Problem 107 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following changes occur when an excess of \(\mathrm{CO}_{2}\) gas is passed into a clear solution of lime water? (a) a white precipitate containing both \(\mathrm{CaCO}_{3}\) and \(\mathrm{Ca}\left(\mathrm{HCO}_{3}\right)_{2}\) is formed (b) initially a white precipitate of \(\mathrm{CaCO}_{3}\) is formed which changes into soluble \(\mathrm{Ca}\left(\mathrm{HCO}_{3}\right)_{2}\) on passing \(\mathrm{CO}_{2}\) gas (c) a white precipitate of \(\mathrm{Ca}\left(\mathrm{HCO}_{3}\right)_{2}\) is formed (d) a white precipitate of \(\mathrm{CaCO}_{3}\) is formed

Step-by-Step Solution

Answer

Option (b): Initially, \( \text{CaCO}_3 \) forms, then changes into soluble \( \text{Ca(HCO}_3)_2 \) with excess \( \text{CO}_2 \) .

Step 1: Understanding Lime Water Reaction with CO2

Lime water (\( \text{Ca(OH)}_2 \)) reacts with carbon dioxide to form calcium carbonate (\( \text{CaCO}_3 \)) which is insoluble in water, and this results in a white precipitate.

Step 2: Formation of White Precipitate

When \( \text{CO}_2 \) is introduced, it reacts with \( \text{Ca(OH)}_2 \) to initially form \( \text{CaCO}_3 \) as a solid precipitate:\[ \text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 (\text{solid, white precipitate}) + \text{H}_2\text{O} \]

Step 3: Reaction with Excess CO2

If excess \( \text{CO}_2 \) is passed through the solution, the initially formed \( \text{CaCO}_3 \) reacts further with \( \text{CO}_2 \) and water to form soluble calcium bicarbonate \( \text{Ca(HCO}_3)_2 \): \[ \text{CaCO}_3 + \text{CO}_2 + \text{H}_2\text{O} \rightarrow \text{Ca(HCO}_3)_2 (\text{soluble}) \]

Step 4: Choice Verification

Option (b) correctly describes the process: initially, a white precipitate of \( \text{CaCO}_3 \) is formed, which redissolves as \( \text{CO}_2 \) passes through to become the soluble \( \text{Ca(HCO}_3)_2 \). This eliminates options (a), (c), and (d) as they do not completely describe the sequence of reactions.

Key Concepts

Calcium Carbonate Formation

Lime water is a common name for a dilute solution of calcium hydroxide, denoted as \( \text{Ca(OH)}_2 \). When carbon dioxide \( \text{CO}_2 \) is passed through it, a fascinating chemical reaction occurs. The carbon dioxide reacts with the calcium hydroxide to form calcium carbonate \( \text{CaCO}_3 \), which is an insoluble compound. This reaction can be observed as it results in the formation of a white precipitate. The chemical equation for this reaction is as follows:

• \( \text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 (\text{solid, white precipitate}) + \text{H}_2\text{O} \)

The appearance of this white precipitate is a classic demonstration in chemistry. It's an excellent example of a visual reaction that shows students the process of a solid forming in a liquid solution. This initial step is crucial because it sets the stage for further reactions when more \( \text{CO}_2 \) is introduced.

Calcium Bicarbonate Solubility

Once the white precipitate of calcium carbonate \( \text{CaCO}_3 \) is formed, the reaction doesn’t stop there. If you continue to add carbon dioxide \( \text{CO}_2 \) to the solution, the original precipitate can dissolve again, forming a new compound, calcium bicarbonate \( \text{Ca(HCO}_3)_2 \). This fascinating shift occurs because calcium bicarbonate is soluble in water. As a result, the once cloudy solution becomes clear as the solid dissolves into the liquid. The chemical process can be outlined as:

• \( \text{CaCO}_3 + \text{CO}_2 + \text{H}_2\text{O} \rightarrow \text{Ca(HCO}_3)_2 (\text{soluble}) \)

This conversion demonstrates the dynamic nature of chemical reactions and the solubility properties of different compounds. The disappearance of the white precipitate is an indication that calcium bicarbonate has formed. Understanding this offers insights into the solubility rules and the impact carbon dioxide can have on chemical equilibria.

Carbon Dioxide Excess Reaction

The introduction of excess carbon dioxide \( \text{CO}_2 \) into lime water not only forms calcium bicarbonate \( \text{Ca(HCO}_3)_2 \) but also illustrates the principle of excess reagent in chemical reactions. Initially, the carbon dioxide reacts with calcium hydroxide to form a solid precipitate, but when an overabundance of \( \text{CO}_2 \) enters the system, it dissolves the precipitated calcium carbonate. This demonstrates how a reaction can shift under different conditions.
In real-world scenarios, such as in nature, this reaction is important. It explains processes like the formation of stalactites and stalagmites in caves, where bicarbonate-rich water drips and deposits calcium carbonate over time. Additionally, the ability of \( \text{CO}_2 \) to change the solubility of compounds is vital in understanding carbon cycles and environmental chemistry. It’s a perfect example of how a chemical reaction can vary with the concentration of the reacting substances and how an excess element can alter the outcome significantly.