Problem 21 - A Complete Resource Book in Chemistry for JEE Main

Question

A chloride dissolves appreciably in cold water. When placed on platinum wire in Bunsen flame no distinctive colour in noticed, the cation would be (a) \(\mathrm{Ba}^{2+}\) (b) \(\mathrm{Ca}^{2+}\) (c) \(\mathrm{Mg}^{2+}\) (d) \(\mathrm{Pb}^{2+}\)

Step-by-Step Solution

Answer

The cation is Mg^{2+} .

Step 1: Analyze the Properties

Consider the information given: the compound dissolves in cold water and does not show a distinctive color in a Bunsen flame test. These properties are key to identifying the metal cation from the given options.

Step 2: Consider Solubility in Water

We know that magnesium chloride ( MgCl_2) is highly soluble in cold water. Checking solubility data, Mg^{2+} is the cation most likely associated with a chloride that thoroughly dissolves in water compared to some of the other options.

Step 3: Check Flame Test Results

Understand how the cations exhibit color in a Bunsen flame: Ba^{2+} shows apple green, and Ca^{2+} shows brick red. Meanwhile, Pb^{2+} and Mg^{2+} do not exhibit a distinctive flame color. Since no color is observed, Mg^{2+} is a strong candidate.

Step 4: Verify by Elimination

Finally, confirm by process of elimination: Ba^{2+} and Ca^{2+} both show distinctive flame colors, which are not observed. PbCl_2 does not dissolve appreciably in cold water. Therefore, the only cation satisfying both conditions is Mg^{2+} .

Key Concepts

Solubility in Water

Solubility is how well a substance can dissolve in a solvent, like water. This property is primarily dependent on the intermolecular forces present between particles in the substance and the solvent. For many ionic compounds like chlorides, their solubility in water is largely determined by the nature of the ions.

• Water as a Solvent: Water is a polar solvent, meaning it has a partial positive and negative charge. This allows it to surround and dissociate ions from an ionic compound, leading to dissolving.
• Magnesium Chloride: In this context, magnesium chloride ( MgCl_2 ) is recognized for its high solubility in cold water. This is due to the efficient interaction between its ions ( Mg^{2+} and Cl^- ) and the polar water molecules.

Recognizing the solubility of various metal chlorides helps predict which compounds can dissolve readily in cold water. This understanding is foundational in concluding that Mg^{2+} is the most likely cation responsible for the observations in the problem statement.

Cation Identification

Identifying a cation in a compound involves examining the physical and chemical properties that the cation imparts. In our exercise, a key point is that when the chloride compound is placed in a Bunsen flame, it did not produce a distinctive color.

• Significance of No Color: In a flame test, most metal cations produce characteristic colors due to electron transitions. For example, Ba^{2+} appears apple green, and Ca^{2+} appears brick red. When there is no distinctive color, it's indicative that the cation does not significantly emit within the visible range.
• Magnifying the Choice of Mg^{2+}: Magnesium is known for its lack of color in a flame test, which aligns perfectly with the problem statement. This absence of a visible color emission aids in narrowing down Mg^{2+} as a likely candidate.

By examining these properties, students can pinpoint the cation involved, using techniques like flame tests as confirmation tools.

Metal Chlorides

Metal chlorides are compounds composed of metal cations and chloride anions ( Cl^- ). They are a large group with varied properties depending on the metal they contain.

• Chloride Role: Chlorides allow us to simplify the properties of the metallic cation they accompany, helping in tests like solubility and flame color identification.
• Common Metal Chlorides: Different metals forming chlorides show differing solubility and reactivity. For instance, MgCl_2 is highly soluble in water, a characteristic leveraged to differentiate Mg^{2+} from Pb^{2+} , which forms PbCl_2 , a less soluble compound.
• Application in Testing: Metal chlorides often undergo tests to identify and characterize the metal component. Understanding properties across the metal chloride spectrum is crucial for such identifications.

Recognizing how metal chlorides behave in solution and when subject to flame helps in varied applications, from simple labs to industrial processes.