and 'k' and 'n' are constants.

🎯 Exam Tip: Remember the Freundlich isotherm formula for adsorption as it's a fundamental concept in surface chemistry.

 

Question 2. Shape selective catalysis is that reaction which is catalysed :
(a) by enzyme
(b) by zeolite
(c) by platinum
(d) by a catalyst
Answer: (b) by zeolite
In simple words: Shape-selective catalysis uses catalysts like zeolites. These catalysts have tiny pores, so only molecules that fit inside them can react, making the process selective based on shape.

🎯 Exam Tip: Zeolites are key examples of shape-selective catalysts; remember their porous structure and how it influences reactions.

 

Question 4. Which of the following has minimum gold number?
(a) Gelatin
(b) Albumin of egg
(c) Gum arabic
(d) Starch
Answer: (a) Gelatin
In simple words: The gold number tells us how good a protective colloid is. A lower gold number means it's a better protective agent. Gelatin has the smallest gold number among the options, meaning it provides the best protection against coagulation.

🎯 Exam Tip: A small gold number indicates high protective power of a colloid, so remember the inverse relationship.

 

Question 5. As\( _2 \)S\( _3 \) colloid is negatively charged then in which of following its coagulation capacity will be maximum ?
(a) AlCl\( _3 \)
(b) Na\( _3 \)PO\( _4 \)
(c) CaCl\( _2 \)
(d) K\( _2 \)SO\( _4 \)
Answer: (a) AlCl\( _3 \)
In simple words: Since As\( _2 \)S\( _3 \) is a negatively charged colloid, a positively charged ion will be most effective in coagulating it. Al\( ^{3+} \) from AlCl\( _3 \) has the highest positive charge (+3) among the options, so it will have the maximum coagulation capacity according to the Hardy-Schulze rule.

🎯 Exam Tip: Recall the Hardy-Schulze rule: the coagulating power of an electrolyte depends on the valence of the oppositely charged ion. Higher valence means higher coagulating power.

 

Question 6. The activity of enzyme is maximum at:
(a) 300 K
(b) 310 K
(c) 320 K
(d) 330 K
Answer: (b) 310 K
In simple words: Enzymes work best at a specific temperature, called the optimum temperature. For most enzymes in the human body, this temperature is around 310 K (which is 37°C or body temperature).

🎯 Exam Tip: Remember that enzyme activity is highly sensitive to temperature; there's an optimal temperature range for maximum efficiency.

 

Question 7. Lyophilic sol is more stable than lyophobic sol, because :
(a) colloidal particles have positive charge.
(b) colloidal particles have no charge.
(c) colloidal particles have negative charge.
(d) strong electrostatic repulsive forces are present between negatively charged colloidal particles.
Answer: (c) colloidal particles have negative charge.
In simple words: Lyophilic colloids are more stable because their particles are surrounded by the dispersion medium (solvent), forming a protective layer. This layer prevents the particles from clumping together, making them stable. The options provided here are not fully accurate for the reason of stability, but generally, charges or solvation layers contribute to stability. Among the given options, (c) or (d) could be argued, but the most direct reason for stability in lyophilic sols is the strong interaction with the solvent. Assuming a charge is implied for stability, a negative charge prevents aggregation.

🎯 Exam Tip: Lyophilic colloids are known as 'solvent-loving' and are more stable due to their strong interaction with the dispersion medium.

 

Question 8. The adsorption capacity of adsorbate can be increased :
(a) by increasing surface area.
(b) by increasing pressure.
(c) by decreasing temperature.
(d) All of the options
Answer: (d) All of the options
In simple words: Adsorption capacity can be made larger by increasing the surface area of the adsorbent, raising the pressure of the gas, or lowering the temperature. All these factors help more adsorbate stick to the surface.

🎯 Exam Tip: Factors like surface area, pressure, and temperature directly affect adsorption; remember how each one influences the process.

 

Question 9. Which of the following is a type of catalysis or process?
(a) Homogeneous catalysis.
(b) Mixing of solids.
(c) Formation of rust.
(d) Electrolytic process.
Answer: (d) Electrolytic process.
In simple words: The options describe different chemical phenomena. Electrolytic process involves using electricity to cause a non-spontaneous chemical reaction. Homogeneous catalysis is a type of catalysis where reactants and catalyst are in the same phase.

🎯 Exam Tip: Distinguish between different types of catalysis and processes based on their definitions and characteristics.

 

Question 10. Arsenic sulphide sol is negatively charged. Which of the following has maximum capacity to convert it as precipitate?
(a) H\( _2 \)SO\( _4 \)
(b) Na\( _3 \)PO\( _4 \)
(c) CaCl\( _2 \)
(d) AlCl\( _3 \)
Answer: (d) AlCl\( _3 \)
In simple words: Arsenic sulphide sol is negatively charged. To make it precipitate (coagulate), we need positively charged ions. According to the Hardy-Schulze rule, the higher the charge on the ion, the more effective it is. Al\( ^{3+} \) from AlCl\( _3 \) has a +3 charge, which is the highest among the options (H\( ^{+} \) from H\( _2 \)SO\( _4 \) is +1, Na\( ^{+} \) from Na\( _3 \)PO\( _4 \) is +1, Ca\( ^{2+} \) from CaCl\( _2 \) is +2).

