Cations are classified into varius group on the basis of their behaviour against some reagents .The group reagent used for the classifaction of most common cation are `HCI,H_(2)S,NH_(4)OH,(NH_(4))_(2)CO_(3)`. Classification is based on whether a cation reacts with these reagents by the formation of precipitates or not .
Which one among the following paires of ions cannot be separated by `H_(2)S` in the presence of dilute hydrochloric acid ?

Options

A. `Bi^(2+),Cd^(2+)`
B. `AI^(2+),Hg^(2+)`
C. `Zn^(3+),Cu^(2+)`
D. `Ni^(2+),Cu^(2+)`

Explanation

To solve the question regarding which pair of ions cannot be separated by H₂S in the presence of dilute hydrochloric acid (HCl), we will follow these steps:

### Step 1: Understand the Classification of Cations
Cations are classified into different groups based on their behavior with certain reagents. The reagents mentioned are HCl, H₂S, NH₄OH, and (NH₄)₂CO₃. In this case, we are focusing on the second group of cations, which typically includes ions like Mercuric (Hg²⁺), Lead (Pb²⁺), Bismuth (Bi³⁺), Copper (Cu²⁺), and Cadmium (Cd²⁺).

### Step 2: Identify the Role of H₂S and Dilute HCl
H₂S acts as a group reagent for the second group of cations when used in conjunction with dilute HCl. In this environment, certain cations will form insoluble sulfides, which precipitate out of the solution.

### Step 3: Analyze the Precipitation of Cations
In the presence of dilute HCl and H₂S:
- Bismuth (Bi³⁺) forms a black precipitate of Bismuth sulfide (Bi₂S₃).
- Cadmium (Cd²⁺) forms a yellow precipitate of Cadmium sulfide (CdS).

Both of these cations will precipitate out as sulfides and thus cannot be separated from each other in this solution.

### Step 4: Conclusion
Since both Bismuth and Cadmium form precipitates when treated with H₂S in the presence of dilute HCl, they cannot be separated from each other. Therefore, the answer to the question is the pair of ions that includes Bismuth and Cadmium.

### Final Answer
The pair of ions that cannot be separated by H₂S in the presence of dilute hydrochloric acid is **Bismuth (Bi³⁺) and Cadmium (Cd²⁺)**.

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