Phosphorus react with concentrated \(HNO_3\) to form
A. Phosphoric acid
B. Phosphorus acid
C. \(NO_2\)
D. NO
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → A and C only
Concentrated nitric acid is a strong oxidising agent. It oxidises phosphorus to its highest oxidation state in phosphoric acid and is itself reduced to nitrogen dioxide. Each P goes from 0 to +5 (5 electrons lost, 20 for one \( \ce{P4} \)); each N goes from +5 in \( \ce{HNO3} \) to +4 in \( \ce{NO2} \) (1 electron gained). So 20 \( \ce{HNO3} \) are reduced for each \( \ce{P4} \):
\( \displaystyle \ce{P4 + 20HNO3(conc.) -> 4H3PO4 + 20NO2 + 4H2O} \)
A. Phosphoric acid — Correct. \( \ce{H3PO4} \), with P in the +5 state, is the oxidation product.
B. Phosphorus acid — Incorrect. \( \ce{H3PO3} \) has P in the +3 state; concentrated \( \ce{HNO3} \) oxidises phosphorus beyond this.
C. NO2 — Correct. Concentrated nitric acid is reduced to \( \ce{NO2} \).
D. NO — Incorrect. NO is the reduction product of dilute nitric acid (as with copper), not of the concentrated acid.
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.
NCERT (Pre-rationalised edition, 2022-23):
Nitric Acid – “… Concentrated nitric acid also oxidises non-metals and their compounds. Iodine is oxidised to iodic acid, carbon to carbon dioxide, sulphur to H2SO4, and phosphorus to phosphoric acid. P4 + 20HNO3(conc.) → 4H3PO4 + 20NO2 + 4H2O”