Question

Which one of the following salts is blue in colour?
(A) Ferrous sulphate
(B) Copper sulphate
(C) Cobalt chloride
(D) Alum

Answer 1

Salt is a term which is used to describe the compound which is just made by joining a positively charged acid with a negatively charged base. Moreover salts may exist in many different colours, which arise either from the anions or cations.

Complete answer:
-So with the help of acids, bases and salts we are going to do questions very easily.
-The colour of ferrous sulphate crystals is green. Ferrous sulphate contains water molecules $(FeS{O_4}.7{H_2}O).$ On heating ferrous sulphate $(FeS{O_4})$ is formed. So their colour changes from light green to white. On further heating, anhydrous ferrous sulphate decomposes to from ferric oxide $(F{e_2}{O_3}),$ sulphur dioxide $(S{O_2})$ and sulphur trioxide $(S{O_3})$. So the gas emitted smells like burning sulphur moreover, in this reaction, the single reacting $FeS{O_4}$ decomposes to form three different products. So the reaction is a decomposition reaction.
$2FeS{O_4} \to F{e_2}{O_3} + S{O_2} + S{O_3}$
-The colour of copper sulphate solution. However the solid may be white or blue. The white form of copper sulphate powder is the dehydrated form, which contains no water. So the hydrated form of copper sulphate powder is blue in colour. And during heating, the salt loses its water of crystallization and then turns white.
-When an iron nail is immersed in the solution of copper sulphate then iron displaces copper from the solution of copper sulphate because iron is more reactive than copper. Therefore copper sulphate solution colour changes from blue to pale green.
So, option (B) is correct.

Note:
In hydrated $CuS{O_4}$ , the water molecules surrounding the central method $(Cu)$ act as ligands which result in $d - \alpha$ transition and therefore, limiting blue colour in the visible region due to which hydrated $CuS{O_4}$ appears blue.