what is the color of precipitate when silver nitrate (BaCl2) is added to S2O3-2 (thiosulfate)?
The correct answer and explanation is:
Correct Answer: White precipitate (initially), which may dissolve or turn yellowish with time due to complex formation.
Explanation:
When silver nitrate (AgNO₃) is added to a solution containing thiosulfate ions (S₂O₃²⁻), a reaction occurs between the silver ions (Ag⁺) and thiosulfate ions.
Initially, a white precipitate forms. This white solid is silver thiosulfate (Ag₂S₂O₃). However, the chemistry doesn’t stop there—thiosulfate is a complexing agent, and it has the ability to form soluble complexes with silver ions.
The primary reaction is: 2Ag++S2O32−→Ag2S2O3(s)(white precipitate)2Ag^+ + S_2O_3^{2-} \rightarrow Ag_2S_2O_3(s) \quad \text{(white precipitate)}
But in excess thiosulfate, this precipitate can dissolve, forming a soluble complex: Ag2S2O3(s)+S2O32−→2[Ag(S2O3)2]3−(soluble complex)Ag_2S_2O_3(s) + S_2O_3^{2-} \rightarrow 2[Ag(S_2O_3)_2]^{3-} \quad \text{(soluble complex)}
This means that while a white precipitate may form initially, it may disappear upon standing or in excess thiosulfate due to complexation. In some cases, the solution may appear yellowish due to formation of silver-thiosulfate complexes or due to decomposition of thiosulfate to elemental sulfur, which can also give a milky or yellow appearance.
Summary:
- Initial reaction gives a white precipitate of Ag₂S₂O₃.
- In excess thiosulfate, the precipitate dissolves forming a soluble silver-thiosulfate complex.
- Over time or under certain conditions, solution may turn yellowish due to side reactions or decomposition.
This reaction is an important demonstration of how complex ion formation can affect solubility and precipitate behavior in aqueous chemistry.