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) is correct.

Step 1: Understanding the Reaction

When carbon dioxide (\(\mathrm{CO}_2\)) is passed into lime water, which contains calcium hydroxide (\(\mathrm{Ca(OH)_2}\)), it first reacts to form calcium carbonate (\(\mathrm{CaCO_3}\)) as a white precipitate. The reaction is given by: \[\mathrm{Ca(OH)_2 (aq) + CO_2 (g) \rightarrow CaCO_3 (s) + H_2O (l)}\].

Step 2: Reaction with Excess CO2

If an excess amount of carbon dioxide is passed through, the calcium carbonate precipitate further reacts with the carbon dioxide and water to form soluble calcium bicarbonate \(\mathrm{Ca(HCO_3)_2}\). The equation for this reaction is: \[\mathrm{CaCO_3 (s) + CO_2 (g) + H_2O (l) \rightarrow Ca(HCO_3)_2 (aq)}\].

Step 3: Conclusion Based on Reaction Steps

Initially, a white precipitate of calcium carbonate is formed, and upon continuing to pass carbon dioxide, the calcium carbonate dissolves to form soluble calcium bicarbonate. This matches option (b) from the choices; a white precipitate of \(\mathrm{CaCO_3}\) is formed, which changes into soluble \(\mathrm{Ca(HCO_3)_2}\) upon the passage of excess \(\mathrm{CO_2}\).

Key Concepts

Calcium Carbonate Precipitation

Calcium carbonate precipitation is an important chemical process. When carbon dioxide (\(\mathrm{CO}_2\)) enters a lime water solution, it interacts with calcium hydroxide (\(\mathrm{Ca(OH)_2}\)). The result is a white solid that we know as calcium carbonate (\(\mathrm{CaCO_3}\)). This transformation can be expressed with the equation:\[\mathrm{Ca(OH)_2 (aq) + CO_2 (g) \rightarrow CaCO_3 (s) + H_2O (l)}\]It's a simple reaction. When you see the cloudy appearance, that's the calcium carbonate precipitate taking shape.
It's one of those classic chemical reactions that show visible change, making it easier to observe and understand.The formation of \(\mathrm{CaCO_3}\)) as a precipitate is crucial since it sets the stage for further reactions, depending on what happens next.
In many natural processes, calcium carbonate precipitation is seen in places like caves, where stalactites and stalagmites grow over time.

Formation of Calcium Bicarbonate

The formation of calcium bicarbonate, \(\mathrm{Ca(HCO_3)_2}\), happens when calcium carbonate reacts with even more carbon dioxide in the presence of water. Initially, when \(\mathrm{CO}_2\) is added to lime water, we get calcium carbonate. But, if we keep adding more \(\mathrm{CO}_2}\), a shift happens.Here's the equation that describes this transformation:\[\mathrm{CaCO_3 (s) + CO_2 (g) + H_2O (l) \rightarrow Ca(HCO_3)_2 (aq)}\]The calcium carbonate dissolves in the added \(\mathrm{CO}_2}\), forming soluble calcium bicarbonate.
This change is significant because it transitions the newly formed precipitate back into a dissolved state, altering the solution's properties.
You won't see the white precipitate anymore as it becomes part of the clear liquid once again. This process is a great example of how changing conditions alter chemical forms. Understanding this reaction helps illustrate dynamic chemical equilibriums in the natural world.

Reaction of Calcium Hydroxide with Carbon Dioxide

The reaction between calcium hydroxide and carbon dioxide is the foundational reaction here. Lime water, a solution of calcium hydroxide, looks completely clear until we introduce \(\mathrm{CO}_2}\).When mixed, these two react to form calcium carbonate:\[\mathrm{Ca(OH)_2 (aq) + CO_2 (g) \rightarrow CaCO_3 (s) + H_2O (l)}\]But this isn't the whole story.
As we discussed, the reaction doesn't stop with the first formation. If you keep passing \(\mathrm{CO}_2}\) through the system, the reaction evolves further into having calcium bicarbonate in the solution.
This entire process is an excellent demonstration of how gases like \(\mathrm{CO}_2}\) can change and influence different chemical reactions. In practical applications, this same reaction explains phenomena such as the temporary hardness of water.
Understanding the complete reaction helps in both academic and real-world settings.