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

X is \( ext{H}_2 ext{O}_2\) and Y is \( ext{H}_2 ext{O}\).

Step 1: Identify Diamagnetic Properties

Both liquids X and Y are stated to be diamagnetic. This indicates that both X and Y do not have unpaired electrons in their atomic or molecular structures.

Step 2: Analyze the Litmus Test

Liquid X turns blue litmus red, which means X must be acidic. Liquid Y does not affect litmus, implying it is neutral. This is an important clue that helps differentiate between the two.

Step 3: Evaluate Reaction with Iodide and Acid

For liquid X, the reaction \( ext{X} + ext{KI} + ext{H}^+ \rightarrow\) leads to a blue color when starch is present. This reaction is characteristic of hydrogen peroxide (\( ext{H}_2 ext{O}_2\)), which oxidizes \( ext{I}^-\) ions to \( ext{I}_2\), giving a blue color with starch. Liquid Y does not show this color change, indicating it does not produce \( ext{I}_2\). This behavior is characteristic of water (\( ext{H}_2 ext{O}\)).

Step 4: Determine Acidic and Neutral Liquid

Given that X turns litmus red, it is acidic, and we know \( ext{H}_2 ext{O}_2\) is slightly more acidic than \( ext{H}_2 ext{O}\), which is neutral. Hence, \( ext{X} = ext{H}_2 ext{O}_2\) and \( ext{Y} = ext{H}_2 ext{O}\).

Step 5: Conclusion

Based on the properties and reaction results, \( ext{X} = ext{H}_2 ext{O}_2\) and \( ext{Y} = ext{H}_2 ext{O}\). This matches option (a) in the given choices.

Key Concepts

Understanding Diamagnetism

Diamagnetism is a fascinating property of certain substances. It arises from the paired electrons in atomic or molecular structures. When a substance is diamagnetic, it does not have any unpaired electrons. As a result, it slightly repels an external magnetic field.
Diamagnetism is the opposite of paramagnetism and ferromagnetism, where unpaired electrons do interact with magnetic fields.
In the context of the given problem, both liquids X and Y being diamagnetic means neither is influenced by a magnetic field because all their electrons are paired. This provides a clue to the nature of these substances and how they might behave in chemical reactions.

• Substances with no unpaired electrons exhibit diamagnetism.
• Diamagnetism causes a weak repulsion to magnetic fields.
• It helps identify molecular structures with completely paired electrons.

This property aids chemists in determining specific characteristics of elements and compounds, such as stability and reactivity within magnetic influences, which is crucial for applications in various scientific fields.

Exploring Acid-Base Reactions

Acid-base reactions are a fundamental concept in chemistry. They involve the transfer of protons (H⁺ ions) between substances.
An acid, like liquid X in the problem, is a substance that donates a proton in a reaction, turning blue litmus paper red, indicative of its acidic nature.
Meanwhile, a base would typically turn the red litmus paper blue, although this isn't observed with liquid Y.

• Acids donate H⁺ ions.
• Bases accept H⁺ ions.
• Litmus tests help identify acidic or basic properties.

In many reactions, acids and bases can neutralize each other, leading to the formation of water and salts. Litmus tests are a simple yet effective method for determining whether a solution is acidic or basic, as seen in identifying liquid X as an acidic substance in the given scenario. Knowing these reactions can allow predictions of behavior in various chemical processes.

The Role of Hydrogen Peroxide

Hydrogen peroxide (H₂O₂) is an interesting chemical compound, known for its role as both a bleaching agent and a disinfectant. It stands out due to its unique properties as a reactive oxygen species.
Unlike water (H₂O), hydrogen peroxide has an additional oxygen atom, resulting in its slightly acidic nature and ability to decompose into water and oxygen. This decomposition is crucial in its chemical reactions, such as the one involving potassium iodide (KI) and an acidic medium in our problem.

• H₂O₂ breaks down into H₂O and O₂.
• It can act as both an oxidizing and reducing agent.
• Important in synthesis and industrial applications.

In the exercise, the reaction of H₂O₂ with KI in acidic conditions forms iodine, which turns starch blue, a classic test for peroxides. This transformation distinguishes hydrogen peroxide from water, emphasizing its vital role in various chemical processes and industries. Its ability to engage in oxidation-reduction reactions makes it a versatile compound in both laboratory and industrial settings.