Question

Which of the following on reaction with hypo solution gives black precipitate?
A.\[\;FeC{l_3}\]
B.\[AgN{O_3}\]
C.\[\;C{l_2}\]
D.\[\;HCl\]

Answer 1

Hint:Hypo solution is also called as Sodium thiosulphate solution which contains thiosulphate group and when it reacts with the solution, it generally gives sulfide ions so ultimately sulfides are formed (depending upon the reactant) and sulfides have dark color like brown, black etc. So molecules whose sulfide is darker in color will form black precipitate.

Complete step by step answer:
Hypo solution is also called as Sodium Thiosulphate which has chemical formula \[N{a_2}{S_2}{O_3}\]and it has wide range of uses in medicinal field, photography, gold extraction, titration etc.
In this particular question we will observe the product and chemical reaction of Hypo solution.
With FeCl3, reaction of Sodium thiosulphate occurs as: -
\[2FeC{l_3} + {\text{ }}2N{a_2}{S_2}{O_3} \to 2FeC{l_2} + {\text{ }}N{a_2}{S_4}{O_6} + {\text{ }}2NaCl\]
In this reaction the final product formed is FeCl2 which is generally pale green (light) in color. So, this is not forming black precipitate with hypo and also salts of Iron are generally white or pale yellow, green
With\[AgN{O_3}\], reaction of sodium thiosulphate occurs as: -
\[2AgN{O_3} + {\text{ }}N{a_2}{S_2}{O_3} \to A{g_2}{S_2}{O_3} + {\text{ }}2NaN{O_3}\]
\[A{g_2}{S_2}{O_3} + {\text{ }}{H_2}O{\text{ }} \to A{g_2}S{\text{ }}\left( {black{\text{ }}ppt.} \right){\text{ }} + {\text{ }}{H_2}S{O_4}\]
In this reaction \[AgN{O_3}\]reacts with hypo solution and forms initially Silver thiosulphate (light white precipitate) and it has low solubility so it settles down after few minutes it gets converted to black ppt of formula\[A{g_2}S\]. Also Silver generally shows precipitation reactions and in this reaction Silver sulfide is formed at the end which is of black color. Generally, sulfides are darker in color like black, brown etc.
So, you can also tell color through knowing the final product of sulfide.
With \[C{l_2}\]reaction of sodium thiosulphate occurs as: -
\[N{a_2}{S_2}{O_3}\; + {\text{ }}C{l_2}\; + {\text{ }}{H_2}O{\text{ }} \to {\text{ }}S{\text{ }} + {\text{ }}2HCl{\text{ }} + {\text{ }}N{a_2}S{O_4}\]
So, in this final product is Sodium sulphate along with sulfur and \[HCl\]is formed. Sodium sulphate is white colored salt which will settle down in reaction.
Reaction of \[HCl\]with sodium thiosulphate occurs as: -
\[N{a_2}{S_2}{O_3} + {\text{ }}2HCl \to {\text{ }}2NaCl{\text{ }} + {\text{ }}S{O_2} + {\text{ }}{H_2}O\]
So, in this reaction final product formed are \[NaCl\]and Water and \[S{O_2}\]along with sulfur and \[NaCl\]is also white colored salt which will settle down in reaction and \[S{O_2}\]gas will evolve out.
So, among all we see that \[AgN{O_3}\]is the one which forms Silver Sulfide precipitate which is of black color.

Therefore, correct answer is option B i.e. \[AgN{O_3}\]

Note:
Generally, the simple compounds like Sodium thiosulphate, Calcium oxychloride, phenolphthalein, methyl orange are important and few indicators and their color change upon addition of acid or base is important to know which can invite few questions. Hypo solution is also used in titration and there are indicators also in titration so their color change is important to know.