Problem 172 - A Complete Resource Book in Chemistry for JEE Main

Question

Assertion: When freshly precipitated nickel sulphide (NiS) is shaken with a solution of hydrogen sulphide, a colloidal solution is obtained. Reason: A colloidal solution is stable because all the colloidal particles carry same charge

Step-by-Step Solution

Answer

Both assertion and reason are correct, and the reason explains the assertion.

Step 1: Understanding the Assertion

The assertion states that when nickel sulphide (NiS), which has been recently precipitated, is mixed with hydrogen sulphide ( H_2 S) solution, it forms a colloidal solution. This implies NiS becomes dispersed enough in the H_2 S to form a colloidal system, which consists of very small particles evenly distributed throughout the solution.

Step 2: Examining the Reason

The reason provided is that colloidal solutions are stable because the particles within them carry the same type of charge. This uniform charge on particles provides them with mutual repulsion, preventing them from aggregating and settling out, thus maintaining the colloidal dispersion.

Step 3: Relating the Assertion and Reason

The assertion describes the formation of a colloidal solution from NiS and H_2 S. The reason explains a characteristic property of colloids: stability due to particles having the same charge. If the NiS particles formed a colloid with H_2 S, these particles would carry similar charges, supporting the reason given.

Step 4: Conclusion on Assertion and Reason Relationship

The reason provided is indeed the correct explanation for the assertion. The stability of the colloidal solution of NiS in H_2 S is attributed to the particle charges being similar, which matches the given reason.

Key Concepts

NiS precipitation

When nickel sulfide (NiS) is freshly precipitated, it can become part of a colloidal solution when mixed with a hydrogen sulfide solution (\( H_2S \)). This process involves transforming NiS into tiny particles that can disperse evenly throughout the solution.

A colloidal solution is formed because the particles become sufficiently small and are distributed uniformly in the liquid. These particles are usually in the size range between 1 nm and 1000 nm. Such small size allows them to remain suspended without settling for a significant period.

The precipitation of NiS itself involves a chemical reaction. NiS forms when nickel ions (Ni²⁺) react with sulfide ions (S²⁻) to produce an insoluble compound, nickel sulfide. This compound can precipitate out of the solution as solid particles. However, when shaken with \( H_2S \), these particles get dispersed to form a colloid.

Particle charge in colloids

One key characteristic of colloids is the presence of charged particles. Particles in a colloidal solution carry the same type of charge, which influences their behavior in the mixture. This similarity in charge is crucial because it results in mutual repulsion among the particles.

- **Same charge:** The particles in the colloidal solution of NiS and hydrogen sulfide are similarly charged. This can be negative or positive, but in most cases, colloidal particles tend to carry a negative charge. - **Electrostatic repulsion:** Because all particles bear the same charge, they repel each other, preventing them from coming close enough to aggregate and form larger, settleable particles. This charge-induced repulsion is the essential force that keeps the colloidal solution stable.

The stability of colloids, therefore, is heavily reliant on this charged nature. It's what prevents particles from clumping together and precipitating out, maintaining them in suspension.

Colloidal stability chemistry

In chemistry, the stability of colloidal solutions is paramount because it determines how long particles remain dispersed without settling. For NiS in a colloidal form, its stability is primarily governed by electrostatic interactions and the particle charge.

- **Prevention of aggregation:** The electrical charges on the particles provide a protective barrier, making aggregation less likely. When particles attempt to come close, their like-charges generate repulsion that keeps them at a distance.- **Influences on stability:** Factors like ionic strength, pH levels, and the presence of electrolytes in the solution can influence the charge and affect stability. In the case of NiS in \( H_2S \), the surrounding sulfide solution helps maintain the necessary environment for stability.- **Applications:** Understanding colloidal stability is crucial in fields like material science, food chemistry, and pharmaceuticals, where controlled particle dispersion is often required.

Thus, mastering the concept of colloidal stability not only helps in understanding why NiS forms a stable colloidal solution upon precipitation but also opens the door to numerous practical applications.