Which one is a chemical change: Electrolysis of water or Sodium chloride exposed to sunlight?

Why do we store silver chloride in dark-coloured bottles?

Which among the following is(are) double displacement reaction$(s)$?
$(i)$ $Pb + CuCl_2 \longrightarrow PbCl_2 + Cu$
$(ii)$ $Na_2SO_4 + BaCl_2 \longrightarrow BaSO_4 + 2NaCl$
$(iii)$ $C + O_2 \longrightarrow CO_2$
$(iv)$ $CH_4 + 2O_2 \longrightarrow CO_2 + 2H_2O$

When a green iron salt is heated strongly, its colour finally changes to black and the odour of burning sulphur is given out.
$(a)$ Name the iron salt.
$(b)$ Name the type of reaction that takes place during the heating of iron salt.

$A$ substance $X$, which is an oxide of a group $2$ element, is used intensively in the cement industry. This element is present in bones also. On treatment with water, it forms a solution which turns red litmus blue. Identify $X$ and write the chemical reactions involved.

Give differences between the displacement and double displacement reactions.