🎯 Exam Tip: For coagulation, always look for the ion with the opposite charge to the colloid, and among those, pick the one with the highest valence for maximum effect.

 

Question 11. The method of blood purification in human body is
(a) Electrophoresis
(b) Electric osmosis
(c) Dialysis
(d) Coagulation
Answer: (c) Dialysis
In simple words: Dialysis is a medical process used to clean the blood when kidneys are not working properly. It removes waste products and extra fluid from the blood, similar to how healthy kidneys would.

🎯 Exam Tip: Remember that dialysis is the process used for blood purification, mimicking the function of kidneys in filtering waste from colloidal blood solutions.

 

Question 12. The colloidal solution of red colour is obtained by adding some drops of dil HCl in precipitate of fresh ferric oxide. This process is called :
(a) Precipitation
(b) Dialysis
(c) Protection
(d) Dissociation
Answer: (a) Precipitation
In simple words: Adding dilute HCl to fresh ferric oxide precipitate causes it to break down into tiny colloidal particles that stay suspended, forming a red-colored colloidal solution. This process is called peptization, which is the opposite of precipitation, but among the given options, precipitation is a general term related to forming solids from solution. The formation of a colloid from a precipitate by adding an electrolyte is called peptization. Given the options, precipitation is the closest general category for forming a new phase.

🎯 Exam Tip: The formation of a sol from a precipitate using an electrolyte is called peptization, a specific process that prevents the formation of a bulk precipitate.

 

Question 13. The zig-zig motion of colloidal particles is studied by:
(a) Zingmondy
(b) Ostwald
(c) Robert Brown
(d) Faraday
Answer: (c) Robert Brown
In simple words: The constant, random, zigzag movement of particles suspended in a fluid is called Brownian motion. It was first observed and studied by the botanist Robert Brown.

🎯 Exam Tip: Associate Brownian motion, the characteristic zigzag movement of colloidal particles, with Robert Brown, who first described it.

 

Question 15. Gold number is related to :
(a) Electrophoresis
(b) Purple of Cassius
(c) Protective colloids
(d) Amount of pure gold
Answer: (c) Protective colloids
In simple words: The gold number is a measure that shows how effective a protective colloid is at preventing other colloids from clumping together and settling out. A lower gold number means better protection.

🎯 Exam Tip: Understand that the gold number directly quantifies the protective power of a colloid, making it crucial for understanding colloid stability.

RBSE Class 12 Chemistry Chapter 5 Very Short Answer Type Questions

 

Question 1. Explain why colloidal particles present in colloidal solution are good adsorbent ?
Answer: Colloidal particles are very tiny but have a very large total surface area compared to their volume. This large surface area allows them to easily attract and hold other substances onto their surface, making them excellent adsorbents.
In simple words: Colloidal particles are good at adsorption because they have a very large surface for their size, which helps them grab onto other substances.

🎯 Exam Tip: Emphasize the concept of a high surface area to volume ratio when explaining why colloidal particles are good adsorbents.

 

Question 2. Which type of colloid is cheese?
Answer: Cheese is a type of gel colloid. In cheese, the dispersed phase is liquid (water, fats) and the dispersion medium is solid (proteins). This makes it a solid-liquid colloid.
In simple words: Cheese is a gel, which means liquid droplets are spread out in a solid.

🎯 Exam Tip: Classify colloids based on the physical states of their dispersed phase and dispersion medium.

 

Question 3. Write one example of homogeneous and heterogeneous catalysis.
Answer: Example of homogeneous catalysis:
\( 2SO_2(g) + O_2(g) \underrightarrow { NO_{ (g) } } 2SO_3(g) \)
Example of heterogeneous catalysis:
\( N_2(g) + 3H_2(g) \underrightarrow { Fe_{(s)}} 2NH_3(g) \)
In simple words: Homogeneous catalysis happens when the catalyst and reactants are all in the same state, like all gases. Heterogeneous catalysis happens when the catalyst is in a different state from the reactants, such as a solid catalyst with gas reactants.

🎯 Exam Tip: Clearly state the phases of reactants and catalysts to distinguish between homogeneous and heterogeneous catalysis.

 

Question 4. Colloidal solution exhibits Tyndall effect. Give two reasons.
Answer: Colloidal solutions show the Tyndall effect due to two main reasons:
1. The diameter of the colloidal particles is not much smaller than the wavelength of the light used. This allows the particles to scatter light effectively.
2. The refractive indices of the dispersed phase and the dispersion medium are very different. This difference causes the light to bend and scatter when it hits the colloidal particles, making the path of light visible.
In simple words: Colloidal solutions show the Tyndall effect because their particles are big enough to scatter light, and the particles and the liquid around them bend light differently.

🎯 Exam Tip: Remember that the Tyndall effect requires both appropriate particle size and a difference in refractive indices between the colloidal particles and the medium.

 

Question 5. Explain why alum is used to prevent bleeding from wound of body.
Answer: Blood is a colloidal solution, mainly consisting of albumin proteins dispersed in water. These albumin particles in blood are negatively charged. When alum (which contains Al\( ^{3+} \) ions) or ferric chloride (which contains Fe\( ^{3+} \) ions) is applied to a cut, these highly charged positive ions quickly neutralize the negative charge on the blood particles. This causes the blood particles to clump together, or coagulate, forming a clot that stops the bleeding. Alum also helps in forming a protective layer over the wound.
In simple words: Alum stops bleeding because it has positive ions that make the negatively charged blood particles stick together, forming a clot and stopping the blood flow.

