Problem 94 - A Complete Resource Book in Chemistry for JEE Main

Question

Gas ' \(\mathrm{A}\) ' is bubbled through slaked lime when a white precipitate is formed. On prolonged bubbling the precipitate dissolved. On heating the resultant solution, the white precipitate reappeared with solution of gas 'B'. The gases 'A' and 'B' respectively are (a) \(\mathrm{CO}\) and \(\mathrm{CO}_{2}\) (b) \(\mathrm{CO}_{2}\) and \(\mathrm{CO}_{2}\) (c) \(\mathrm{CO}\) and \(\mathrm{CO}\) (d) \(\mathrm{CO}_{2}\) and \(\mathrm{CO}\)

Step-by-Step Solution

Answer

The gases 'A' and 'B' are both CO_2, so option (b) is correct.

Step 1: Identify Gas A and Initial Reaction

Gas 'A' is bubbled through slaked lime (Ca(OH)_2), resulting in the formation of a white precipitate. This reaction suggests that Gas 'A' must be carbon dioxide (CO_2), which causes the formation of calcium carbonate (CaCO_3), as shown in the equation: \( ext{Ca(OH)}_2 + ext{CO}_2 \rightarrow ext{CaCO}_3 + ext{H}_2 ext{O} \). This reaction leads to the formation of the white precipitate, calcium carbonate.

Step 2: Prolonged Bubbling of Gas A

When the gas is bubbled for a longer period, the initially formed calcium carbonate precipitate dissolves. This indicates that excess CO_2 is being bubbled, leading to the formation of calcium bicarbonate \( \text{Ca(HCO}_3)_2 \), which is soluble in water. The reaction is: \( ext{CaCO}_3 + ext{CO}_2 + ext{H}_2 ext{O} \rightarrow ext{Ca(HCO}_3)_2 \).

Step 3: Identify Gas B and Reappearance of Precipitate

Upon heating the resultant solution, the calcium bicarbonate decomposes back to form the white precipitate of calcium carbonate and releases CO_2 gas. The reaction can be described as: \( ext{Ca(HCO}_3)_2 \rightarrow ext{CaCO}_3 + ext{CO}_2 + ext{H}_2 ext{O} \). This confirms that Gas 'B' is also CO_2.

Step 4: Conclusion

From the analysis, it is clear that both gases 'A' and 'B' are carbon dioxide (CO_2). Therefore, the correct answer is option (b) CO_2 and CO_2.

Key Concepts

Gas Reactions

Gas reactions involve reactants in the gaseous state interacting with other substances. These reactions are key in many natural and industrial processes. For example, in the original exercise, gas 'A' is carbon dioxide \( \text{CO}_2 \). When \( \text{CO}_2 \) is bubbled through slaked lime, or calcium hydroxide \( \text{Ca(OH)}_2 \), a chemical reaction occurs.
\[ \text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O} \]
This reaction forms a white precipitate of calcium carbonate \( \text{CaCO}_3 \), illustrating a typical gas-solid reaction. Gas reactions can yield solids, liquids, or even more gases as products, depending on the nature of the reactants involved.
Important points about gas reactions:

• They can vary based on temperature and pressure.
• Soluble or insoluble compounds can result.
• They are reversible under certain conditions.

Understanding these reactions helps us comprehend numerous chemical and environmental processes.

Precipitation Reactions

Precipitation reactions involve the formation of a solid from a solution when two soluble substances react. The formation of a precipitate indicates that a new substance is created that is insoluble in water. This is evident in the reaction between carbon dioxide and slaked lime in the original exercise.
In this case, the initial formation of a white precipitate of calcium carbonate \( \text{CaCO}_3 \) indicates a precipitation reaction between \( \text{CO}_2 \) and \( \text{Ca(OH)}_2 \). Here's the equation:
\[ \text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O} \]
Through prolonged exposure, however, the calcium carbonate dissolves as more \( \text{CO}_2 \) is added, indicating:

• Excess reactants can dissolve precipitates.
• Precipitation is often initially reversible.

Learning about precipitation reactions helps individuals predict the outcomes of mixing different ionic compounds in solutions.

Carbon Dioxide Behavior

Carbon dioxide, \( \text{CO}_2 \), is a versatile gas involved in many chemical processes. Its interactions with water and calcium hydroxide provide great examples of its behavior. Initially, \( \text{CO}_2 \) reacts with \( \text{Ca(OH)}_2 \) to form a precipitate. Upon continued bubbling of \( \text{CO}_2 \), this precipitate dissolves due to the formation of calcium bicarbonate \( \text{Ca(HCO}_3)_2 \), which is soluble in water.
Equation illustrating this behavior:
\[ \text{CaCO}_3 + \text{CO}_2 + \text{H}_2\text{O} \rightarrow \text{Ca(HCO}_3)_2 \]
Key aspects of carbon dioxide behavior include:

• Ability to form both soluble and insoluble compounds.
• Behavior changes with concentration and conditions.
• Reactions can be reversed upon heating or changing concentrations.

This illustrates a fundamental aspect of CO2's role in geological and biological processes.