(a) Give reasons: (i) $\mathrm{H}_3\mathrm{PO}_3$ undergoes disproportionation reaction but $\mathrm{H}_3\mathrm{PO}_4$ does not. (ii) When $\mathrm{Cl}_2$ reacts with excess of $\mathrm{F}_2, \mathrm{ClF}_3$ is formed and not $\mathrm{FCl}_3$. (iii) Dioxygen is a gas while sulphur is a solid at room temperature. (b) Draw the structures of the following: (i) $\mathrm{XeF}_4$ (ii) $\mathrm{HClO}_3$ OR(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 :$\mathrm{HF}, \mathrm{H}, \mathrm{HBr}, \mathrm{HI}$(c) Complete the following reaction:$\mathrm{XeF}_4 + \mathrm{SbF}_5 \rightarrow$