There are some deposits of nitrates and phosphates in the earth's crust. Nitrates are more soluble in water. Nitrates are difficult to reduce under laboratory conditions, but microbes do it easily. Ammonia, forms a large number of complexes with transition metals. Hybridization easily explains the ease of sigma donation capability of \(NH_3\) and \(PH_3\).
Phosphine is an inflammable gas, it is prepared from white phosphorus.
White phosphorus on reacting with \(NaOH\) gives \(NaH_2PO_2\) and \(PH_3\) as products. This is an example of
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Disproportionation reaction
White phosphorus dissolves in boiling NaOH solution (in an inert atmosphere) to give phosphine and sodium hypophosphite:
\( \ce{P4 + 3NaOH + 3H2O -> PH3 + 3NaH2PO2} \)
The oxidation state of phosphorus changes from 0 in \( \ce{P4} \) to –3 in \( \ce{PH3} \) (reduction) and to +1 in \( \ce{NaH2PO2} \) (oxidation). The same element is oxidised and reduced at the same time, so this is a disproportionation reaction.
Why the other options are incorrect
Option (1) Dimerization — Incorrect. Two identical molecules are not joining to form a dimer; \( \ce{P4} \) is broken up.
Option (3) Condensation — Incorrect. No two molecules combine with loss of a small molecule such as water.
Option (4) Precipitation — Incorrect. No insoluble solid separates; \( \ce{PH3} \) is a gas and \( \ce{NaH2PO2} \) stays in solution.
Note 1: 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):
Preparation
Note 2: The same/similar question has been asked in different years by NTA as per the details given below:
| 2026 | 2025 | 2024 | 2023 | 2022 |
|---|---|---|---|---|
| – | – | – | 02-06 S3 31-05 S1 | – |