Question

A blue coloured crystal (A) is heated strongly. On heating this crystal turns white. On adding this white residue to the water solution again becomes blue. On adding a grayish metal (B) into this blue solution, turns light green. Identify ‘A’ and ‘B’ and give the reactions involved.

Answer 1

Compound A is a metal sulphate; we usually prepare them in the laboratory too. And it is penta-hydrated. The metal B is attracted by magnet and easily gets rusted in the presence of air or moisture. The displacement reaction occurs in between the compound A and the metal B.

Complete step by step answer:
The compound A is copper sulphate pentahydrate the formula of copper sulphate pentahydrate is: ${\text{CuS}}{{\text{O}}_4}.5{{\text{H}}_2}{\text{O}}$ .
If we heat the compound A, that is copper sulphate pentahydrate, strongly then it loses all its water of crystallization. The colour of copper sulphate pentahydrate is blue due to the presence of its water of crystallization. When the water of crystallization is lost upon strong heating, the crystal turns white, but if we again put the same crystals in water they again become blue in colour. This is because they get hydrated again and gain their water of crystallization. The colour of the crystal again changes to Blue from white.
The metal B is iron metal. Iron is grayish in colour, when B that is iron reacts with copper sulphate pentahydrate then the displacement reaction occurs and the iron displaces copper from copper sulphate. The formation of iron sulphate and copper occurs. This happens due to the more reactivity of iron and copper and iron displaces copper.

The compound A is ${\text{CuS}}{{\text{O}}_4}.5{{\text{H}}_2}{\text{O}}$ and the metal B is iron.

Note:
Displacement reaction is the reaction where a more reactive metal displaces a less reactive metal. This is the reason the copper sulphate cannot be stored in an iron vessel .This is because iron will react with copper sulphate and form iron sulphate and will destroy the property of copper sulphate.