Write the essential condition for the following reaction to take place: $2 AgCl \rightarrow 2 Ag + Cl_{2}$ and write one use of this reaction.

Give the characteristic tests for the following gases:
$(a)$ $CO_2$
$(b)$ $SO_2$

Give reason why:
$(i)$ White coloured silver chloride turns grey when kept in sunlight.

Which among the following is(are) double displacement reaction$(s)$?
$(i)$ $Pb + CuCl_{2} \rightarrow PbCl_{2} + Cu$
$(ii)$ $Na_{2}SO_{4} + BaCl_{2} \rightarrow BaSO_{4} + 2NaCl$
$(iii)$ $C + O_{2} \rightarrow CO_{2}$
$(iv)$ $CH_{4} + 2O_{2} \rightarrow CO_{2} + 2H_{2}O$

State what happens when:
$(a)$ Gypsum is heated at $373 \, K$.
Also,write the chemical equation for the reaction.

What happens when a piece of
$(a)$ zinc metal is added to copper sulphate solution?
$(b)$ aluminium metal is added to dilute hydrochloric acid?
$(c)$ silver metal is added to copper sulphate solution?
Also,write the balanced chemical equation if the reaction occurs.