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

Question

Gas '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

(b) \( \text{CO}_2 \) and \( \text{CO}_2 \)

Step 1: Understanding the Initial Reaction

When gas 'A' is bubbled through slaked lime, a white precipitate forms. Slaked lime is chemically calcium hydroxide, or \( \text{Ca(OH)}_2 \). If the white precipitate is \( \text{CaCO}_3 \), carbon dioxide (\( \text{CO}_2 \)) is the likely gas responsible for this reaction, as it reacts with calcium hydroxide to form calcium carbonate (\( \text{CaCO}_3 \)).\[ \text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O} \]

Step 2: Understanding Prolonged Bubbling

On prolonged bubbling, the precipitate dissolves. This suggests the formation of calcium bicarbonate (\( \text{Ca(HCO}_3)_2 \)), which is soluble in water. This is formed when excess \( \text{CO}_2 \) reacts with \( \text{CaCO}_3 \).\[ \text{CaCO}_3 + \text{CO}_2 + \text{H}_2\text{O} \rightarrow \text{Ca(HCO}_3)_2 \]

Step 3: Heating the Solution

Upon heating the resultant solution, the white precipitate reappears. When calcium bicarbonate solution is heated, it reverts back to calcium carbonate and releases carbon dioxide gas. This means gas 'B' is also \( \text{CO}_2 \).\[ \text{Ca(HCO}_3)_2 \rightarrow \text{CaCO}_3 + \text{CO}_2 + \text{H}_2\text{O} \]

Step 4: Analyzing Final Results

Both the initial and reappearing reactions involve \( \text{CO}_2 \). Thus, both gases 'A' and 'B' are carbon dioxide, i.e., option (b) \( \text{CO}_2 \) and \( \text{CO}_2 \).

Key Concepts

Slaked Lime

Slaked lime, also known as calcium hydroxide, is a crucial chemical compound often used in various industrial and chemical processes. It is represented chemically as \( \text{Ca(OH)}_2 \). This compound is formed when quicklime or calcium oxide (CaO) reacts with water, resulting in a slightly soluble substance that has a wide array of uses.
Some of the key properties of slaked lime include:

• High alkalinity, which makes it a potent neutralizing agent for acids.
• White, powdery appearance in its pure form.
• Slightly soluble in water, creating what is known as lime water.

Slaked lime is often used in the construction industry to produce mortar and plaster. It also finds applications in water treatment for softening and purifying water, and in agriculture to correct soil acidity. Moreover, due to its reactivity with carbon dioxide, it plays a significant role in certain chemical reactions.
Understanding slaked lime's properties and reactivity with gases like carbon dioxide is essential for many practical applications in chemistry and industries.

Carbon Dioxide Reactions

Carbon dioxide \((\text{CO}_2)\) is a colorless and odorless gas that is integral to many chemical processes, especially involving slaked lime \((\text{Ca(OH)}_2)\). When carbon dioxide is bubbled through slaked lime, a specific chemical interaction takes place that forms calcium carbonate \((\text{CaCO}_3)\), a white precipitate. This reaction is succinctly represented as:
\[ \text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O} \]
In this equation, calcium hydroxide (or slaked lime) reacts with carbon dioxide to yield calcium carbonate, illustrating a simple yet significant reaction.
Despite its role in forming calcium carbonate, carbon dioxide goes further to react with the precipitate when excessive amounts are present. In such cases, it can convert calcium carbonate into calcium bicarbonate \((\text{Ca(HCO}_3)_2)\), which is water-soluble and thus dissolves the precipitate:
\[ \text{CaCO}_3 + \text{CO}_2 + \text{H}_2\text{O} \rightarrow \text{Ca(HCO}_3)_2 \]
This pivotal transition is what causes the white precipitate to dissolve under prolonged exposure to carbon dioxide, demonstrating the gas's chemical versatility and its significant impacts on various chemical systems.

Calcium Carbonate Formation

Calcium carbonate \((\text{CaCO}_3)\) is a common compound resulting from reactions involving carbon dioxide and calcium compounds like slaked lime. It typically forms when carbon dioxide \((\text{CO}_2)\) comes into contact with calcium hydroxide \((\text{Ca(OH)}_2)\), producing a characteristic white precipitate:
\[ \text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O} \]
This formation of calcium carbonate is notable for several reasons:

• Its ubiquitous presence in nature, often found in rocks, shells, and the ocean.
• It is extensively used in the manufacture of cement, ceramics, and concrete.
• Environmental implications, as it plays a role in the carbon cycle, helping to capture and store carbon.

When excess \(\text{CO}_2\) is present, calcium carbonate can transform into calcium bicarbonate, leading to its dissolution in water. However, heating of calcium bicarbonate can reverse this dissolution, re-precipitating calcium carbonate, and releasing carbon dioxide. This reversible process provides insights into the dynamic nature of chemical reactions involving these common substances and underlines their importance in both natural and industrial contexts.