Question

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) Quicklime

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 turns black.
(c) On heating green coloured ferrous sulphate crystals, reddish brown solid is left and smell of a gas having odour of burning sulphur is experienced.
(d) Iron nails when left dipped in blue colour copper sulphate solution become brownish in colour and the blue colour of copper sulphate fades away.
(e) Quick lime reacts vigorously with water releasing a large amount of heat.
Write chemical equations for each one of the above reactions.

Answer 1

(a) When barium chloride solution is mixed with the copper sulphate solution, we get barium sulphate as the precipitate. The reaction is precipitation reaction or double displacement reaction.
${\mathrm{BaCl}}_2\left(\mathrm{aq}\right)+{\mathrm{CuSO}}_4\left(\mathrm{aq}\right)\to {\mathrm{BaSO}}_4\left(\mathrm{s}\right)+{\mathrm{CuCl}}_2\left(\mathrm{aq}\right)$

(b) When copper oxide is heated, it combines with oxygen in the air to form copper oxide. This copper oxide is black in colour, because of which the surface of copper powder turns black. This reaction is a redox reaction.
$\mathrm{Cu}\left(\mathrm{s}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)\to 2\mathrm{CuO}$

(c) Ferrous sulphate is green in colour. On being heated, it loses water of crystallization and is converted to anhydrous ferrous sulphate. The anhydrous ferrous sulphate on further heating decomposes to ferric oxide, sulphur dioxide and sulphur trioxide. Ferric oxide is reddish brown in colour. This is an example of a decomposition reaction.
$2{\mathrm{FeSO}}_4\to \mathrm{FeO}+{\mathrm{SO}}_2+{\mathrm{SO}}_3$

(d) When iron nails are dipped in copper sulphate solution, then the blue colour of solution will slowly fade away also a brown substance will be deposited on the iron nail. This substance is nothing but copper metal. This reaction takes place because iron is more reactive than copper and is placed above it in the reactivity series. Hence, iron displaces copper from copper sulphate and instead forms FeSO₄ and copper metal. It is a displacement reaction.
${\mathrm{CuSO}}_4\left(\mathrm{aq}\right)+\mathrm{Fe}\left(\mathrm{s}\right)\to {\mathrm{FeSO}}_4\left(\mathrm{aq}\right)+\mathrm{Cu}\left(\mathrm{s}\right)$

(e) When quick lime is added to water, then calcium hydroxide is formed. A large amount of heat is released in the process. This reaction is a combination reaction.
$\mathrm{CaO}\left(\mathrm{s}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\to \mathrm{Ca}\left(\mathrm{OH}{)}_2\right(\mathrm{aq})$