Question

For testing thiosulphate solution if we use mercuric nitrate as reagent then:
A. We get a brown precipitate of mercuric thiosulphate
B. The compound formed is thermally unstable
C. The precipitate formed is soluble in excess of thiosulphate
D. On heating the precipitate formed we see the color transition from white to black precipitate

Answer 1

Sodium thiosulphate is an inorganic compound with the formula $N{a_2}{S_2}{O_3}.x{H_2}O$. It is an efflorescent crystalline substance that dissolves well in water. It undergoes different reactions as well. To solve this question, we need to know the different reactions and tests of thiosulphate.

Complete step by step answer:
Basically, sodium thiosulphate is available as white or colorless pentahydrate $N{a_2}{S_2}{O_3}.5{H_2}O$. It is an efflorescent i.e. loses water rapidly crystalline substance that dissolves well in water. Further upon heating to around ${300^ \circ }C$, it decomposes to sodium sulfate and sodium polysulfide. The reaction is as shown:
$4N{a_2}{S_2}{O_3} \to 3N{a_2}S{O_4} + N{a_2}{S_5}$.
Now, let’s discuss mercuric nitrate. It is a white soluble crystalline salt of nitric acid. It is made by
reacting hot concentrated nitric acid with mercury metal. Although it is not flammable, it can speed up
flames since it acts as an oxidizer. Moreover, it can form explosive compounds when combined with
alcohol.
Further, when silver nitrate is added to sodium thiosulfate, a white ppt of silver thiosulfate is formed.
This silver thiosulfate is disproportionate to give silver sulfide and sulfuric acid. This silver sulfide is black in color. On adding mercuric nitrate, a black precipitate is obtained which changes to brown and finally to black color. Moreover, in presence of an excess of thiosulfate ions, the ppt of silver thiosulfate dissolves and gives a colorless complex.

Hence, option B, C and D are correct.

Note: Generally, the solutions of sodium thiosulfate are used as a precaution in chemistry laboratories when working with bromine. Further, they are used for the safe disposal of many strong oxidizing agents as well.