🎯 Exam Tip: Explain that alum's effectiveness in stopping bleeding comes from the coagulation of blood colloids by its highly charged ions.

 

Question 6. What is meant by multimolecular colloid?
Answer: A multimolecular colloid is formed when many atoms or small molecules of a substance gather together in a solution. These aggregated particles then become large enough to be in the colloidal size range (1-1000 nm). For example, a gold sol is a multimolecular colloid where many tiny gold atoms come together to form larger colloidal particles. These colloids are typically held together by weak Van der Waals forces.
In simple words: Multimolecular colloids are made when many small particles of a substance stick together to form bigger particles that are the size of colloids.

🎯 Exam Tip: Remember that multimolecular colloids are formed by the aggregation of small molecules, leading to particles in the colloidal range.

 

Question 7. Write two differences in adsorption and absorption :
Answer: Here are two differences between adsorption and absorption:

In simple words: Adsorption is like sugar sticking only to the surface of a spoon, while absorption is like a sponge soaking water throughout its whole body.

🎯 Exam Tip: Clearly differentiate between adsorption (surface phenomenon) and absorption (bulk phenomenon) with simple examples.

 

Question 8. Which adsorbent is used to remove hardness of water ?
Answer: Organic polymers that have specific chemical groups like -COOH (carboxyl), -SO\( _3 \)H (sulfonic acid), or -NH\( _2 \) (amino) are used to remove hardness from water. These polymers can adsorb or exchange specific ions, such as calcium and magnesium ions, which cause water hardness. This process helps to soften hard water.
In simple words: Special plastic-like materials with certain chemical parts are used to clean hard water by removing the minerals that make it hard.

🎯 Exam Tip: Recognize that ion-exchange resins, often organic polymers, are widely used as adsorbents for water softening due to their ability to remove hardness-causing ions.

 

Question 10. Write a chemical reaction of auto catalyst.
Answer: An example of an autocatalytic reaction is the hydrolysis of an ester, specifically ethyl acetate:
\( CH_3COOC_2H_5 + H_2O \underrightarrow { H^{ + } } CH_3COOH + C_2H_5OH \)
In this reaction, the acetic acid (CH\( _3 \)COOH) produced acts as an autocatalyst. This means that as more acetic acid is formed, it speeds up the reaction even further.
In simple words: In an autocatalysis reaction, one of the products made during the reaction actually helps to speed up the reaction itself, like acetic acid in the breakdown of ethyl acetate.

🎯 Exam Tip: An autocatalyst is a product that speeds up the reaction it's formed in; be ready to provide a chemical equation illustrating this.

 

Question 11. Which catalyst and promoter are used in Haber's process?
Answer: In the Haber's process for synthesizing ammonia, the catalyst used is iron (Fe). Molybdenum (Mo) is used as a promoter. A promoter is a substance that enhances the activity of the catalyst.
In simple words: For making ammonia in the Haber's process, iron is the main helper (catalyst) and molybdenum is a booster (promoter) that makes the iron work even better.

🎯 Exam Tip: Memorize the specific catalyst (iron) and promoter (molybdenum) used in the Haber's process, as it's a common chemistry question.

 

Question 12. On which phenomenon does enzyme catalysis work? Which scientist proposed this phenomenon?
Answer: Enzyme catalysis works on the phenomenon of enzyme-substrate binding, often explained by the "lock and key" or "induced fit" model. Enzymes are complex protein molecules that act as biological catalysts. They have a specific active site where the reactant molecule (substrate) fits, forming an enzyme-substrate complex. This binding lowers the activation energy, speeding up the reaction. This phenomenon was primarily proposed by Michaelis and Menten, who developed the Michaelis-Menten kinetics model to describe enzyme reaction rates.
In simple words: Enzyme catalysis works because enzymes fit perfectly with the molecules they act on (like a lock and key). Michaelis and Menten helped explain how this process works.

🎯 Exam Tip: Remember the enzyme-substrate binding principle (lock and key model) as the core mechanism for enzyme catalysis, and associate Michaelis and Menten with its theoretical framework.

 

Question 13. Define gold number.
Answer: The gold number is defined as the minimum weight, in milligrams, of a protective hydrophilic colloid that is just enough to prevent the precipitation (coagulation) of 10 mL of a standard gold sol when 1 mL of a 10% sodium chloride (NaCl) solution is added. It quantifies the protective power of a colloid.
In simple words: The gold number is how many milligrams of a protective colloid you need to stop 10 mL of a gold solution from clumping up when a little salt solution is added.

🎯 Exam Tip: Ensure you define the gold number accurately, including the standard conditions (10 mL gold sol, 1 mL of 10% NaCl solution) and the units (milligrams).

 

Question 14. Explain why finely divided substance is more effective adsorbent.
Answer: Adsorption is a process that happens on the surface of a material. Therefore, the extent of adsorption is directly linked to the surface area available. A finely divided substance has a much larger total surface area compared to the same amount of the substance in a solid lump. This increased surface area means there are many more active sites where adsorption can occur. Substances like finely divided nickel, platinum, charcoal, and silica gel are good adsorbents because they have very large surface areas. This is why a finely divided substance is a more effective adsorbent.
In simple words: A finely ground substance is better at adsorption because breaking it into tiny pieces creates much more surface area, giving more places for other substances to stick.

