Problem 47 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Amongst the following, the total number of compounds whose aqueous solution turns red litmus paper blue is \(\mathrm{KCN}, \mathrm{K}_{2} \mathrm{SO}_{4},\left(\mathrm{NH}_{4}\right)_{2} \mathrm{C}_{2} \mathrm{O}_{4}, \mathrm{NaCl}, \mathrm{Zn}\left(\mathrm{NO}_{3}\right)_{2}, \mathrm{FeCl}_{3}, \mathrm{~K}_{2} \mathrm{CO}_{3}, \mathrm{NH}_{4} \mathrm{NO}_{3}\) and \(\mathrm{LiCN}\)

Step-by-Step Solution

Answer

3 compounds turn red litmus paper blue: KCN, K₂CO₃, and LiCN.

Step 1: Understand the Problem

We need to identify which of the given compounds will turn red litmus paper blue when dissolved in water. This indicates that the solution is basic (alkaline).

Step 2: Analyze Each Compound's Nature

- **KCN (Potassium cyanide):** CN⁻ is a strong base, so KCN turns red litmus blue. - **K₂SO₄ (Potassium sulfate):** Neutral compound, does not change litmus color. - **(NH₄)₂C₂O₄ (Ammonium oxalate):** Weakly acidic; does not turn red litmus blue. - **NaCl (Sodium chloride):** Neutral solution, does not change litmus color. - **Zn(NO₃)₂ (Zinc nitrate):** Acidic due to hydrolysis; does not turn red litmus blue. - **FeCl₃ (Ferric chloride):** Acidic due to hydrolysis; does not turn red litmus blue. - **K₂CO₃ (Potassium carbonate):** CO₃²⁻ is a strong base, turns red litmus blue. - **NH₄NO₃ (Ammonium nitrate):** Acidic solution; does not change litmus color. - **LiCN (Lithium cyanide):** CN⁻ is a strong base, so LiCN turns red litmus blue.

Step 3: Count the Basic Solutions

From the analysis, the compounds that turn red litmus paper blue are KCN, K₂CO₃, and LiCN. Thus, the number of such basic compounds is 3.

Key Concepts

Litmus Paper Test

A litmus paper test is a simple yet effective way to determine the acidity or basicity of a solution. Litmus paper is made from specially treated paper that changes color when exposed to different pH levels. When you immerse litmus paper in a solution:

• Red litmus paper will turn blue if the solution is basic (alkaline).
• Blue litmus paper turns red in the presence of an acidic solution.

These color changes occur because dyes in the litmus paper react with the hydrogen or hydroxide ions in the solution. For example, in our exercise, red litmus paper turned blue when exposed to basic solutions like those of KCN, K₂CO₃, and LiCN. This change indicated that these solutions were indeed basic because they produced more hydroxide ions in water. The litmus test is very helpful in quickly identifying the nature of a solution without needing elaborate equipment.

Aqueous Solutions

An aqueous solution is simply a solution where water is the solvent. This is the most common type of solution in chemistry because water is an excellent solvent due to its ability to dissolve many substances. In aqueous solutions:

• The solute is the substance that is dissolved (e.g., KCN, NaCl).
• The solvent is water, which facilitates the dissolution process.

When compounds like KCN, K₂SO₄, and NaCl dissolve in water, they dissociate into their respective ions. These ions interact with water, and the nature of this interaction determines if the solution will be acidic, neutral, or basic. For instance, if a compound releases hydroxide ions (OH⁻), the solution becomes basic, as seen with KCN and LiCN in our exercise. On the other hand, substances that release hydrogen ions (H⁺) tend to create acidic solutions, although that wasn't the focus here.

Basic Compounds

Basic compounds are substances that, when dissolved in water, increase the concentration of hydroxide ions (OH⁻) in the solution. This increase results in a pH greater than 7, indicating basicity. Such compounds often contain ions known to be bases, such as carbonate ( ext{CO}_3^{2-}) and cyanide ( ext{CN}^-).

• Compounds like KCN and LiCN dissociate to form cyanide ions, which strongly attract protons and create hydroxide ions.
• K₂CO₃, or potassium carbonate, dissociates to form carbonate ions, which also results in a basic solution.

In practical scenarios, these basic solutions can turn red litmus paper blue, which is a clear visual indication of their basic nature. Understanding the properties of basic compounds helps predict their behavior in various chemical reactions and solutions, just like in our exercise where we identified the basic characteristics of certain compounds.