Question

When phosphorus is heated with conc. HNO3, it reduces the acid to

• A. NO
• B. NO2
• C. N2O3
• D. N2O5

Answer 1

Answer: (B)

NO2

Answer 2

When phosphorus (P) is heated with concentrated nitric acid (conc. HNO₃), phosphorus gets oxidized to phosphoric acid (H₃PO₄), and concentrated nitric acid gets reduced to nitrogen dioxide (NO₂).

The balanced chemical reaction is: $\mathrm{P} + 5\mathrm{HNO_3} \text{ (conc.)} \longrightarrow \mathrm{H_3PO_4} + 5\mathrm{NO_2} + \mathrm{H_2O}$

In this reaction:

• Phosphorus (P) goes from oxidation state 0 to +5 in H₃PO₄.
• Nitrogen in HNO₃ (oxidation state +5) goes to nitrogen in NO₂ (oxidation state +4).

Therefore, concentrated nitric acid is reduced to NO₂.