Oxoacids of phosphorus having strong reducing properties
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Hypophosphorous acid & Phosphorous acid
The oxoacids of phosphorus that contain P–H bonds are strong reducing agents: the P–H bonds are not ionisable, and the P atom, in a low oxidation state, is readily oxidised to +5.
Hypophosphorous (phosphinic) acid, H3PO2: P is +1 (3 + x − 4 = 0) with two P–H bonds; it reduces \( \ce{AgNO3} \) to metallic silver:
\( \displaystyle \ce{4AgNO3 + 2H2O + H3PO2 -> 4Ag + 4HNO3 + H3PO4} \)
Phosphorous (phosphonic) acid, H3PO3: P is +3 with one P–H bond; it is also a reducing agent.
Why the other options are incorrect
Option (1) Phosphoric Acid & Orthophosphoric acid — Incorrect. Both names denote \( \ce{H3PO4} \), with P in its highest state (+5) and no P–H bond, so it has no reducing property.
Option (3) Phosphoric acid & Cyclotrimetaphosphoric acid — Incorrect. Both have P in the +5 state and no P–H bond.
Option (4) Pyrophosphoric acid & orthophosphoric acid — Incorrect. \( \ce{H4P2O7} \) and \( \ce{H3PO4} \) are both +5 acids without P–H bonds.
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.”