Question is based on following passage:
p-block elements are placed in groups 13 to 18 of the periodic table. Group 15 is nitrogen family. Nitrogen makes various compounds like \(NH_3\), \(HNO_3\), \(N_2O\), \(NO_2\) etc. Phosphorus exists in the form of white and red phosphorus. White phosphorus glows in dark. \(PH_3\), gas is used as smoke screen. Group 16 is called oxygen family. Dry air contains 21% \(O_2\) gas by volume. Combined sulphur exists as sulphates such as gypsum, Epsom salt, baryte etc. The allotropic forms of sulphur are Rhombic and monoclinic sulphur. \(S_2\) is paramagnetic like \(O_2\) and it is stable at high temperature. Group 17 is Halogen family. \(F_2\), is highly reactive, \(Cl_2\), is greenish yellow gas. Both \(SO_2\), and \(Cl_2\), are used for bleaching. Group 18 is Noble gas family but some noble gases also form compounds. Complete hydrolysis of \(XeF_6\) gives \(HF\) and \(XeO_3\), as main products.
The permanent bleaching effect is caused by:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Chlorine gas
Moist chlorine bleaches by oxidation. It reacts with water to give nascent oxygen, which oxidises the coloured matter to a colourless product:
\( \displaystyle \ce{Cl2 + H2O -> 2HCl + [O]} \)
\( \displaystyle \text{Coloured substance} + \ce{[O]} \longrightarrow \text{Colourless substance} \)
The oxidised product does not regain its colour, so the bleaching effect of chlorine is permanent.
Why the other options are incorrect
Option (2) SO2 gas — Incorrect. \( \ce{SO2} \) bleaches by reduction; the reduced matter is re-oxidised by air and the colour returns, so its bleaching is temporary:
\( \displaystyle \ce{SO2 + 2H2O -> H2SO4 + 2[H]} \)
\( \displaystyle \text{Coloured substance} + \ce{[H]} \longrightarrow \text{Colourless substance} \)
Option (3) Aqua Regia — Incorrect. Aqua regia (3 HCl : 1 \( \ce{HNO3} \)) is used to dissolve noble metals such as gold and platinum, not as a bleaching agent.
Option (4) COCl2 — Incorrect. Phosgene is a poisonous gas used in organic synthesis; it is not a bleaching 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):
Chlorine – …