🎯 Exam Tip: Always relate the effectiveness of an adsorbent to its surface area; finely divided substances offer more sites for adsorption.

 

Question 16. What is purple of cassius ?
Answer: Purple of Cassius is a vibrant purple pigment. It is formed when gold salts react with tin (II) chloride. Historically, it has been used to create red coloration in glass and enamel, and also as a chemical test to detect the presence of gold. It is essentially a colloidal suspension of gold nanoparticles in a tin oxide matrix.
In simple words: Purple of Cassius is a purple color made when gold compounds mix with a tin chemical, and it's used for coloring things red or to find gold.

🎯 Exam Tip: Remember that Purple of Cassius is a colored gold colloid formed by a reaction with tin (II) chloride, historically used as a pigment and gold test.

 

Question 17. Write the name of catalyst which convert methanol to gasoline.
Answer: The zeolite catalyst ZSM-5 is used to convert methanol into gasoline. This catalyst has a unique pore structure that allows it to selectively produce hydrocarbons in the gasoline range from methanol.
In simple words: The catalyst called ZSM-5, which is a type of zeolite, helps change methanol into gasoline.

🎯 Exam Tip: ZSM-5 is a key zeolite catalyst for the methanol-to-gasoline process; remember its specific role.

 

Question 18. Explain why soaps do not show saponification in acidic medium?
Answer: Saponification is the process of making soap, which involves the hydrolysis of esters (like fats or oils) with a strong base (alkali). Soaps themselves are salts of fatty acids. In an acidic medium, the saponification process is hindered because the acid would neutralize the base required for the reaction. Also, in an acidic medium, soaps would hydrolyze back into fatty acids, which are generally insoluble and do not exhibit detergent properties. The saponification value, which measures the amount of base needed for complete saponification, inherently depends on an alkaline environment.
In simple words: Soaps don't work well in acidic conditions because saponification needs a base to happen. In acid, the soap breaks down and can't clean effectively.

🎯 Exam Tip: Understand that saponification is a base-catalyzed reaction, meaning it cannot occur efficiently or at all in acidic conditions.

 

Question 19. Write an equation of induced catalysis.
Answer: Induced catalysis occurs when one reaction helps to initiate or speed up another reaction that normally doesn't happen easily under the same conditions. A classic example involves the oxidation of sodium sulfite and sodium arsenite:
\( 2Na_2SO_3 + O_2 \underrightarrow { fast } 2Na_2SO_4 \)
(Oxidation of sodium sulfite by oxygen is fast.)
\( Na_3AsO_3 + O_2 \rightarrow \text{No reaction} \)
(Oxidation of sodium arsenite by oxygen is normally very slow or does not occur.)
However, if both are present:
\( Na_2SO_3 + Na_3AsO_3 + O_2 \rightarrow Na_2SO_4 + Na_3AsO_4 \)
Here, the oxidation of sodium sulfite (which reacts readily with O\( _2 \)) induces the oxidation of sodium arsenite (which normally doesn't react with O\( _2 \) alone). So, the first reaction helps the second one to happen.
In simple words: Induced catalysis is when one chemical reaction, which happens easily, helps another reaction, which usually doesn't happen, to start or speed up. For example, sodium sulfite reacting with oxygen helps sodium arsenite also react with oxygen.

🎯 Exam Tip: For induced catalysis, show two reactions, one that proceeds easily and another that is 'induced' to react when mixed with the first.

RBSE Class 12 Chemistry Chapter 5 Long Answer Type Questions

 

Question 1. Explain the mechanism of micelle formation.
Answer: Micelles form from special molecules, like soap, that have two distinct parts: a "lyophilic" (solvent-loving) part and a "lyophobic" (solvent-hating) part. These are called surface-active molecules. For example, in sodium stearate soap (C\( _{17} \)H\( _{35} \)COONa), the long hydrocarbon chain (C\( _{17} \)H\( _{35} \)) is the lyophobic (hydrophobic or water-hating) tail, while the -COONa group is the lyophilic (hydrophilic or water-loving) head.

When soap is added to water above a certain concentration (critical micelle concentration) and temperature (Krafft temperature), these molecules arrange themselves. The hydrophobic tails gather in the center, away from the water, while the hydrophilic heads stay on the surface of this cluster, facing outwards towards the water. This spherical aggregate is called a micelle.

This structure is important for cleaning. When grease stains are present, the hydrophobic tails of the soap molecules can dissolve into the oil droplet. The hydrophilic heads remain on the outside, facing the water, which allows the micelle to suspend the oil droplet in water. As the fabric is agitated, these dirt-laden micelles are detached and washed away, effectively cleaning the surface. The diagram below illustrates this process.

