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 \(\mathrm{HCl}\) or \(\mathrm{NaOH}\). When \(\mathrm{H}_{2} \mathrm{~S}\) is passed through solution \(\mathrm{B}\), and 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

Compound A is ZnO.

Step 1: Understanding the properties of compound A

Compound A is described as a white, water-insoluble solid that turns yellow upon heating and reverts back to white upon cooling.

Step 2: Reaction with dilute HCl or NaOH

When compound A is treated with dilute HCl or NaOH, it forms a clear solution B. This suggests that A is amphoteric, meaning it can react with both acids and bases.

Step 3: Reaction upon passing H2S into solution B

Passing H2S through solution B and making it neutral results in the formation of a white precipitate C. This is indicative of a metal sulfide precipitate.

Step 4: Identifying compound A

Known amphoteric oxides that show this color change upon heating include ZnO. ZnO is white at room temperature and turns yellow upon heating. Furthermore, ZnO is an amphoteric oxide, which reacts with both acids and bases to form a clear solution.

Key Concepts

ZnO properties

Zinc Oxide (ZnO) is a fascinating chemical compound with multiple unique properties. At room temperature, ZnO appears as a white solid. Interestingly, when heated, it undergoes a physical change and turns yellow. Upon cooling, it reverts back to its original white color. This reversible color change is due to changes in the energy levels within the compound when heated.

ZnO is known to be insoluble in water, making it distinct from many other compounds. Yet, it reacts readily with strong acids like hydrochloric acid (HCl) and strong bases like sodium hydroxide (NaOH), which leads us to its next important property.

Metal sulfide precipitation

When you react certain solutions with hydrogen sulfide ( H_2S ) and adjust them to a neutral pH, a precipitate can form. This is a common method to identify metal ions in a solution.

In the context of this chemistry problem, when ZnO is dissolved in either an acidic or basic environment to create solution B, and then H_2S is passed through it, a white precipitate of zinc sulfide ( ZnS ) forms. This precipitate confirms the presence of zinc ions, as zinc sulfide is the expected and identifiable product from this reaction.

• Solution B reacts with H_2S to form ZnS, the white precipitate.
• Adjusting the pH to neutral is crucial for precipitate formation.

This property of forming a precipitate is useful in qualitative analysis within chemistry labs.

Chemistry problem solving

Tackling chemistry problems requires a logical approach. Begin by noting the properties and reactions involved, as seen with Zinc Oxide in this exercise.

Breaking down the problem into steps can simplify it significantly:

• Identify physical properties: Note whether changes occur upon heating or cooling, as these can offer clues about the compound's identity.
• Reactivity with acids and bases: Experiments involving reactions with HCl or NaOH can pinpoint amphoteric nature.
• Formation of precipitates: Determine what happens when H_2S is introduced; the precipitate formed is an essential clue.

Using these steps, compound A was identified as ZnO, confirmed by the specific reactions that occurred and the appearance of the precipitate.

Reaction with HCl and NaOH

Zinc Oxide's interaction with both HCl and NaOH is a key point in understanding amphoteric behavior. Amphoteric substances can react with both acids and bases, producing salts and water.

When ZnO reacts with HCl:

• The reaction forms zinc chloride ( ZnCl_2 ) and water.
• This provides a clear solution, showcasing the dissolution of ZnO.

With NaOH, ZnO forms sodium zincate ( Na_2ZnO_2 ) and water, another clear solution. This behavior confirms ZnO's amphoteric nature.

This capacity for dual reactivity is notable because it sets amphoteric oxides apart from typical oxides. It also provides a broader range of applications and experimental possibilities in chemistry.