State the main characteristics of chemical reactions. Give at least one example in each case.

The main characteristics of chemical reactions are :

1. Evolution of gas
   Zn(NO₃)₂ $\xrightarrow{\Delta}$ 4NO₂ + 2ZnO + O₂ ↑

2. Change of colour
   $\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}}

3. Change of state
   2H₂O (l) $\xrightarrow[\text{current}]{\text{electric}}$ 2H₂ (g) + O₂ (g)

4. Formation of precipitate
   NaCl (aq.) + AgNO₃ (aq.) ⟶ AgCl (white ppt.) ↓ + NaNO₃ (aq.)