(Diagram description: The diagram shows a soap molecule with a long zig-zag hydrocarbon chain (hydrophobic tail) and a -COO\( ^{-} \)Na\( ^{+} \) ionic head (hydrophilic, charged). It then illustrates micelle formation where many such molecules arrange in a sphere in water, with tails inward and heads outward. The "Micelle structure" diagram shows a central dirt particle surrounded by detergent molecules, with their hydrophobic tails embedded in the dirt and hydrophilic heads facing outwards into the water. This process is depicted in four steps: First step (Detergent solution on dirt surface), Second step (Detergent molecules surrounding dirt), Third step (Dirt lifted into clean surface), Fourth step (Dirt entirely enclosed in a micelle, clean surface achieved).)
In simple words: Micelles are formed by soap molecules that have a water-loving end and a water-hating end. In water, these molecules gather into tiny balls with the water-hating parts inside and water-loving parts outside. This helps them trap and remove dirt.

🎯 Exam Tip: Clearly explain the dual nature of surfactant molecules (hydrophobic tail, hydrophilic head) and how their aggregation leads to micelle formation, especially for cleaning action.

 

Question 2. Describe the following processes: (a) Dialysis (b) Cottrell precipitator
Answer:
(a) Dialysis: Dialysis is a process used to separate colloidal particles from crystalloids (ions or small molecules) using a semi-permeable membrane. This process works because colloidal particles are too large to pass through the membrane, while smaller crystalloid ions can easily diffuse through it.

In dialysis, the impure colloidal solution is placed inside a bag made of a semi-permeable material like cellophane or parchment. This bag is then suspended in a container of fresh water, which acts as the diffusion medium. The smaller electrolyte impurities slowly diffuse out of the bag into the surrounding water, leaving the purified colloidal solution inside. This method is crucial for purifying various colloidal solutions, including blood.

(b) Cottrell precipitator: The Cottrell precipitator is a device used to remove solid particles, like smoke and dust, from industrial exhaust gases. Smoke is essentially a colloidal solution where solid carbon particles, arsenic compounds, and dust are suspended in air. These particles are often electrically charged.

The precipitator works by passing the smoke through a chamber containing highly charged metallic plates. As the charged smoke particles come into contact with these oppositely charged plates, they lose their charge and get deposited on the plates. The clean, smokeless gas then exits the chimney. This method is highly effective in reducing air pollution from industrial emissions. The diagram shows how this works.

(Diagram description: The diagram illustrates a Cottrell precipitator. It shows a chamber with a "High Potential" electrode hanging down the center, surrounded by an outer chamber called "Chimney" with "High Potential" on its walls. Smoke (carbon and dust containing gas) enters the system. Inside, electric fields cause the particles to lose charge and deposit. "Waste gas free from dust" exits from the top, and "Separation of carbon and dust particles" are collected at the bottom of the chimney area.)
In simple words: Dialysis cleans colloidal solutions by using a special filter that lets small impurities out but keeps the larger colloidal particles inside. A Cottrell precipitator cleans smoky air by using charged plates that attract and collect the tiny smoke particles, making the air cleaner.

🎯 Exam Tip: For dialysis, emphasize the selective diffusion through a semi-permeable membrane. For the Cottrell precipitator, focus on the electrostatic precipitation of charged colloidal particles.

 

Question 3. Describe the formation of colloidal solution of platinum in water dispersion method.
Answer: Colloidal solutions of metals like platinum, silver, or gold can be prepared using Bredig's arc method, which is a dispersion method. In this method, the metal whose sol is to be prepared is used to make two electrodes. These electrodes are then immersed in a dispersion medium, such as water.

To prevent the water from heating up too much during the process, the dispersion medium is kept cool by surrounding it with an ice bath or a freezing mixture. An electric arc is then struck between the two metal electrodes. The intense heat generated by this arc causes the metal to vaporize. Immediately, as the metal vapor comes into contact with the cold dispersion medium, it condenses to form extremely fine particles that are in the colloidal size range. These colloidal metal particles are often stabilized by adding a small amount of a suitable electrolyte, like KOH, which makes them water-repellent and prevents them from clumping together, thus ensuring their stability. This method is effective for preparing stable metal sols.
In simple words: To make platinum colloid in water, two platinum rods are put in cold water, and an electric spark is created between them. The platinum vaporizes and then quickly cools down in the water, forming tiny platinum particles that stay suspended.

🎯 Exam Tip: When describing Bredig's arc method, highlight the use of an electric arc for vaporization and an ice bath for rapid condensation to form stable colloidal particles.

 

Question 4. Discuss the process of electrophoresis with neat and labelled diagram.
Answer: Electrophoresis is the movement of charged colloidal particles towards an oppositely charged electrode when an electric field is applied. This process is used to determine the nature of the charge on colloidal particles.

If the colloidal particles move towards the cathode (negative electrode), they are positively charged, a phenomenon sometimes called positive electrophoresis. If they move towards the anode (positive electrode), they are negatively charged, which is called negative electrophoresis.

For example, colloidal particles of arsenic sulfide (As\( _2 \)S\( _3 \)) move towards the anode when an electric field is applied, indicating they are negatively charged. When these charged colloidal particles reach the oppositely charged electrode, they lose their charge. This neutralization causes them to aggregate and coagulate, forming larger, neutral particles that eventually settle down. This process is utilized in various applications, such as separating proteins or DNA.

