Problem 43 - A Complete Resource Book in Chemistry for JEE Main

Question

Aqueous solution of carnalite gives a positive test for (a) potassium, magnesium and chloride ions (b) potassium ions only (c) potassium and chloride ions only (d) chloride ions only

Step-by-Step Solution

Answer

Option (a): potassium, magnesium, and chloride ions.

Step 1: Understanding Carnalite's Composition

Carnalite is a mineral composed of potassium, magnesium, and chloride, with the chemical formula \( KCl \cdot MgCl_2 \cdot 6H_2O \). This indicates that an aqueous solution of carnalite contains potassium ions \( (K^+) \), magnesium ions \( (Mg^{2+}) \), and chloride ions \( (Cl^-) \).

Step 2: Identifying Ions from Aqueous Solution

When carnalite is dissolved in water, it dissociates into its constituent ions: potassium ions \( K^+ \), magnesium ions \( Mg^{2+} \), and chloride ions \( Cl^- \). Thus, we expect all these ions to be present in the solution.

Step 3: Determining the Positive Test

A positive test refers to the identification or confirmation of the presence of certain ions in a solution. Since carnalite dissociates into potassium, magnesium, and chloride ions in solution, a positive test would confirm the presence of each of these ions.

Step 4: Choosing the Correct Option

Based on the dissociation of carnalite and the ions identified, the aqueous solution tests positive for all three ions: potassium \( K^+ \), magnesium \( Mg^{2+} \), and chloride \( Cl^- \). Therefore, the correct answer is option (a): potassium, magnesium, and chloride ions.

Key Concepts

Chemical Composition

Carnalite is a fascinating mineral that not only captures interest due to its vibrant chemistry but also due to its crucial composition. This mineral is chemically represented by the formula \( KCl \cdot MgCl_2 \cdot 6H_2O \). This formula can be broken down as follows:

• Potassium Chloride \( (KCl) \)
• Magnesium Chloride \( (MgCl_2) \)
• Water of crystallization, specifically six molecules \( (6H_2O) \)

This composition indicates that when carnalite dissolves, it releases ions into the solution. Knowing this helps in predicting which ions will be present in its aqueous form. Understanding the chemical makeup is critical because without recognizing the components, it would be impossible to anticipate the behavior of carnalite in an aqueous environment.

Ion Identification

Upon dissolution in water, carnalite dissociates into its primary components. This process is known as dissociation, and it results in the formation of individual ions:

• Potassium ions \( (K^+) \)
• Magnesium ions \( (Mg^{2+}) \)
• Chloride ions \( (Cl^-) \)

Each of these ions is critical for various chemical and physical processes that can occur in solution. Identifying these ions is essential for performing further chemical testing, providing insights into reactivity, and understanding the substance's full chemical profile. These ions are the basic units that help chemists predict and describe chemical reactions—for example, determining if precipitation would occur when mixed with another solution.

Aqueous Solutions

When carnalite is dissolved in water, it forms what is known as an aqueous solution. An aqueous solution is a liquid mixture where the solvent is water, which acts as the medium in which substances dissolve. Water molecules aid the separation of carnalite by surrounding its ions and keeping them within the solution. In this context, the solubility of carnalite in water is an essential property because it defines how well the substance dissolves. A high solubility means that plenty of the ions will be present in the solution, making it more reactive. Therefore, understanding how substances behave in aqueous solutions is fundamental to mastering many concepts in chemistry.

Chemical Tests

Chemical tests are experiments carried out to identify or confirm the presence of different ions in a solution. These tests are crucial for analyzing an aqueous solution of carnalite. For carnalite, a positive chemical test could be applied to detect each ion present. - Potassium ions \( (K^+)\) might be identified by their characteristic flame test, which emits a lilac color. - Magnesium ions \( (Mg^{2+})\) could be detected using a reagent like sodium hydroxide \((NaOH)\), forming a white precipitate of magnesium hydroxide \((Mg(OH)_2)\). - Chloride ions \( (Cl^-) \) may be confirmed by adding silver nitrate \((AgNO_3)\), which precipitates as silver chloride \((AgCl)\), a white solid.These chemical tests are essential for students to understand practical applications of theory, enabling them to verify the composition of solutions and learn the significance of ion presence in chemistry.