A metal salt mx when exposed to light splits up to form metal m and ga...
The gas produced X2 from the salt MX is involved in formation of bleaching powder, so, first of all we consider the formation of bleaching powder Ca(OCl)Cl or CaOCl2 involves;
Ca (OH)2 (slaked lime or calcium hydroxide)+ Cl2 (chlorine gas) ---> Ca (OCl)Cl + H2O
Further, the metal M is used in ornaments so ,it may be Au(gold), Ag(silver), Platinum(Pt)etc.
But, the metal salt is MX (Metal chloride) and as Chlorine is monovalent with -1 oxidation state so the metal should also have +1 Oxidation state to form MX salt.
Hence, the metal with +1 oxidation state is silver (Ag) which forms silver chloride with Cl2 gas as AgCl (MX salt), and it is also used in black and white photography.
Thus,
1. Metal M is silver (ag)
2. the gas X2 is chlorine Cl2
3. The metal salt (MX) is silver chloride or AgCl.
4.Chemical reaction of decomposition of AgCl is on exposure to light is known as photolysis
2AgCl(s) —> 2Ag(s)+Cl2(g)
Silver chloride