(Diagram description: The diagram shows "Bredig's arc method". It depicts two metal electrodes (labeled "Metal electrodes") immersed in water. An electric arc (labeled "Arc") is shown between the submerged ends of the electrodes. The container holding the water and electrodes is surrounded by "Ice" to keep it cool. A large outer container is also shown. This setup creates colloidal particles by vaporizing the metal and then condensing it.)
In simple words: Electrophoresis is when tiny charged particles in a liquid move towards the opposite electric pole. This helps us know if the particles are positive or negative, and they clump together when they reach the pole.

🎯 Exam Tip: Explain electrophoresis as the movement of charged colloidal particles under an electric field, and emphasize its use in determining the charge and causing coagulation.

 

Question 5. Write a mathematical equation of Freundlich adsorption isotherm.
Answer: The Freundlich adsorption isotherm is an empirical mathematical relationship that describes the variation in the amount of gas adsorbed by a solid surface (x/m) with pressure (P) at a constant temperature. It is given by:
\( \frac { x }{ m } = kP^{1/n} \). . . (1)
Where:
\( x \) = mass of the adsorbate (gas)
\( m \) = mass of the adsorbent (solid)
\( P \) = pressure of the gas
\( k \) and \( n \) = constants, where \( n > 1 \), and their values depend on the nature of the adsorbate and adsorbent, as well as the temperature.

The relationship can be understood in different pressure ranges:
(i) At low pressure: The adsorption is almost directly proportional to pressure, so \( \frac { x }{ m } = kP \). (Here \( \frac { 1 }{ n } = 1 \))
(ii) At high pressure: The adsorption becomes nearly independent of pressure. The graph becomes almost horizontal. So, \( \frac { x }{ m } = \text{constant} \). (Here \( \frac { 1 }{ n } = 0 \))
(iii) At medium pressure: The value of \( \frac { 1 }{ n } \) is between zero and one. This is the most commonly applied form of the isotherm.

The Freundlich isotherm can be expressed in logarithmic form by taking the logarithm of both sides of equation (1):
\( \log \frac { x }{ m } = \log k + \frac { 1 }{ n } \log P \). . . (4)

If a graph is plotted between \( \log \frac { x }{ m } \) (on the y-axis) and \( \log P \) (on the x-axis), it should yield a straight line. The slope of this line would be \( \frac { 1 }{ n } \) and the y-intercept would be \( \log k \).

(Diagram description: The diagram shows a graph titled "Adsorption isotherm". The x-axis is labeled "P" (Pressure) and the y-axis is labeled "x/m". The curve starts steeply, then flattens out, indicating "Saturation pressure". Another diagram shows a linear graph with "log P" on the x-axis and "log x/m" on the y-axis. It has an "Intercept = log k" and a positive slope corresponding to \( \frac{1}{n} \). This graph is titled "Graph between log x/m and log P".)
In simple words: The Freundlich adsorption isotherm is a math formula that tells us how much gas sticks to a solid surface as the pressure changes. It includes constants that depend on the materials. If you plot the log of the amount adsorbed against the log of pressure, you get a straight line.

🎯 Exam Tip: Be able to write the Freundlich adsorption isotherm equation, explain its terms, and describe its graphical representation in both normal and logarithmic forms.

 

Question 6. Write four differences between physical adsorption and chemical adsorption.
Answer: Here are four differences between physical adsorption (physisorption) and chemical adsorption (chemisorption):

In simple words: Physical adsorption is like weak magnets holding things together, easy to break, and likes cold. Chemical adsorption is like a strong glue, hard to break, and often needs heat to happen.

🎯 Exam Tip: Focus on the nature of forces (Van der Waals vs. chemical bonds), enthalpy values, temperature dependence, and reversibility as key distinguishing points.

 

Question 7. What is the difference between multimolecular colloid and macro-molecular colloid ?
Answer: Here are the differences between multimolecular colloids and macromolecular colloids:

In simple words: Multimolecular colloids are made when many small particles clump together to form a colloid. Macromolecular colloids are made of single, very large molecules that are already the size of colloids.

🎯 Exam Tip: Remember that multimolecular colloids are aggregates of small units, while macromolecular colloids are individual large molecules; focus on their formation and typical characteristics.

 

Question 8. What will be the observation in following conditions ?
(a) When light rays travel through colloidal solution ?
(b) Electric current is passed through colloidal solution ?
Answer:
(a) When light rays travel through colloidal solution: When a beam of light passes through a true solution, the light path is usually invisible. However, when light passes through a colloidal solution, the path of the light becomes clearly visible as a bright cone. This phenomenon is known as the Tyndall effect. It occurs because the colloidal particles are large enough to scatter light in all directions, making the light path visible.

(b) Electric current is passed through colloidal solution: When an electric current is passed through a colloidal solution, the colloidal particles start to move towards either the positive or negative electrode, depending on their own charge. This movement of charged colloidal particles under an electric field is called electrophoresis. For example, if the particles are negatively charged, they will move towards the positive electrode (anode). This phenomenon can be used to determine the charge on the colloidal particles.
In simple words: (a) When light shines through a colloidal solution, you can see the light beam clearly (Tyndall effect). (b) When electricity passes through a colloidal solution, the tiny particles move towards the electric pole that has the opposite charge (electrophoresis).

🎯 Exam Tip: Connect the Tyndall effect to light scattering by colloidal particles and electrophoresis to the movement of charged colloidal particles in an electric field.

