Question

A is a coloured crystalline solid which is easily soluble in water. Addition of aqueous KCN gives a precipitate which dissolves in excess of reagent whereas addition of KI aqueous solution gives a white grey precipitate with the liberation of iodine. On gently heating, A loses \(80 \%\) of its water of crystallization. Identify A here? (a) \(\mathrm{ZnSO}_{4} \cdot 7 \mathrm{H}_{2} \mathrm{O}\) (b) \(\mathrm{FeSO}_{4} \cdot 7 \mathrm{H}_{2} \mathrm{O}\) (c) \(\mathrm{CuSO}_{4} \cdot 5 \mathrm{H}_{2} \mathrm{O}\) (d) \(\mathrm{Hg}_{2} \mathrm{Cl}_{2}\)

Short answer

The given compound \( A \) is \( \mathrm{CuSO}_{4} \cdot 5 \mathrm{H}_{2} \mathrm{O} \).

Step-by-step solution

Analyze the Solubility Property

We know that \( A \) is a colored crystalline solid easily soluble in water. This feature is commonly associated with copper (II) sulfate, \( \mathrm{CuSO}_{4} \cdot 5 \mathrm{H}_{2} \mathrm{O} \), which is known to be blue in color and soluble, eliminating \( \mathrm{Hg}_{2} \mathrm{Cl}_{2} \) which is not readily soluble in water.

Evaluate Reaction with KCN

The problem states that \( A \) in the presence of \( \mathrm{KCN} \) gives a precipitate that dissolves in excess \( \mathrm{KCN} \). \( \mathrm{CuSO}_{4} \cdot 5 \mathrm{H}_{2} \mathrm{O} \), when reacted with \( \mathrm{KCN} \), initially forms a precipitate of \( \mathrm{Cu}_2 [\mathrm{Fe}{(CN)}_6] \) which dissolves in excess forming a soluble complex.

Evaluate Reaction with KI

When \( \mathrm{CuSO}_{4} \cdot 5 \mathrm{H}_{2} \mathrm{O} \) is treated with \( \mathrm{KI} \), it forms white precipitate \( \mathrm{CuI} \) and liberates iodine, just as described in the problem. This aligns with the behavior of copper sulfate in such reactions.

Examine Water Loss on Heating

On heating gently, \( \mathrm{CuSO}_{4} \cdot 5 \mathrm{H}_{2} \mathrm{O} \) will lose about \( 80\% \) of its water of crystallization, which corresponds well with the given information as it loses 4 of 5 water molecules (\( 80\% \)).

Key concepts

Solubility

Solubility refers to the ability of a substance to dissolve in a solvent. It's a key property that determines how substances interact with each other in solutions. Copper(II) sulfate, for example, is known for its vivid blue color and is highly soluble in water. When a substance is soluble, it disperses evenly in the solvent, creating a homogeneous mixture.

In cases like the one discussed, solubility helps determine what a particular solid substance could be based on its ability to dissolve in water. Many salts like copper(II) sulfate (CuSO₄·5H₂O) dissolve easily, making them perfect candidates for identification in solution-based chemistry problems.

Precipitation Reactions

Precipitation reactions occur when two soluble substances in aqueous solution react to form an insoluble compound, known as a precipitate. When aqueous solutions of KCN are added to copper(II) sulfate, a precipitate forms initially. However, this precipitate dissolves again if excess KCN is added.

This double-phase interaction showcases how substances can transition between soluble and insoluble states. Precipitation reactions are useful in separating compounds or identifying specific substance properties based on their behavior in the presence of other reagents.

Water of Crystallization

Water of crystallization is the water molecules that are integral to the crystal structure of certain compounds. It varies from compound to compound, especially with colored crystalline solids like copper(II) sulfate pentahydrate (CuSO₄·5H₂O).

Upon gentle heating, CuSO₄·5H₂O loses a significant portion of its water content, approximately 80%, corresponding to the loss of 4 out of 5 water molecules. This loss of water changes the compound's structure and can significantly alter its physical appearance and chemical behavior.

Copper(II) Sulfate

Copper(II) sulfate, commonly recognized by its chemical formula CuSO₄·5H₂O, is a versatile compound known for its bright blue crystalline appearance. It's widely used in various chemical reactions and laboratory settings.

In reactions, it readily forms new compounds, such as precipitates, and partakes in redox reactions. Its solubility and clear reaction pathways, like forming white copper iodide (CuI) when reacted with KI, make it a useful subject in chemical identification exercises.

Chemical Reactions with KCN and KI

When copper(II) sulfate is involved in chemical reactions with potassium cyanide (KCN) and potassium iodide (KI), distinct changes occur. The reaction with KCN initially yields a precipitate due to the formation of copper cyanide compounds. However, adding more KCN results in a soluble complex.

With KI, copper(II) sulfate exhibits another interesting behavior. It forms a white-gray precipitate of copper iodide (CuI), simultaneously liberating iodine. Such reactions are pivotal for exploring the interaction of metal salts with different reagents, highlighting physical and chemical transformations that serve as basis for chemical identification.