Write the chemical reaction where the following changes are observed.

(a) Gas is evolved

(b) Colour change is noticed

(c) Precipitate is formed

(d) Physical state of reactants is changed

(a) Gas is evolved

Zn(s) + H₂SO₄ (aq) ⟶ ZnSO₄ (aq) + H₂ (g)

(b) Colour change is noticed

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

(c) Precipitate is formed

$\underset{\text{ ferrous sulphate}}{\text{FeSO}$4 \text{ (aq.)}} + \underset{\text{ sodium hydroxide}}{2\text{NaOH}\text{ (aq.)}} \longrightarrow \underset{\text{ dirty green ppt.}}{\text{Fe(OH)}2↓} + \underset{\text{ sodium sulphate}}{\text{Na}2\text{SO}4\text{ (aq.)}}

(d) Physical state of reactants is changed

C (s) + 2S (s) ⟶ CS₂ (l)