(a) When concentrated sulphuric acid was added to an unknown salt present in a test tube a brown gas (A) was evolved . This gas intensified when copper turnings were added to this test tube . On cooling, the gas (A) changed into a colourless solid (B) .
(i) Identify (A) and (B).
(ii) Write the structures of (A) and (B).
(iii) Why does gas (A) change to solid on cooling ?
(b) Arrange the following in the decreasing order of their reducing character :
HF , HCl , HBr , HI
(c) Complete the following reaction :
`XeF_(4) + SbF_(5) to `

Explanation

### Step-by-Step Solution

#### Part (a)
1. **Identify Gas (A) and Solid (B)**:
- When concentrated sulfuric acid is added to sodium nitrate (NaNO3), it produces nitric acid (HNO3). Upon heating, nitric acid decomposes to form nitrogen dioxide (NO2), which is a brown gas (A).
- On cooling, nitrogen dioxide (NO2) dimerizes to form dinitrogen tetroxide (N2O4), which is a colorless solid (B).

**Answer**:
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