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. 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) \(\mathrm{Cu}^{2+} \mathrm{salt}\) (c) \(\mathrm{Pb}^{2+}\) salt (d) \(\mathrm{Hg}_{2}^{2+} \mathrm{salt}\)

Step-by-Step Solution

Answer

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

Step 1: Understanding the Reaction with Dil. HCl

The solution forms a white precipitate when treated with dilute HCl, which dissolves on heating. Compounds of lead, such as lead chloride (\(\text{PbCl}_2\)), form white precipitates with \(\text{HCl}\) that dissolve in hot water, matching this description.

Step 2: Investigating Reaction with H2S

When \(\text{H}_2\text{S}\) is passed through the hot acidic solution, a black precipitate forms. Lead ions (\(\text{Pb}^{2+}\)) react with hydrogen sulfide to form lead sulfide (\(\text{PbS}\)), which is black. This behavior is consistent with lead salts.

Step 3: Comparing Options with Observations

Reviewing the options, \(\text{Ag}^+\) and \(\text{Hg}_2^{2+}\) can also form white precipitates with HCl but do not react as described with \(\text{H}_2\text{S}\). \(\text{Cu}^{2+}\) does not usually form white precipitates with \(\text{HCl}\). The only consistent explanation is \(\text{Pb}^{2+}\), which forms both a white precipitate with \(\text{HCl}\) and a black precipitate with \(\text{H}_2\text{S}\).

Key Concepts

Precipitation Reactions

Precipitation reactions are a fundamental type of chemical reaction, especially relevant in inorganic chemistry. They occur when two soluble salts in aqueous solution react to form an insoluble solid, known as a precipitate. This process is often used in qualitative analysis to identify components in a solution.

Understanding the outcomes and conditions of precipitation reactions is critical. It involves examining solubility rules that help predict whether a precipitate will form when different ions are combined. In simple terms:

• If the product of a reaction is insoluble in water, a precipitate will occur.
• The precipitate can be identified based on its color and solubility characteristics.
• Temperature and concentration can affect the formation and characteristics of the precipitate.

In the given exercise, the formation of a white precipitate when dilute HCl is added suggests the creation of an insoluble chloride compound. Heating helps redissolve some precipitates, providing additional information to identify the substance correctly.

Lead Salts

Lead salts, such as lead chloride ( PbCl_2), feature prominently in precipitation reactions due to their unique properties. Lead salts are versatile in their reactions, forming different products based on the reactants and conditions involved.

In a laboratory setting, recognizing a lead salt is often facilitated by its interaction with hydrochloric acid (HCl) and hydrogen sulfide ( H_2S):

• Addition of HCl to a lead salt solution generally forms lead chloride, appearing as a white precipitate.
• Upon heating, the insoluble lead chloride can dissolve, helping to differentiate it from other metal chlorides.
• With H_2S, lead companounts produce lead sulfide, a black precipitate, distinct in identification processes.

The ability of lead salts to form both colorless and contrasting colored compounds on reaction enhances their usefulness in chemical analysis, allowing for a clear distinction from other possible solutions.

Hydrogen Sulfide Reaction

The hydrogen sulfide reaction is a classic method used extensively to test for metal ions in a qualitative analysis setting. When a solution containing metal ions is treated with hydrogen sulfide (H_2S), different metal sulfides may precipitate based on the metal ions present.

Here is what usually happens during such interactions:

• Hydrogen sulfide acts as a reducing environment and introduces sulfide ions into the solution.
• Certain metal ions, like lead ( Pb^{2+}), react to produce metal sulfides that are usually black, such as lead sulfide ( PbS).
• The characteristic colors of metal sulfides help in identifying the metal ions, as is evident with the black precipitate of lead sulfide.

This type of reaction is significant because it is not only pivotal in confirming the presence of specific metal salts but also aids in understanding the reactivity and selectivity of sulfides in aqueous solutions.

Aqueous Solutions

Aqueous solutions are central to the study of inorganic chemistry, serving as a medium for many reactions including precipitation and redox processes. An aqueous solution simply means that water is the solvent in which the solute is dissolved.

The property of water in supporting ions and enabling reactions highlights its role in chemistry:

• Water's polar nature allows it to dissolve a wide range of substances, making it ideal for reactions requiring ion exchange.
• In these solutions, solubility rules are applied to predict and identify possible precipitates.
• Using aqueous solutions provides a controlled environment to study reactions like the lead salt precipitation described in the exercise.

Thus, when working with aqueous solutions, understanding the interactions between dissolved ions and the solvent helps chemists not only predict outcomes but also manipulate reactions toward desired results in practical applications.