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

Initially, \(\mathrm{CaCO_3}\) forms, then dissolves to \(\mathrm{Ca(HCO_3)_2}\) with excess \(\mathrm{CO_2}\); answer (b) is correct.

Step 1: Understanding the Chemical Reaction

When carbon dioxide (\(\mathrm{CO}_2\)) gas is passed into a clear solution of lime water, which contains calcium hydroxide (\(\mathrm{Ca(OH)_2}\)), a chemical reaction occurs. This reaction initially forms calcium carbonate (\(\mathrm{CaCO}_3\)), which is a white precipitate.

Step 2: Initial Reaction Equation

The initial reaction can be represented by the equation: \[\mathrm{Ca(OH)_2 (aq) + CO_2 (g) \rightarrow CaCO_3 (s) + H_2O (l)}\]. This represents the formation of the white precipitate \(\mathrm{CaCO}_3\).

Step 3: Effect of Excess CO2

When excess \(\mathrm{CO_2}\) is continually passed through the solution, the initially formed \(\mathrm{CaCO_3}\) reacts further with \(\mathrm{CO_2}\) and water to form soluble \(\mathrm{Ca(HCO_3)_2}\). This results in the disappearance of the white precipitate.

Step 4: Secondary Reaction Equation

The secondary reaction is represented by: \[\mathrm{CaCO_3 (s) + CO_2 (g) + H_2O (l) \rightarrow Ca(HCO_3)_2 (aq)}\]. This equation shows that the excess \(\mathrm{CO_2}\) dissolves the \(\mathrm{CaCO_3}\) precipitate, forming the soluble \(\mathrm{Ca(HCO_3)_2}\) solution.

Step 5: Conclusion of Reactions

Based on the reactions, as you pass \(\mathrm{CO_2}\) into lime water, a white precipitate of \(\mathrm{CaCO_3}\) is initially formed and then dissolves to form \(\mathrm{Ca(HCO_3)_2}\) with further \(\mathrm{CO_2}\). Therefore, answer (b) is correct: initially a white precipitate of \(\mathrm{CaCO_3}\) is formed which changes into soluble \(\mathrm{Ca(HCO_3)_2}\) on passing \(\mathrm{CO_2}\) gas.

Key Concepts

Carbon Dioxide

Carbon dioxide, often abbreviated as CO₂, plays a significant role in many chemical reactions. It is a colorless and odorless gas composed of one carbon atom covalently double bonded to two oxygen atoms. Due to its presence in the atmosphere, CO₂ is integral in processes like photosynthesis in plants and the crustal carbon cycle, including ocean-atmosphere exchanges.

In the context of acid-base reactions and lime water, CO₂ acts as an acidic oxide. When it is bubbled through lime water, it can cause calcium hydroxide to react and form calcium carbonate, which is a solid precipitate. If CO₂ is in excess, it continues to react, converting the calcium carbonate into a more soluble form, calcium bicarbonate. This adaptability highlights CO₂’s role in a variety of chemical and environmental systems.

Lime Water

Lime water is a common name for a solution of calcium hydroxide (Ca(OH)₂) in water. It appears as a clear, colorless liquid and is alkaline in nature. Lime water is often used in laboratory settings to test for the presence of carbon dioxide gas.

When carbon dioxide is introduced into lime water, a fascinating reaction occurs. Initially, carbon dioxide reacts with calcium hydroxide to produce calcium carbonate, which creates a cloudy or milky appearance due to the formation of a white precipitate. This simple test exploits the reactivity of lime water with CO₂, serving as a quick and effective method for detecting carbon dioxide.

Calcium Carbonate

Calcium carbonate (CaCO₃) is a widely occurring compound found in nature as a primary component of rocks such as limestone, chalk, and marble. It is often seen and used in its pure mineral forms and contributes to the building materials industry. In the reaction with lime water, it initially forms as a white precipitate.

This compound is significant in various industries, from agriculture, where it acts as a soil conditioner, to medicine, where it's used as a calcium supplement. In the reaction with excess CO₂, calcium carbonate transitions to soluble calcium bicarbonate, an essential process in natural water cycles and carbon capture technologies.

Calcium Bicarbonate

Calcium bicarbonate, with the chemical formula Ca(HCO₃)₂, only exists in aqueous solution. It is formed when carbon dioxide reacts with water and calcium carbonate, which means it is highly soluble and does not precipitate out.

In natural systems, calcium bicarbonate plays a role in the dissolution of rocks and the formation of caves through processes known as karstification. This compound is pivotal in maintaining the balance of carbonate chemistry in water, impacting both environmental systems and industrial processes. Because of its solubility, this form allows the temporary removal of carbon dioxide from the system, which can eventually be released back into the atmosphere or remain in dissolved form.