Problem 20 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

An aqueous solution of a substance gives a white precipitate on treatment with dilute hydrochloric acid, which dissolves on heating. When hydrogen sulfide is passed through the hot acidic solution, a black precipitate is obtained. The substance is a (a) \(\mathrm{Hg}_{2}^{2+}\) salt (b) \(\mathrm{Cu}^{2+}\) salt (c) \(\mathrm{Ag}^{+}\)salt (d) \(\mathrm{Pb}^{2+}\) salt

Step-by-Step Solution

Answer

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

Step 1: Analyze Reaction with Dilute Hydrochloric Acid

The substance gives a white precipitate when treated with dilute hydrochloric acid, which dissolves upon heating. This indicates that the compound forms a chloride that becomes soluble at higher temperatures.

Step 2: Identify Lead Chloride as a Possible Candidate

Lead(II) chloride ( PbCl_2 ) is known to form a white precipitate when dilute HCl is added. This precipitate is soluble in hot water, matching the problem description.

Step 3: Analyze Reaction with Hydrogen Sulfide

Passing hydrogen sulfide through the hot acidic solution results in a black precipitate. Among the options, lead(II) sulfide ( PbS ) is known to form a black precipitate, which fits the observations.

Step 4: Finalize the Substance Identification

Combining the above observations – white precipitate re-dissolvable on heating and black precipitate formation with H_2S – suggests that the original ion is Pb^{2+} .

Key Concepts

Precipitation Reactions

Precipitation reactions occur when two soluble ionic compounds in solution react to form an insoluble solid, or precipitate. The substance that falls out of the solution is the precipitate. The reaction happens when ions in the solution bond together such that they form an insoluble compound.

In the context of the exercise provided, when a white precipitate forms upon the addition of dilute hydrochloric acid to an aqueous solution, it suggests that an insoluble chloride salt is created. The white precipitate dissolving on heating indicates the salt becomes more soluble at higher temperatures.

This behavior is exhibited by certain compounds, such as lead(II) chloride (PbCl_2), which forms a white precipitate. Heating increases the solubility of PbCl_2, leading to its dissolution.

• A visible precipitate is evidence of a chemical reaction.
• Solubility changes with temperature or the presence of different reagents.
• Precipitation reactions are key in identifying ions in solution.

Transition Metal Ions

Transition metals are elements found in the d-block of the periodic table, and they often form ions with variable oxidation states. Transition metal ions are unique because of their ability to form colorful compounds due to d-d electronic transitions.

In precipitation reactions, transition metal ions can form distinct precipitates with different reagents, allowing for easy identification. For example, lead(II) ion (\(\text{Pb}^{2+}\)) can form a variety of precipitates such as lead(II) chloride (\(\text{PbCl}_2\)), which is white, and lead(II) sulfide (\(\text{PbS}\)), which is black. The variety in possible compounds is a characteristic of transition metal ions that makes them useful in qualitative analysis.

• Usually form complexes and precipitates with characteristic colors.
• Widely used in analytical chemistry to detect the presence of specific ions.
• Transition metal ions often demonstrate variable oxidation states.

Aqueous Solutions

An aqueous solution is a solution in which the solvent is water. It's commonly used in chemistry because water is a good solvent that can dissolve many different substances. In an aqueous solution, ions are dissociated and surrounded by water molecules, allowing them to move freely and react with other ions.

The behavior of ions in aqueous solutions is crucial for many reactions, including the precipitation reactions discussed. Solutions like those involving \(\text{PbCl}_2\) are clear until a reagent is added that leads to the formation of a visible precipitate.

• Water's polarity allows it to dissolve ionic compounds effectively.
• Reaction conditions such as temperature and concentration affect solubility.
• Aqueous surroundings facilitate ion mobility and interaction, crucial for precipitation.