Problem 133 - A Complete Resource Book in Chemistry for JEE Main

Question

An aqueous solution containing \(\mathrm{S}^{2}\) ions will not give (a) White precipitate with \(\mathrm{CaCO}_{3}\) suspension (b) Purple colour with sodium thiosulphate solution (c) A yellow precipitate with the suspension of \(\mathrm{CdCO}_{3}\) in water (d) Black precipitate with lead acetate solution

Step-by-Step Solution

Answer

(b) Purple colour with sodium thiosulphate solution

Step 1: Identify the Ions Involved

We need to analyze the chemical reactions between the given aqueous solution containing \(\mathrm{S}^{2-}\) ions and the other substances mentioned in the options.

Step 2: Option Analysis: Calcium Carbonate

In the case of \(\mathrm{CaCO}_{3}\), \(\mathrm{S}^{2-}\) ions do not typically form a precipitate because calcium sulfide is not formed with \(\mathrm{S}^{2-}\) ions in an aqueous medium.

Step 3: Option Analysis: Sodium Thiosulphate

Sodium thiosulphate \((\mathrm{Na}_2\mathrm{S}_2\mathrm{O}_3)\) does not produce a purple color reaction with \(\mathrm{S}^{2-}\) ions. This reaction is more typical with iodine, not sulphide ions.

Step 4: Option Analysis: Cadmium Carbonate

When \(\mathrm{CdCO}_{3}\) interacts with \(\mathrm{S}^{2-}\) ions, cadmium sulfide (\(\mathrm{CdS}\)) is formed, which is a yellow precipitate as \(\mathrm{CdS}\) is insoluble in water.

Step 5: Option Analysis: Lead Acetate

When \(\mathrm{Pb(AcO)_2}\) (lead acetate) is in the presence of \(\mathrm{S}^{2-}\) ions, a black precipitate of lead sulfide \((\mathrm{PbS})\) is formed due to its insolubility.

Step 6: Determine the Correct Option

From the analysis, only option (b), where purple color is expected with sodium thiosulphate, results in no reaction. In this scenario, no visible change indicates the answer.

Key Concepts

Aqueous Solutions

An aqueous solution is one where water acts as the solvent. In such solutions, substances are dissolved at the molecular level, allowing various ions to participate in chemical reactions. For example, when sulfur ions

• (denoted as \( \mathrm{S}^{2-}\) are introduced into aqueous solutions, they can interact with other dissolved ions.
• This environment facilitates reactions, provided the reactive ion pairs are present.

Understanding aqueous solutions is crucial because it sets the stage for an array of chemical interactions. As water is a universal solvent, it is involved in many chemical processes where ionic compounds are concerned. These reactions open insights into solubility, precipitation, and more, giving us a baseline to predict outcomes when ions like \(\mathrm{S}^{2-}\) meet various chemicals in water-based settings.

Grasping the behavior of ions within an aqueous setting enhances both comprehension and predictive abilities regarding reactions, such as those involving calcium carbonate, which doesn’t react with sulfide ions in this medium.

Chemical Precipitation

The concept of chemical precipitation revolves around the formation of a solid in a solution during a chemical reaction. This solid, known as the precipitate, emerges when the product of a reaction is insoluble in the solvent.

• For instance, forming cadmium sulfide (\( \mathrm{CdS}\)), a yellow precipitate, occurs when \(\mathrm{CdCO}_{3}\) interacts with \(\mathrm{S}^{2-}\) ions in an aqueous environment.
• Key factors, such as the concentrations of ions and the solubility product constants, determine if a precipitate will form.

Precipitates are visible signals of chemical change and are pivotal in separating and purifying compounds in lab settings.

They mark crucial points in reactions which indicate completion or a shift in an equilibrium position. Therefore, understanding chemical precipitation can aid in recognizing when a reaction truly occurs, as seen with cadmium sulfide's formation.

Color Reactions

Color reactions are specific changes observed when certain chemicals interact, manifesting visually in a hue shift. These reactions are not only chemical footprints but also serve as qualitative tests for detecting specific ions in a solution.

• An important note is the expectation of a purple color from a reaction of \(\mathrm{S}^{2-}\) ions with sodium thiosulphate, which, in this case, does not occur.
• This highlights how different ions demand distinct tests for confirmation, with color changes often being the strategy.

These reactions tie directly to the qualitative analyses in chemistry labs, offering clues for the presence or absence of particular ions. The lack of reaction in certain settings can be as informative as an expected change, guiding decisions and conclusions regarding the chemical species involved.

Insolubility in Water

The theme of insolubility in water relates to compounds which, when mixed with water, do not dissolve but rather form a distinct phase.

• For example, compounds like lead sulfide (\( \mathrm{PbS}\)) and cadmium sulfide can precipitate due to their insolubility.
• Such insoluble compounds become visible as solids, often requiring removal or further processing.

Understanding insolubility is vital as it underscores limitations in mixing substances and highlights selective barrier reactions.

These occurrences allow chemists to harness these reactions for processes such as purification or extraction. Recognizing these principles aids in predicting and manipulating outcomes in various chemical environments, ensuring effective handling of substances that exhibit these characteristics.