Problem 81 - A Complete Resource Book in Chemistry for JEE Main

Question

A mixture of two salts is not water soluble but dissolves completely in dil. HCl to form a colourless solution. The mixture could be (a) \(\mathrm{BaCO}_{3}\) and \(\mathrm{ZnS}\) (b) \(\mathrm{AgNO}_{3}\) and \(\mathrm{KBr}\) (c) \(\mathrm{Mn}\left(\mathrm{NO}_{3}\right)_{2}\) and \(\mathrm{MgSO}_{4}\) (d) \(\mathrm{FeCl}_{3}\) and \(\mathrm{CaCO}_{3}\)

Step-by-Step Solution

Answer

The mixture could be (d) \( \mathrm{FeCl}_{3} \) and \( \mathrm{CaCO}_{3} \).

Step 1: Understanding the Problem

The problem involves identifying a mixture of two salts that is not soluble in water but will dissolve in dilute HCl to form a colorless solution. We need to determine which combination of salts could fit this description.

Step 2: Analyzing Option (a)

Consider the salts \( \mathrm{BaCO}_{3} \) and \( \mathrm{ZnS} \). Both are insoluble in water. \( \mathrm{BaCO}_{3} \) reacts with \( \mathrm{HCl} \) to form \( \mathrm{BaCl}_{2} \), \( \mathrm{CO}_{2} \), and \( \mathrm{H}_{2}\mathrm{O} \), while \( \mathrm{ZnS} \) reacts with \( \mathrm{HCl} \) to form \( \mathrm{ZnCl}_{2} \) and \( \mathrm{H}_{2}\mathrm{S} \). Neither forms a colorless solution due to the evolution of \( \mathrm{H}_{2}\mathrm{S} \) gas, which smells.

Step 3: Analyzing Option (b)

Consider \( \mathrm{AgNO}_{3} \) and \( \mathrm{KBr} \). \( \mathrm{AgNO}_{3} \) is water-soluble, making this option invalid as both salts in the mixture must be insoluble in water.

Step 4: Analyzing Option (c)

Consider \( \mathrm{Mn}(\mathrm{NO}_{3})_{2} \) and \( \mathrm{MgSO}_{4} \). Both are soluble in water, so they cannot form an insoluble mixture as required by the problem's conditions.

Step 5: Analyzing Option (d)

Consider \( \mathrm{FeCl}_{3} \) and \( \mathrm{CaCO}_{3} \). \( \mathrm{CaCO}_{3} \) reacts with \( \mathrm{HCl} \) producing \( \mathrm{CaCl}_{2} \), \( \mathrm{CO}_{2} \), and \( \mathrm{H}_{2}\mathrm{O} \), leaving a clear solution. \( \mathrm{FeCl}_{3} \) is soluble in water, hence the combination of the two in presence of \( \mathrm{HCl} \) results in a clear, colorless solution.

Key Concepts

Solubility

Solubility is a measure of how well a substance dissolves in a solvent, like water. When a substance dissolves, it separates into individual ions or molecules in the solvent. Some salts are highly soluble, meaning they dissolve easily in water. Others are insoluble, meaning they do not dissolve. Solubility depends on various factors:

• Temperature: Higher temperatures usually increase solubility.
• Nature of the solute and solvent: Certain combinations dissolve better than others.
• Presence of other chemicals: Some reactions can increase solubility.

In our exercise, the key is finding salts that are insoluble in water but react with hydrochloric acid (HCl) to form a clear solution.

Insoluble Salts

Insoluble salts do not dissolve in water, remaining as a solid. These salts require a chemical reaction to dissolve. For instance, in water, barium carbonate (\( \mathrm{BaCO}_{3} \)) and zinc sulfide (\( \mathrm{ZnS} \)) do not dissolve, making them insoluble. However, these salts can react with acids to form products that are soluble.When an insoluble salt like \( \mathrm{CaCO}_{3} \) reacts with dilute HCl, it forms \( \mathrm{CaCl}_{2} \), \( \mathrm{CO}_{2} \) gas, and water. This reaction leads to a clear solution. The ability of these salts to undergo reactions and change into soluble forms is a significant property in chemistry.

Reaction with HCl

Hydrochloric acid (HCl) is a strong acid often used to react with insoluble salts. This reaction can form soluble chlorides, releasing gases and water as byproducts. For example, calcium carbonate (\( \mathrm{CaCO}_{3} \)) reacts with HCl to dissolve and become calcium chloride (\( \mathrm{CaCl}_{2} \)), carbon dioxide (\( \mathrm{CO}_{2} \)), and water (\( \mathrm{H}_{2}O \)).Some salts will release gases upon reaction with HCl, such as hydrogen sulfide (\( \mathrm{H}_{2}S \)) in the case of zinc sulfide (\( \mathrm{ZnS} \)). These gases can be a clue to the nature of the chemical reaction taking place. Knowing how salts and acids react helps predict whether a solution remains clear and colorless.

Colorless Solutions

When salts dissolve in water or any other solvent, they form solutions. The color of these solutions depends on the ions present. Colorless solutions contain ions that do not produce color in visible light. For example, calcium chloride (\( \mathrm{CaCl}_{2} \)) in water appears colorless, which is why it's often a result of reactions with dilute HCl.The presence of colored ions, such as those from transition metals, typically displays vivid colors. However, in our exercise, a colorless solution suggests the contained ions are not absorbing visible light, indicating the absence of colored complexes.Understanding the types of ions that produce color versus those which do not, can help scientists and students alike deduce the composition of an unknown solution.