Problem 140 - A Complete Resource Book in Chemistry for JEE Main

Question

A white, water insoluble solid A turns yellow on heating and becomes white on cooling. A gives a clear solution B when treated with dilute HCl or \(\mathrm{NaOH}\). When \(\mathrm{H}_{2} \mathrm{~S}\) is passed through solution \(\mathrm{B}\), and \(\mathrm{made}\) neutral, a white precipitate \(\mathrm{C}\) is formed. Identify \(\mathrm{A} ?\) (a) \(\mathrm{ZnO}\) (b) \(\mathrm{CuO}\) (c) \(\mathrm{Fe}_{2} \mathrm{O}_{3}\) (d) \(\mathrm{Ag}_{2} \mathrm{O}\)

Step-by-Step Solution

Answer

A is \(\mathrm{ZnO}\) because it matches the described changes in color and reactions with HCl, NaOH, and \(\mathrm{H}_{2} \mathrm{S}.\)

Step 1: Analyze the Given Properties of A

The solid A is described as a white, water-insoluble solid that turns yellow when heated and reverts to white upon cooling. This is a characteristic property of zinc oxide (\(\mathrm{ZnO}\)), which undergoes a reversible color change from white to yellow when heated.

Step 2: Solubility in Dilute HCl or NaOH

Solid A dissolves in both dilute hydrochloric acid (HCl) and sodium hydroxide (NaOH) to form solution B. Zinc oxide (\(\mathrm{ZnO}\)) is amphoteric, meaning it can react with both acids and bases to form soluble compounds, a property that aligns with A's behavior.

Step 3: Reaction with H2S to Form Precipitate C

When \(\mathrm{H}_{2} \mathrm{S}\) is passed through neutral solution B, a white precipitate C is formed. Zinc ions (\(\mathrm{Zn}^{2+}\)) can react with \(\mathrm{H}_{2} \mathrm{S}\) to form zinc sulfide (\(\mathrm{ZnS}\)), which is a white precipitate, further confirming the identity of A as zinc oxide.

Key Concepts

Amphoteric behavior

Zinc oxide, represented as ZnO, is known for its amphoteric behavior. This unique chemical property means that zinc oxide can react with both acids and bases.
When ZnO is treated with dilute hydrochloric acid (HCl), it dissolves to form soluble zinc chloride (ZnCl extsubscript{2}). Meanwhile, when it interacts with sodium hydroxide (NaOH), it forms sodium zincate.

• In an acidic reaction: \( ext{ZnO} + 2 ext{HCl} ightarrow ext{ZnCl}_2 + ext{H}_2 ext{O}\)
• In a basic reaction: \( ext{ZnO} + 2 ext{NaOH} + ext{H}_2 ext{O} ightarrow ext{Na}_2 ext{[Zn(OH)]}_4\)

This ability of ZnO to form different compounds when reacting with acids or bases is what makes it amphoteric. Understanding this property helps to predict the behavior of zinc oxide in various chemical environments, making it valuable for applications where reactions with both acids and bases are encountered.

Color change of zinc oxide

One of the fascinating physical properties of zinc oxide is its color change upon heating. In its natural, cooled state, ZnO appears white.
When zinc oxide is heated, it transitions to a yellow hue. However, when the material cools, it returns to its original white color.
This phenomenon is due to changes at the atomic level.

• The color change occurs because heating ZnO causes a rearrangement of electrons and the formation of defects in the lattice structure of the solid.
• This change temporarily alters the way the material absorbs and reflects light, giving it the yellow appearance on heating.
• Upon cooling, the electrons return to their original placement, and the solid turns white again.

This reversible color change is primarily a characteristic feature of ZnO, making it easily identifiable by this property in various experiments and applications.

White precipitate formation

The formation of a white precipitate when passing hydrogen sulfide (H extsubscript{2}S) through a solution containing zinc ions further confirms the presence of zinc oxide. H extsubscript{2}S reacts with these zinc ions to form zinc sulfide (ZnS), which appears as a white precipitate in solution.
The reaction can be described as follows:

• \( ext{Zn}^{2+} + ext{H}_2 ext{S} ightarrow ext{ZnS} + 2 ext{H}^+\)

The white precipitate, ZnS, is highly insoluble in water, confirming the reaction's occurrence. This reaction is a useful method for detecting zinc ions in a solution, as the formation of a white precipitate is a strong visual indicator of zinc's presence. Moreover, the formation of zinc sulfide from zinc oxide showcases the inert nature of ZnS in basic or neutral mediums, validating its identification and confirming the properties outlined under zinc oxide reactions.