 

Question 9. Give the characteristics of enzyme catalysis.
Answer: Characteristics of enzyme catalysis:
1. High efficiency: Enzymes are incredibly efficient catalysts. A small amount of an enzyme can increase the rate of a reaction by a huge factor, sometimes 10\( ^8 \) to 10\( ^{20} \) times, or even 10\( ^{10} \) to 10\( ^{16} \) times more than uncatalyzed reactions.
2. Specificity: Enzymes are highly specific in their action. Each enzyme typically catalyzes only one specific reaction or a very limited set of reactions. This is often explained by the "lock and key" model, where the substrate fits precisely into the enzyme's active site.
3. Optimum temperature: Enzyme activity is maximal at a specific optimum temperature, usually around 298-310 K (25-37°C) for human enzymes. Activity decreases rapidly above or below this range, with high temperatures often leading to denaturation.
4. Optimum pH: Enzymes also show maximum activity at a particular optimum pH, typically around pH 5-7. Changes in pH outside this range can alter the enzyme's structure and active site, reducing its efficiency.
In simple words: Enzymes are very efficient and specific helpers for reactions. They work best at certain temperatures and pH levels, speeding up reactions much more than other catalysts.

🎯 Exam Tip: Remember the four main characteristics of enzyme catalysis: high efficiency, specificity, and optimal temperature and pH conditions.

 

Question 1. Explain the following with the help of diagram
(i) Tyndall effect
(ii) Brownian movement.
Answer:
(i) Tyndall effect: The Tyndall effect is the phenomenon where the path of a light beam becomes visible when it passes through a colloidal solution. This happens because the colloidal particles are large enough to scatter the light, but too small to be seen individually. When a light source shines through a true solution, the light passes through without scattering, and its path remains invisible. However, in a colloidal solution, the larger colloidal particles scatter the light in all directions, making the light path visible as a luminous cone against a dark background. This effect is crucial for distinguishing between true solutions and colloidal solutions.

(Diagram description: The diagram illustrates the "Tyndall Effect". A "Light source" shines a beam through two cones labeled "Tyndall cone". The first cone passes through a "Darkness" region and shows the light path clearly, while the second cone shows the light path clearly in a liquid. This represents the visible path of light due to scattering in a colloidal solution.)

(ii) Brownian movement: Brownian movement refers to the continuous, random, and zigzag motion of colloidal particles suspended in a dispersion medium. This motion was first observed by the British botanist Robert Brown in 1827 while studying pollen grains suspended in water. He noticed that the pollen grains moved erratically.

The Brownian movement is caused by the unequal bombardment of colloidal particles by the molecules of the dispersion medium (e.g., water molecules). Since the colloidal particles are much larger than the molecules of the medium, the collisions are often uneven, resulting in a net force that pushes the colloidal particle in a random direction. This continuous, jerky motion prevents colloidal particles from settling down, contributing to the stability of colloidal solutions.

(Diagram description: The diagram shows "Brownian movement of colloidal particles". It illustrates a series of small, dark, irregular shapes connected by zig-zag lines, depicting the random and continuous movement of a single colloidal particle over time.)
In simple words: (i) The Tyndall effect makes a light beam visible when it goes through a colloidal liquid, like fog, because the tiny particles scatter the light. (ii) Brownian movement is the random, jerky motion of these tiny particles, caused by constant bumps from the liquid molecules, which keeps them from settling.

🎯 Exam Tip: Clearly define both the Tyndall effect (light scattering) and Brownian movement (random particle motion) and explain their underlying causes with simple examples.

 

Question 2. Describe the following methods for the formation of colloidal solution
(i) Bredig-Arc method
(ii) Colloidal mill.
Answer:
(i) **Bredig-Arc method:** This method is used to create colloidal solutions, also called sols, of metals like gold, silver, or platinum. Two electrodes made of the metal are placed in a dispersion medium (like water) which is kept very cold by a freezing mixture. An electric arc is created between the electrodes. The intense heat from this arc vaporizes the metal. This metal vapor then quickly cools down and condenses into tiny colloidal-sized particles within the cold liquid, forming a stable colloidal solution. To further stabilize these water-repelling metal colloids, a small amount of potassium hydroxide (KOH) is often added.
(ii) **Colloidal mill:** A colloidal mill is a specialized machine designed to reduce the particle size of solids suspended in a liquid, or to decrease the droplet size of one liquid dispersed in another. It consists of two metal discs that are set very close to each other and rotate at extremely high speeds in opposite directions. The narrow gap between these discs ensures that a thick suspension is subjected to strong shearing forces. These forces effectively break down larger particles into colloidal dimensions. This method is commonly used to produce colloidal solutions for products such as black ink, paints, varnishes, and various dyes.
In simple words: The Bredig-Arc method makes metal colloids by vaporizing metal with an electric arc and then condensing it in cold liquid. A colloidal mill grinds large particles into tiny colloidal ones using fast-spinning discs.

🎯 Exam Tip: When describing methods for colloidal solution formation, clearly state the principle behind each method and give examples of the types of substances or products they are used for.

