Give an example of each of the following chemical changes.

(a) A reaction involving

(i) change of state

(ii) formation of a precipitate

(b) An exothermic and an endothermic reaction involving carbon as one of the reactants.

(c) A reaction where colour change is noticed.

(a) An example of:

(i) A reaction involving change of state — Ammonia gas reacts with hydrogen chloride gas to produce solid ammonium chloride.

NH₃ (g) + HCl (g) ⇌ NH₄Cl (s)

(ii) A reaction involving formation of a precipitate — Take a solution of silver nitrate in a test tube and add solution of sodium chloride, a curdy white ppt. is formed.

AgNO₃ (aq) + NaCl (aq) ⟶ AgCl ↓ + NaNO₃ (aq)

(b) Exothermic reaction involving carbon — Combustion of methane gives out large amount of energy.

CH₄ + 2O₂ ⟶ CO₂ ↑ + 2H₂O + Heat

Endothermic reaction involving carbon — When carbon is heated with sulphur at high temperature, liquid carbon disulphide is formed.

C + 2S $\xrightarrow \Delta$ CS₂

(c) A reaction where colour change is noticed — When a few pieces of iron are dropped into a blue coloured copper sulphate solution, the blue colour of the solution fades and eventually turns into light green due to the formation of ferrous sulphate.

$\underset{\text{ iron}}{\text{Fe}} + \underset{\text{ blue sol.}}{\text{CuSO}$4} \longrightarrow \underset{\text{ green sol.}}{\text{FeSO}4} + \underset{\text{ copper-brown red deposit}}{\text{Cu}}