Problem 39 - A Complete Resource Book in Chemistry for JEE Main

Question

The gas that turns lime water milky is (a) \(\mathrm{SO}_{2}\) (b) \(\mathrm{CO}_{2}\) (c) both of these (d) none of these

Step-by-Step Solution

Answer

The gas that turns lime water milky is (b) CO2.

Step 1: Understanding the Chemical Reaction

When carbon dioxide, CO2, is passed through lime water (a clear solution of calcium hydroxide), it reacts to form calcium carbonate, which is insoluble in water and causes the solution to turn milky. The chemical reaction is: \[ \text{Ca(OH)}_2 (\text{aq}) + \text{CO}_2 (\text{g}) \rightarrow \text{CaCO}_3 (\text{s}) + \text{H}_2\text{O} (\text{l}) \] This is a known test for the presence of \( \text{CO}_2 \).

Step 2: Considering SO2 Reaction

Sulfur dioxide, SO2, can also react with lime water, but instead of turning it milky right away, it forms calcium sulfite, which might stay in solution or eventually precipitate depending on concentrations and conditions. The immediate milky effect is not as characteristic as the one caused by \( \text{CO}_2 \). The reaction is: \[ \text{Ca(OH)}_2 (\text{aq}) + \text{SO}_2 (\text{g}) \rightarrow \text{CaSO}_3 (\text{s}) + \text{H}_2\text{O} (\text{l}) \]

Step 3: Identifying the Correct Option

Given the more consistent and pronounced reaction of \( \text{CO}_2 \) with lime water that results in a milky solution due to calcium carbonate, the correct answer is that the gas that turns lime water milky is \( \text{CO}_2 \). \( \text{SO}_2 \) does not produce the same immediate milky effect.

Key Concepts

Calcium Carbonate Formation

When carbon dioxide (\(\text{CO}_2\)) interacts with lime water, a fascinating transformation takes place. Lime water is a diluted solution of calcium hydroxide (\(\text{Ca(OH)}_2\)). Upon adding \(\text{CO}_2\), a chemical reaction occurs, resulting in the formation of calcium carbonate (\(\text{CaCO}_3\)), a compound that is not soluble in water and appears as a milky suspension. This creates the characteristic cloudy appearance.The reaction can be illustrated by the following chemical equation:\[\text{Ca(OH)}_2 (\text{aq}) + \text{CO}_2 (\text{g}) \rightarrow \text{CaCO}_3 (\text{s}) + \text{H}_2\text{O} (\text{l})\]Here are some noteworthy points about this process:

• Calcium carbonate is the reason for the milky appearance, which indicates a successful reaction.
• It's a straightforward test often used in labs to detect the presence of carbon dioxide.
• The formation of calcium carbonate is a hallmark of a basic-acid reaction, where the basic nature of lime water neutralizes the acidic carbon dioxide gas.

Understanding this simple yet crucial reaction is essential in grasping how gases interact with solutions to form precipitates.

Reaction with Carbon Dioxide

Carbon dioxide (\(\text{CO}_2\)) is a well-known atmospheric gas that plays significant roles in various chemical reactions. One pivotal interaction is its reaction with lime water. When \(\text{CO}_2\) is bubbled through lime water, the lime water turns milky due to the formation of calcium carbonate (\(\text{CaCO}_3\)).This reaction is not just a lab experiment but has real-world implications. For instance:

• It mirrors natural processes like the formation of limestone in marine environments.
• It highlights how carbon dioxide can be effectively captured and stored as a solid, a principle utilized in carbon capture strategies.
• The milky appearance of lime water when reacting with \(\text{CO}_2\) is also a visible demonstration of the gas throughout various teaching scenarios.

Recognizing this behavior of \(\text{CO}_2\) is vital for understanding both environmental science and basic principles of chemistry, as it demonstrates how gases can be transformed and utilized in chemical reactions.

Chemical Tests for Gases

Chemical tests are crucial in identifying and analyzing the presence of various gases. The lime water test is one such method used to detect carbon dioxide (\(\text{CO}_2\)). It leverages the characteristic formation of calcium carbonate to indicate the presence of \(\text{CO}_2\).Here is how the lime water test stands out among chemical tests for gases:

• Simple Procedure: The test involves passing the suspected gas through lime water. If \(\text{CO}_2\) is present, the solution turns milky.
• Visual Clue: The milky appearance is a straightforward visual indicator, making it accessible and easy to interpret without complex instruments.
• Selectivity: Although other gases like sulfur dioxide (\(\text{SO}_2\)) can also react with lime water, their immediate effects are different and less pronounced, particularly in relation to producing a milky solution.

The lime water test is therefore not only a classic demonstration in education but also a reliable method to ascertain the presence of carbon dioxide. This makes it an invaluable tool in both scientific exploration and practical applications.