3. Mixing of which of the following will result in a white precipitate which turns blue on standing ? (A) CuSO4 and HgCl2 (B) MgCl2 and Cu (C) Fe2(SO4)3 and K3[Fe(CN)6] (D) FeSO4 and K4[Fe(CN)6]

To determine which combination of substances results in a white precipitate that turns blue upon standing, we need to analyze the chemical reactions involved in each option. The key here is to identify the precipitate formed and its subsequent behavior when exposed to air or left to stand.

Analyzing the Options

Option A: CuSO4 and HgCl2

This combination does not produce a white precipitate. Instead, it may lead to the formation of a complex or other products, but not a white precipitate that changes color.

Option B: MgCl2 and Cu

Mixing magnesium chloride with copper does not yield a precipitate. Copper is a metal and does not react with magnesium chloride in a way that would produce a white precipitate.

Option C: Fe2(SO4)3 and K3[Fe(CN)6]

This combination results in the formation of a precipitate known as Prussian blue, but it does not start as a white precipitate. Instead, it directly forms a blue precipitate.

Option D: FeSO4 and K4[Fe(CN)6]

When iron(II) sulfate reacts with potassium ferrocyanide, it produces a white precipitate of ferrous ferrocyanide, which is also known as Turnbull's blue. Initially, this precipitate appears white, but upon standing, it can turn blue due to oxidation of Fe(II) to Fe(III) in the presence of air.

Conclusion

The correct answer is Option D: FeSO4 and K4[Fe(CN)6. This reaction is a classic example of how a precipitate can change color due to oxidation processes. The initial formation of a white precipitate that turns blue upon exposure to air is a fascinating aspect of inorganic chemistry, showcasing the dynamic nature of chemical reactions.

Further Explanation

To elaborate on the chemistry behind this, when FeSO4 (iron(II) sulfate) is mixed with K4[Fe(CN)6] (potassium ferrocyanide), the ferrous ions (Fe²⁺) react with the ferrocyanide ions to form a white precipitate of Fe3[Fe(CN)6]2. Over time, especially in the presence of oxygen, the ferrous ions can oxidize to ferric ions (Fe³⁺), leading to the formation of a blue complex, which is the characteristic color of Prussian blue.

This transformation is a great example of how environmental factors, such as oxygen in the air, can influence chemical reactions and the properties of the products formed. It highlights the importance of understanding not just the reactants, but also the conditions under which reactions occur.