In the laboratory preparation of nitric acid, nitre is heated with :

1. conc. HCl
2. conc. H₂SO₄
3. dil. H₂SO₄
4. conc. H₃PO₄

conc. H₂SO₄

Reason — In the laboratory preparation of Nitric acid by distillation process, potassium nitrate (nitre) is heated with conc. sulphuric acid (H₂SO₄).

$\underset{[\text{conc.}]}{\text{KNO}$3} + \underset{[\text{conc.}]}{\text{H}2\text{SO}4} \xrightarrow{\lt 200 \degree\text{C}} \underset{[\text{acid salt}]}{\text{KHSO}4} + \text{HNO}_3