Problem 95 - A Complete Resource Book in Chemistry for JEE Main

Question

If gold number of \(\mathrm{A}, \mathrm{B}, \mathrm{C}\) and \(\mathrm{D}\) are \(0.005,0.05,0.5\) and 5 respectively, then which of the following will have the greatest protective value? (a) \(\mathrm{A}\) (b) \(\mathrm{B}\) (c) \(\mathrm{C}\) (d) \(\mathrm{D}\)

Step-by-Step Solution

Answer

Substance A has the greatest protective value.

Step 1: Understanding the Gold Number

The Gold Number is used to describe the protective power of colloids. It is the minimum amount (in milligrams) of a protective colloid needed to prevent the precipitation of a certain amount of gold sol from the addition of 1 mL of NaCl. A smaller Gold Number indicates greater protective power.

Step 2: Analyzing Given Numbers

The Gold Numbers given for substances A, B, C, and D are 0.005, 0.05, 0.5, and 5, respectively.

Step 3: Comparing the Gold Numbers

The smaller the Gold Number, the greater the protective power. Compare the numbers 0.005, 0.05, 0.5, and 5.

Step 4: Identifying the Greatest Protective Value

Among the given Gold Numbers, 0.005 is the smallest, which means substance A has the greatest protective power.

Key Concepts

Gold Number

In colloid chemistry, the Gold Number is a critical concept that helps in the understanding of how protective colloids work. It is a measurement of the efficiency or potency of a protective colloid to prevent the precipitation or coagulation of another colloid, typically a gold sol, when an electrolyte is added. The Gold Number is defined as the minimum amount in milligrams of a colloid that must be added to prevent the coagulation of 10 mL of a gold sol when 1 mL of 10% sodium chloride solution is introduced.

A key aspect of the Gold Number is its inverse relationship with protective power. The smaller the Gold Number, the more effective the colloid is at preventing precipitation, which means it offers greater protection. In the given exercise, colloid A with a Gold Number of 0.005 exhibits the highest protection since it requires the least amount of colloid to prevent precipitation—indicating high efficiency in stabilizing the gold sol.

Protective Colloids

Protective colloids are substances that, when added to a colloidal system, enhance its stability against coagulation. They essentially shield colloidal particles from clumping together when electrolytes are introduced, which would otherwise destabilize the suspension and lead to precipitation.

Several factors contribute to the effectiveness of a protective colloid, such as its surface activity and molecular size. These colloids typically work through a process called dsorption, where protective molecules form a barrier around the colloidal particles. This layer increases the repulsive forces between particles, thus preventing coagulation.

• Adsorption: Protective colloids adsorb onto the surface of colloidal particles, forming a stabilizing layer that prevents them from aggregating.
• Size and Charge: Larger molecules with a significant charge can cover more surface area, thereby offering better protection.

In the context of the exercise, colloid A, with the lowest Gold Number, demonstrates these characteristics most effectively, hence offering the greatest level of protection.

Precipitation Prevention

Precipitation prevention is a key target when dealing with colloidal systems, as it ensures the stability of colloids in various applications. Precipitation, or coagulation, occurs when colloidal particles aggregate due to the presence of electrolytes or changes in physical conditions.

Preventing this process is crucial in industries such as pharmaceuticals, food, and materials science, where stable colloidal dispersions are necessary. Protective colloids help in this prevention by acting as barriers that prevent the aggregation of colloidal particles.

• Electrolytes: Electolytes in a solution have a tendency to neutralize charges on the colloidal particles, causing them to clump together and precipitate. Protective colloids counteract this by providing an additional layer of protection.
• Stability Mechanisms: The stability of a colloidal system relies on mechanisms such as steric stabilization, where physical barriers are formed, and electrostatic stabilization, where charges are managed to increase repulsion between particles.

Gold Number values, such as those discussed in the exercise, indicate the efficacy of different protective colloids in preventing precipitation. A lower Gold Number suggests stronger preventive measures, as fewer colloids are needed to achieve stability.