You are given the following materials :
`{:((i), "Iron nails",, (ii), "Copper sulphate solution"),((iii), "Barium chloride solution",, (iv), "Copper powder"),((v), "Ferrous sulphate crystals",, (vi), "Quick lime"):}`
Identify the type of chemical reaction taking place when :
(a) Barium chloride solution is mixed with copper sulphate solution and a white precipitate is observed.
(b) On heating, copper powder in air in a china dish, the surface of copper powder becomes black.
(c) On heating green ferrous sulphate crystals, reddish brown solid is left and a gas having smell of burning sulphur is noticed.
(d) Iron nails when left dipped in blue copper sulphate solution becomes brownish in colour and blue colour of copper sulphate solution fades away.

Explanation

(a) The reaction is double displacement in nature.
`{:(BaCl_(2)(aq) + CuSO_(4)(aq)to, BaSO_(4)(s) + CuCl_(2)(aq)),(,"(White ppt.)"):}`
(b) It is an example of combination reaction.
`{:(2Cu(s) + O_(2)(g) to, 2CuO(s)),(" ""(in air)", "Copper (II) oxide"),(," ""(Black)"):}`
(c) The green crystals of ferrous sulphate have the chemical formula `FeSO_(4).7H_(2)O.` Upon heating, they lose molecules of water of crystallisation and it is an example of dehydration reaction.
`underset("(Green)")(FeSO_(4)).7H_(2)Ooverset("heat")to FeSO_(4) + 7H_(2)O`
Upon further heating, ferrous sulphate undergoes decomposition reaction as follows :
`{:(2FeSO_(4)(s)overset("heat")to, Fe_(2)O_(3)(s) + SO_(2)(g) + SO_(3)(g)),(,"Ferric oxide(Reddish brown)"):}`
Both the gases evolved have the small of burning sulphur.
(d) This happens because of displacement reaction. Iron displaces copper from copper sulphate solution. Brownish coating of copper gets deposited on the iron nails. As the concentration of coppe sulphate in the solution decreases, the blue colour of the solution slowly fades.
`{:(Fe(s) + CuSO_(4)(aq) to, FeSO_(4)(aq) + Cu(s)),(," ""(Brownish)"):}`