(a) When concentrated sulphuric acid was added to an unknown salt present in a test tube a brown

(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 `