Which of the following reduce silver nitrate to silver?

The correct answer is Option (3) → Pyrophosphorous acid

The oxoacids of phosphorus that contain P–H bonds have strong reducing properties; the P–H hydrogen is not ionisable, but the P(+1) or P(+3) atom is easily oxidised to P(+5). Such acids reduce \( \ce{AgNO3} \) to metallic silver.

Pyrophosphorous acid, \( \ce{H4P2O5} \): 4(+1) + 2x + 5(−2) = 0 gives P = +3, and each P carries one P–H bond. It is oxidised to phosphoric acid:

    \( \ce{H4P2O5} \) + \( \ce{4AgNO3} \) + \( \ce{3H2O} \) → \( \ce{2H3PO4} \) + 4Ag + \( \ce{4HNO3} \)

(2 P atoms lose 2 electrons each, +3 → +5; 4 \( \ce{Ag^{+}} \) gain 4 electrons.)

Why the other options are incorrect

• Option (1) Orthophosphoric acid — Incorrect. \( \ce{H3PO4} \) (3 + x − 8 = 0, P = +5) has no P–H bond; phosphorus is already in its highest oxidation state.

• Option (2) Pyrophosphoric acid — Incorrect. \( \ce{H4P2O7} \) (4 + 2x − 14 = 0, P = +5) has only P–OH and P=O bonds; it cannot be oxidised further.

• Option (4) Metaphosphoric acid — Incorrect. (HPO₃)_n (1 + x − 6 = 0, P = +5) has no P–H bond, so it is not a reducing agent.

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):
Oxoacids of Phosphorus – “… The acids which contain P–H bond have strong reducing properties. Thus, hypophosphorous acid is a good reducing agent as it contains two P–H bonds and reduces, for example, AgNO₃ to metallic silver.
4 AgNO₃ + 2H₂O + H₃PO₂ → 4Ag + 4HNO₃ + H₃PO₄”