Question

Which of the following radicals evolve gas turning lime water milky? (a) \(\mathrm{C}_{2} \mathrm{O}_{4}^{2-}\) (b) \(\mathrm{CO}_{3}^{2-}\) (c) \(\mathrm{SO}_{3}^{2-}\) (d) All of these

Short answer

Option (b) \( \mathrm{CO}_{3}^{2-} \).

Step-by-step solution

Understand the Problem

We need to identify which ion, when combined with an acid, will produce a gas that can make lime water (a solution of calcium hydroxide) turn milky. This typically indicates the production of carbon dioxide gas.

Analyze Option (a) \( \mathrm{C}_{2} \mathrm{O}_{4}^{2-} \)

The oxalate ion \( \mathrm{C}_{2} \mathrm{O}_{4}^{2-} \) does not release \( \mathrm{CO}_2 \) gas when treated with dilute acids. Instead, it usually forms soluble complexes or reacts differently.

Analyze Option (b) \( \mathrm{CO}_{3}^{2-} \)

The carbonate ion \( \mathrm{CO}_{3}^{2-} \) reacts with acids to produce carbon dioxide gas: \( \mathrm{CO}_{3}^{2-} + 2\mathrm{H}^+ \rightarrow \mathrm{CO}_{2}(g) + \mathrm{H}_2\mathrm{O} \). Carbon dioxide turns lime water milky due to the formation of calcium carbonate.

Analyze Option (c) \( \mathrm{SO}_{3}^{2-} \)

The sulfite ion \( \mathrm{SO}_{3}^{2-} \) reacts with acids to produce sulfur dioxide gas: \( \mathrm{SO}_{3}^{2-} + 2\mathrm{H}^+ \rightarrow \mathrm{SO}_{2}(g) + \mathrm{H}_2\mathrm{O} \). Sulfur dioxide does not turn lime water milky.

Conclude from the Analysis

Based on the reactions, only the carbonate ion \( \mathrm{CO}_{3}^{2-} \) produces carbon dioxide gas which turns lime water milky. Therefore, the correct answer is option (b).

Key concepts

Carbonate Ion Reactions

Carbonate ions, represented by the chemical formula \( \mathrm{CO}_{3}^{2-} \), are specialized ions characterized by the presence of both carbon and oxygen atoms. When carbonate ions are exposed to acids, a chemical reaction occurs that releases carbon dioxide gas. This is a classic example of the interaction between carbonate ions and acids. In this process, the acid supplies hydrogen ions \( \mathrm{H}^+ \), which react with the carbonate ions as follows:\[ \mathrm{CO}_{3}^{2-} + 2\mathrm{H}^+ \rightarrow \mathrm{CO}_2(g) + \mathrm{H}_2\mathrm{O}\]As the equation shows, this reaction results in the formation of carbon dioxide gas \( \mathrm{CO}_2(g) \) and water \( \mathrm{H}_2\mathrm{O} \). The liberation of carbon dioxide gas is crucial for certain detection experiments and is a foundational element in many chemical processes, including those found in environmental science and industrial applications.

Carbon Dioxide Detection

Detecting carbon dioxide involves identifying its presence through its ability to react with certain compounds. In chemistry, carbon dioxide gas can be detected by its characteristic reactions that change the appearance of a solution or substance. A common way to confirm the presence of carbon dioxide is through the lime water test. When carbon dioxide gas is bubbled through lime water, a solution made from calcium hydroxide \( \mathrm{Ca(OH)_2} \), the solution becomes cloudy or milky. This occurs due to the formation of calcium carbonate \( \mathrm{CaCO_3} \), a white, insoluble solid:\[ \mathrm{Ca(OH)_2} + \mathrm{CO}_2 \rightarrow \mathrm{CaCO_3} + \mathrm{H}_2\mathrm{O}\]This visible change provides a convenient and quick method to detect carbon dioxide gas in a laboratory or classroom setting. The lime water test is frequently illustrated in educational experiments to demonstrate the chemical properties of gases and reactions.

Chemical Reactions with Acids

The interaction between acids and other substances is an essential concept in chemistry. Acids can react with various ions, leading to the formation of gases, salts, or water as end products. One notable interaction is between carbonate ions \( \mathrm{CO}_{3}^{2-} \) and acids. When these ions react with acids, they release carbon dioxide gas, which is a typical byproduct. This reaction forms part of a broader category of acid-base reactions, where acids donate hydrogen ions \( \mathrm{H}^+ \) to react with other substances. Such reactions are central to understanding how different compounds interact and transform. They have applications across numerous fields such as geology, where acids break down carbonate minerals, and in industrial contexts like the manufacture of calcium carbonate.

Lime Water Test

The lime water test is a classic chemistry experiment used to detect the presence of carbon dioxide gas. The test involves passing the suspected gas through lime water, which is a diluted solution of calcium hydroxide \( \mathrm{Ca(OH)_2} \). Upon contact with carbon dioxide, the lime water turns milky.This milky appearance is due to the formation of calcium carbonate \( \mathrm{CaCO_3} \), a solid precipitate that is insoluble in water, according to the reaction:\[ \mathrm{Ca(OH)_2} + \mathrm{CO}_2 \rightarrow \mathrm{CaCO_3} + \mathrm{H}_2\mathrm{O}\]The lime water test is not only a simple and rapid method for carbon dioxide detection but also a practical educational tool to help students visualize chemical reactions. It's used in fields ranging from environmental science to industrial processes to test for the presence and concentration of carbon dioxide.