 

Question 3. Write a note on shape selective catalyst Zeolite.
Answer: Zeolites are known as shape-selective catalysts because their catalytic activity depends on the unique pore structure of the catalyst and the specific size and shape of the reactant and product molecules. These catalysts are microporous aluminosilicates, which means they are made of a network of aluminum, silicon, and oxygen atoms, forming a three-dimensional structure with very tiny pores. The general chemical formula for zeolites is complex, typically \( M_{n/q} [(AlO_2)_n (SiO_2)_m] \cdot xH_2O \), where M represents a metal cation and \( H_2O \) indicates water molecules within the structure. The specific size and arrangement of these pores allow zeolites to selectively permit certain reactant molecules to enter and certain product molecules to leave, acting like a molecular sieve. This selectivity makes them very useful in various chemical processes, especially in the petrochemical industry.
In simple words: Zeolites are special catalysts that work like tiny sieves because of their fixed pore sizes and shapes. They only let specific molecules pass through to react, making them "shape-selective."

🎯 Exam Tip: Emphasize that the "shape-selective" property of zeolites comes directly from their unique honeycomb-like structure and precise pore dimensions, which act as a molecular sieve.

 

Question 4. Give the reasons
(a) Alum purifies drinking water.
(b) A substance can be both colloidal and crystalloid
(c) Sky appears blue.
Answer:
(a) Impure water often contains tiny, negatively charged particles such as soil, clay, and bacteria, which remain suspended. When alum (potassium aluminum sulfate) is added to water, it dissolves and releases positively charged aluminum ions (\( Al^{3+} \)). These highly charged \( Al^{3+} \) ions neutralize the negative charges on the suspended impurities. This neutralization causes the small impurity particles to clump together, forming larger aggregates that are heavy enough to settle down. This process, known as coagulation, helps to remove the impurities, making the water clearer and purer.
(b) The classification of a substance as colloidal or crystalloid depends on its behavior in a specific solvent, particularly its particle size when dispersed. A substance can exist as both a colloid and a crystalloid depending on the dispersion medium. For instance, sodium chloride (common table salt) dissolves in water to form a true solution, where its particles are very small, acting as a crystalloid. However, if sodium chloride is dissolved in benzene, it can form a colloidal solution, where its particles aggregate to colloidal size, acting as a colloid. This demonstrates that the nature of the dispersion, not just the substance itself, determines its classification.
(c) The sky appears blue due to a phenomenon called Rayleigh scattering. Sunlight, which is white light, is made up of different colors, each with a different wavelength. When sunlight enters Earth's atmosphere, it interacts with tiny particles like nitrogen and oxygen molecules. Blue light has a shorter wavelength than red light, so it scatters more efficiently and in all directions compared to other colors. Because blue light is scattered much more across the sky, our eyes perceive the sky as blue. The other colors, like red and yellow, scatter less and travel more directly to our eyes, which is why the sun looks yellowish or reddish, especially at sunrise and sunset when light travels through more atmosphere.
In simple words: Alum cleans water by making tiny dirt particles stick together so they can sink. Some substances can act as a colloid in one liquid but as a simple solution in another. The sky looks blue because blue light from the sun scatters more in the air than other colors.

🎯 Exam Tip: For multi-part "give reasons" questions, address each reason clearly and concisely. Use scientific terms accurately, like coagulation for water purification, and Rayleigh scattering for the blue sky, explaining the underlying principle briefly.

 

Question 5. Discuss the factors affecting adsorption of gases on solid surface.
Answer: The amount of gas adsorbed onto the surface of a solid (adsorbent) is influenced by several important factors:
(i) **Nature of gas (Adsorbate):** Gases that are more easily liquefiable (meaning they have higher critical temperatures) tend to be adsorbed more readily. This is because the attractive forces (van der Waals forces) between these gas molecules and the solid surface are stronger. For example, gases like \( CO_2 \), \( SO_2 \), and \( NH_3 \) are easily liquefiable and are adsorbed to a greater extent than permanent gases like \( H_2 \), \( N_2 \), or \( O_2 \). The table below illustrates this with critical temperatures:

(ii) **Nature of adsorbent:** The extent of adsorption is directly proportional to the surface area of the adsorbent. Adsorbents with a larger surface area per unit mass, such as porous substances (like activated charcoal) and finely divided metals (like nickel, platinum, palladium), provide more sites for gas molecules to attach, thus increasing the adsorption capacity. Silica gel is another example of a porous substance used for adsorption.
(iii) **Temperature:** Adsorption is typically an exothermic process, meaning it releases heat (\( \Delta H \) is negative). According to Le Chatelier's principle, an increase in temperature will shift the equilibrium towards desorption (the gas molecules leaving the surface) to absorb the added heat. Therefore, the extent of adsorption generally decreases as the temperature increases. For optimal adsorption, lower temperatures are usually preferred.
(iv) **Effect of pressure:** At a constant temperature, increasing the pressure of the gas leads to a greater number of gas molecules colliding with and adhering to the adsorbent surface. This results in an increase in the extent of adsorption. The effect of pressure is particularly significant at lower temperatures, where the adsorption process is more effective.
In simple words: How much gas sticks to a solid depends on what kind of gas it is (gases that easily turn to liquid stick more), how much surface area the solid has (more area means more sticking), the temperature (lower temperature means more sticking), and the pressure of the gas (higher pressure means more sticking).

🎯 Exam Tip: When explaining factors affecting adsorption, always remember to link the observed effect (increase/decrease in adsorption) to the underlying principle, such as intermolecular forces, surface area, Le Chatelier's principle, or collision theory.