A gas 'A' reacts with another gas 'B' in the presence of a catalyst to give a colourless gas 'C'. the gas 'C' when comes in contact with air produces a brown gas 'D'. The solution of 'A' in water turns red litmus blue. Explain the observations.

Explanation

To explain the observations regarding the gases 'A', 'B', 'C', and 'D', we will analyze each part step by step.

### Step 1: Identify Gas A
- **Observation**: The solution of gas 'A' in water turns red litmus blue.
- **Explanation**: This indicates that gas 'A' is a base. The only common gas that fits this description is ammonia (NH₃). When ammonia dissolves in water, it forms ammonium hydroxide (NH₄OH), which is a basic solution.

### Step 2: Identify Gas B
- **Observation**: Gas 'A' reacts with another gas 'B' in the presence of a catalyst.
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