Problem 97 - A Complete Resource Book in Chemistry for JEE Main

Question

Identify the two liquids \((\mathrm{X})\) and \((\mathrm{Y})\) made up of same atoms, both \((\mathrm{X})\) and \((\mathrm{Y})\) are diamagnetic. \((\mathrm{X})\) turns blue litmus to red, but (Y) does not. Also \(\mathrm{X}+\mathrm{KI}+\mathrm{H}^{+} \stackrel{\text { Starch }}{\longrightarrow}\) Blue colour \(\mathrm{Y}+\mathrm{KI}+\mathrm{H}^{+} \stackrel{\text { Starch }}{\longrightarrow}\) No colour (a) \(\mathrm{X}=\mathrm{H}_{2} \mathrm{O}_{2}, \mathrm{Y}=\mathrm{H}_{2} \mathrm{O}\) (b) \(\mathrm{X}=\mathrm{H}_{2} \mathrm{O}, \mathrm{Y}=\mathrm{H}_{2} \mathrm{O}_{2}\) (c) Both \(\mathrm{X}\) and \(\mathrm{Y}\) are \(\mathrm{H}_{2} \mathrm{O}\) (d) \(\mathrm{X}=\mathrm{H}_{2} \mathrm{O}_{2}, \mathrm{Y}=\mathrm{O}_{3}\)

Step-by-Step Solution

Answer

The answer is (a): X = H2O2, Y = H2O.

Step 1: Analyze the Properties of X and Y

X is a diamagnetic liquid that turns blue litmus to red, which indicates that it behaves as an acid. Y is also diamagnetic but does not change blue litmus, suggesting it is neutral. Both are made of the same atoms.

Step 2: Analyze the Reaction with KI, Starch, and H+

When X is combined with KI and H+, in the presence of starch, it turns blue. This indicates the production of iodine, often from the decomposition of peroxides like H2O2. When Y is subjected to the same treatment, no blue color is formed, indicating it does not produce iodine under these conditions.

Step 3: Apply Known Chemical Properties

H2O2 (hydrogen peroxide) is known to decompose in the presence of iodine and H+ to form iodine, resulting in a blue color with starch. H2O (water), however, does not react in this manner and is neutral, thus does not turn blue litmus red.

Step 4: Match Properties to Given Options

Option (a): X = H2O2 turns blue litmus red, reacts with KI to form a blue solution. Y = H2O does not react with KI and H+, and does not change the litmus color, matching the properties described.

Key Concepts

Acid-Base Reactions

Acid-base reactions are a fundamental aspect of chemistry. They involve the transfer of hydrogen ions (\(H^+\)), which can result in the formation of water and salt. Such reactions are typically characterized by their ability to change the color of acid-base indicators, like litmus paper.

• An acid, such as hydrogen peroxide (\(H_2O_2\)), can donate \(H^+\) ions. When it interacts with blue litmus paper, the acid donates a proton, turning the paper red.
• A base, or a neutral substance like water (\(H_2O\)), does not donate or accept \(H^+\) ions in this way, so it keeps the blue litmus unchanged.

By understanding these principles, one can deduce the nature of a liquid in an unknown sample through simple tests.

Hydrogen Peroxide Properties

Hydrogen peroxide (\(H_2O_2\)) is a well-known compound in chemistry, acting as both a weak acid and a strong oxidizing agent.

• As a diamagnetic liquid, \(H_2O_2\) does not have unpaired electrons. This property means it slightly repels magnetic fields and is an essential factor in identifying unknown substances.
• It is capable of decomposing into water and oxygen. This decomposition can be catalyzed in various reactions, including when mixed with potassium iodide (\(KI\)) and an acid. This catalysis results in the formation of iodine, which provides a visible color change when starch is present, thereby confirming the presence of hydrogen peroxide.

The distinctive properties of \(H_2O_2\) make it a versatile tool in both experimental and practical chemistry.

Litmus Test

The litmus test is a convenient and widely used method in chemistry to determine the acidic or basic nature of a substance.

• In its simplest form, litmus paper is treated with a natural dye derived from lichens. Blue litmus paper turns red in the presence of an acid, while red litmus turns blue in the presence of a base.
• For example, when evaluating the properties of a substance like hydrogen peroxide (\(H_2O_2\)), placing blue litmus paper in the liquid leads to a color change to red, confirming its acidic nature.
• In contrast, a neutral substance like water (\(H_2O\)) will not change the color of the litmus paper, making it neutral in this context.

This easy yet powerful test provides instant feedback on the chemical character of solutions or substances in diverse scientific and industrial settings.