Illustrate the following characteristics of a chemical reaction by giving a suitable example of each:

(a) evolution of a gas

(b) change of colour

(c) change of state

(a) In many chemical reactions one of the product is a gas.

When zinc reacts with dilute sulphuric acid, hydrogen gas is evolved with an effervescence.

Zn + H₂SO₄ (dil) ⟶ ZnSO₄ + H₂ (g)

(b) Certain chemical reactions are characterised by a change in the colour of the reactants.

When a few pieces of iron are dropped into a blue coloured copper sulphate solution, the blue colour of the solution fades and turns into light green.

Fe + CuSO₄ (aq) ⟶ FeSO₄ + Cu

(c) In many chemical reactions a change of state is observed.

Ammonia and hydrogen chloride gases react to produce a white solid of ammonium chloride.

NH₃ (g) + HCl (g) ⟶ NH₄Cl (s)