Sodium thiosulphate (hypo) is used in black and white photography because it
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → All of the above
In black and white photography, the developed film is fixed: the undecomposed AgBr left in the film is removed. Hypo does this by forming a soluble complex with the silver ion:
\( \displaystyle \ce{AgBr + 2Na2S2O3 -> Na3[Ag(S2O3)2] + NaBr} \)
(1) Complex formation: the thiosulphate ions act as ligands and bind \( \ce{Ag^+} \) as the complex ion \( \ce{[Ag(S2O3)2]^3-} \). — True.
(2) Removal of AgBr: the complex is soluble, so the undecomposed AgBr is washed out of the film. — True.
(3) Products: the equation above gives \( \ce{Na3[Ag(S2O3)2]} \) and NaBr. — True.
NCERT (2026-27 edition):
5.7 Importance and Applications of Coordination Compounds – “In black and white photography, the developed film is fixed by washing with hypo solution which dissolves the undecomposed AgBr to form a complex ion, [Ag(S2O3)2]3–.”