Problem 24 - A Complete Resource Book in Chemistry for JEE Main

Question

A mixture of sodium oxide and calcium oxide are dissolved in water and saturated with excess carbon dioxide gas. The resulting solution is ......It contains \(\ldots\) (a) acidic \(\mathrm{NaOH}\) and \(\mathrm{CaCO}_{3}\) (b) basic \(\mathrm{Na}_{2} \mathrm{CO}_{3}\) and \(\mathrm{CaCO}_{3}\) (c) neutral \(\mathrm{Na}_{2} \mathrm{CO}_{3}\) and \(\mathrm{CaCO}_{3}\) (d) basic \(\mathrm{NaOH}\) and \(\mathrm{Ca}(\mathrm{OH})_{2}\)

Step-by-Step Solution

Answer

The solution contains basic \(\text{Na}_2\text{CO}_3\) and \(\text{CaCO}_3\), so the answer is (b).

Step 1: Dissolution of Sodium and Calcium Oxides

When sodium oxide (\(\text{Na}_2\text{O}\)) and calcium oxide (\(\text{CaO}\)) are dissolved in water, they form sodium hydroxide (\(\text{NaOH}\)) and calcium hydroxide (\(\text{Ca(OH)}_2\)) respectively. This reaction can be represented as: \[\text{Na}_2\text{O} + \text{H}_2\text{O} \rightarrow 2 \text{NaOH}\] and \[\text{CaO} + \text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2\]. These compounds are both basic in nature.

Step 2: Saturation with Carbon Dioxide

When the solution containing \(\text{NaOH}\) and \(\text{Ca(OH)}_2\) is saturated with carbon dioxide (\(\text{CO}_2\)), a reaction occurs. The carbon dioxide reacts with the bases to form sodium carbonate (\(\text{Na}_2\text{CO}_3\)) and calcium carbonate (\(\text{CaCO}_3\)). The reactions are: \[2 \text{NaOH} + \text{CO}_2 \rightarrow \text{Na}_2\text{CO}_3 + \text{H}_2\text{O}\] and \[\text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O}\].

Step 3: Characteristics of the Resulting Solution

The resulting solution contains sodium carbonate (\(\text{Na}_2\text{CO}_3\)) and calcium carbonate (\(\text{CaCO}_3\)). Sodium carbonate is a basic salt, while calcium carbonate is a water-insoluble substance that often precipitates out of the solution. The basic nature of \(\text{Na}_2\text{CO}_3\) makes the overall solution basic rather than neutral.

Step 4: Identifying the Correct Answer

From the reactions and characteristics of the resulting compounds, the correct answer is (b) basic \(\text{Na}_2\text{CO}_3\) and \(\text{CaCO}_3\) because the solution is basic due to the presence of sodium carbonate.

Key Concepts

Sodium Oxide Reactions

When sodium oxide (\( \text{Na}_2\text{O} \)) comes into contact with water, it undergoes a fascinating reaction. This reaction is characterized as an exothermic process, meaning it releases heat. In simple terms, sodium oxide reacts with water to produce sodium hydroxide (\( \text{NaOH} \)). This reaction can be represented as:

• \( \text{Na}_2\text{O} + \text{H}_2\text{O} \rightarrow 2 \text{NaOH} \)

Sodium hydroxide is well-known for being basic, bringing a high pH to the resulting solution.
This is why sodium oxide is often referred to as a basic anhydride. The creation of sodium hydroxide in this context plays a crucial role in further reactions involving carbon dioxide.

Calcium Oxide Reactions

Calcium oxide (\( \text{CaO} \)) behaves in a similar fashion to sodium oxide when it interacts with water. The reaction forms calcium hydroxide (\( \text{Ca(OH)}_2 \)), a compound that is also basic in nature. The chemical reaction is shown as follows:

• \( \text{CaO} + \text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 \)

Calcium hydroxide is known to form a suspension in water, often referred to as limewater. Importantly, this compound maintains a basic environment, similar to sodium hydroxide.
It's important to note that calcium hydroxide has limited solubility, which means it can form a precipitate. This makes it slightly different from sodium hydroxide, affecting how calcium oxide reacts in different scenarios.

Carbonate Formation

Carbonate formation is an interesting journey when carbon dioxide (\( \text{CO}_2 \)) is introduced into the mix. When this gas is bubbled through a solution of sodium hydroxide and calcium hydroxide, fascinating transformations occur.
Sodium carbonate (\( \text{Na}_2\text{CO}_3 \)) is formed when carbon dioxide reacts with sodium hydroxide according to the equation:

• \( 2 \text{NaOH} + \text{CO}_2 \rightarrow \text{Na}_2\text{CO}_3 + \text{H}_2\text{O} \)

Simultaneously, calcium carbonate (\( \text{CaCO}_3 \)) forms through the reaction:

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

This entire process highlights the transformation of basic solutions into salts.
While sodium carbonate remains in the solution, calcium carbonate precipitates out, due to its low solubility in water. This precipitation is a common feature in natural processes and industrial applications. Understanding these transformations is key in both educational and practical chemistry settings.