Problem 91 - A Complete Resource Book in Chemistry for JEE Main

Question

An aqueous solution of a substance gives a white precipitate on treatment with dil. \(\mathrm{HCl}\) which dissolves on heating. When \(\mathrm{H}_{2} \mathrm{~S}\) is passed through the hot acidic solution, a black precipitate is obtained. The substance is a (a) \(\mathrm{Ag}^{+}\)salt (b) \(\mathrm{Cu}^{2+}\) salt (c) \(\mathrm{Pb}^{2+}\) salt (d) \(\mathrm{Hg}_{2}{\underline{\phantom{xx}}}^{2+}\) salt

Step-by-Step Solution

Answer

The substance is a \( \mathrm{Pb}^{2+} \) salt (option c).

Step 1: Analyze the Description for Precipitate Behavior

The problem states that upon treatment with dilute \( \mathrm{HCl} \), a white precipitate forms which dissolves upon heating. This behavior is characteristic of \( \mathrm{Pb}^{2+} \) ions. The white precipitate is likely \( \mathrm{PbCl}_2 \), which is soluble in hot water.

Step 2: Examine Reaction with \( \mathrm{H}_2\mathrm{S} \)

The passage of \( \mathrm{H}_2 \mathrm{S} \) through the hot acidic solution results in a black precipitate. This behavior is typical for \( \mathrm{PbS} \), which is a black precipitate, confirming the presence of \( \mathrm{Pb}^{2+} \) ions.

Step 3: Eliminate Other Options

Based on chemical properties: \( \mathrm{Ag}^{+} \) forms a white precipitate with \( \mathrm{Cl}^- \) that does not dissolve on heating (option a). \( \mathrm{Cu}^{2+} \) does not typically form a white precipitate with \( \mathrm{HCl} \) (option b), and \( \mathrm{Hg}_{2}^{2+} \) would form a black precipitate with \( \mathrm{HCl} \), but not dissolve on heating (option d). Thus, they can be ruled out.

Step 4: Conclude Based on Evidence

The evidence provided perfectly matches the known reactions of \( \mathrm{Pb}^{2+} \) ions with \( \mathrm{HCl} \) and \( \mathrm{H}_2 \mathrm{S} \). Therefore, the substance must be a \( \mathrm{Pb}^{2+} \) salt.

Key Concepts

Precipitation Reactions

Precipitation reactions are a fundamental concept in inorganic chemistry. They occur when two soluble substances in an aqueous solution react to form an insoluble product, known as the precipitate.
This process can be observed visually as a solid forms and settles out of the solution. The general mechanism behind precipitation reactions involves ion exchange between the reactants. When solutions containing ions are mixed, if the product of their concentrations exceeds the solubility of the compound, a precipitate will form. It's like mixing baking ingredients where, under the right conditions, the new ingredient solidifies out of the solution.
For instance, when aqueous solutions containing lead ions (\(\mathrm{Pb}^{2+}\)) and chloride ions (\(\mathrm{Cl}^-\)) come together, they form lead chloride (\(\mathrm{PbCl}_2\)), which is insoluble. Some important points about precipitation reactions include:

• Precipitates can often be identified by their color and texture, which provides clues about the reactants involved.
• The formation of a precipitate can often confirm the presence of specific ions in a solution.

Thus, in the problem above, the formation and dissolving of a white precipitate upon heating indicates a characteristic behavior of lead chloride.

Qualitative Analysis

Qualitative analysis is a branch of chemistry focused on identifying the composition of substances, rather than quantifying them. This is crucial in detecting what ions or molecules are present in a sample.
The process often involves stepwise testing and observation to derive conclusions based on chemical properties and reactions. In our context, qualitative analysis is used to identify the presence of lead ions (\(\mathrm{Pb}^{2+}\)) in an aqueous solution.Qualitative analysis employs a systematic approach:

• Reacting the substance with various reagents to form observable products.
• In this example, reagents like dilute hydrochloric acid (\(\mathrm{HCl}\)) and hydrogen sulfide (\(\mathrm{H}_2\mathrm{S}\)) are used to provoke characteristic reactions.
• Observations are then compared against expected outcomes to identify the ions present.

This analysis reveals lead's presence through the distinct step of a white precipitate forming and then dissolving with heat, followed by forming a black precipitate when hydrogen sulfide is added.

Lead Compounds

Lead compounds possess distinctive chemical properties that make them easily identifiable through chemical reactions. The most notable is lead (II) chloride (\(\mathrm{PbCl}_2\)), which forms a white precipitate, characteristic of lead compounds.Understanding the behavior of lead compounds involves recognizing how they interact with popular reagents:

• Lead chloride not only forms a white precipitate but also dissolves upon heating, which is unusual for many salts.
• When sulfur-containing reagents are introduced, such as hydrogen sulfide (\(\mathrm{H}_2\mathrm{S}\)), lead forms lead sulfide (\(\mathrm{PbS}\)), which is a black precipitate. This is a strong indicator of lead's presence.

Lead compounds are highly relevant in qualitative analysis due to their distinctive reactions with such simple tests. Recognizing these reactions is vital because they provide clear evidence for lead ions in a solution, simplifying the process of confirming the presence of lead.