Problem 127 - A Complete Resource Book in Chemistry for JEE Main

Question

When brine solution is saturated with \(\mathrm{NH}_{3}\) and \(\mathrm{CO}_{2}\) a slightly alkaline white sodium salt (A) is formed which has \(\mathrm{pH}\) of nearly 8.4. (A) on heating liberates a gas (B) leaving a highly alkaline residue (C) of \(\mathrm{pH}\) nearly 10 to 11 . Gas (B) is colourless and turns a solution of \(\mathrm{Ca}(\mathrm{OH})_{2}\) milky. Identify (B). (a) \(\mathrm{Na}_{2} \mathrm{CO}_{3}\) (b) \(\mathrm{NaHCO}_{3}\) (c) \(\mathrm{Na}_{2} \mathrm{~S}\) (d) \(\mathrm{Na}_{2} \mathrm{SO}_{4}\)

Step-by-Step Solution

Answer

The correct identification of (B) is gas \( \mathrm{CO_2} \) from \( \mathrm{NaHCO}_{3} \). Option (b) is correct.

Step 1: Identify Substance A

The brine solution saturated with \( \mathrm{NH}_{3} \) and \( \mathrm{CO}_{2} \) forms a slightly alkaline white sodium salt with a \( \mathrm{pH} \approx 8.4 \). This corresponds to \( \mathrm{NaHCO}_{3} \) (sodium bicarbonate), which is known for these properties.

Step 2: Analyze Reaction upon Heating

When substance (A), \( \mathrm{NaHCO}_{3} \), is heated, a decomposition reaction occurs producing \( \mathrm{CO_2} \) gas (substance B) and \( \mathrm{Na}_{2} \mathrm{CO}_{3} \) (substance C) as residue: \ \( 2 \mathrm{NaHCO}_{3} \rightarrow \mathrm{Na}_{2} \mathrm{CO}_{3} + \mathrm{CO_{2}} + \mathrm{H}_{2} \mathrm{O} \). This residue is very alkaline with a \( \mathrm{pH} \approx 10-11 \).

Step 3: Identify Gas B

Gas (B) is \( \mathrm{CO_2} \), which is colorless. It turns a solution of \( \mathrm{Ca}( ext{OH})_{2} \) milky due to the formation of \( \text{CaCO}_3 \) (calcium carbonate), confirming its identity as \( \mathrm{CO_{2}} \).

Step 4: Choose the Correct Option

Since gas (B) from the decomposition of (A) is \( \mathrm{CO_2} \), originating from \( \mathrm{NaHCO}_{3} \), the correct option identifying (A) is (b) \( \mathrm{NaHCO}_{3} \).

Key Concepts

Brine Solution Chemistry

A brine solution is a high-concentration solution of salt, usually sodium chloride (NaCl), in water. Brine solutions are significant in industrial processes and the food industry. But in this context, we focus on the chemical properties of brine solution when saturated with gases like ammonia (NH_{3}) and carbon dioxide (CO_{2}).

This combination leads to the formation of sodium bicarbonate (NaHCO_{3}), also known as baking soda. When dissolved in water, baking soda exhibits a slightly alkaline nature. Alkalinity is measured by pH values; substances with a pH greater than 7 are alkaline. Sodium bicarbonate has a pH of about 8.4, confirming its mildly alkaline property.

Brine solutions like this are key in various chemical reactions, primarily because they provide a medium for reactions like the formation of sodium bicarbonate and facilitate gas saturation processes. This foundation in brine solution chemistry helps us understand the initial steps leading to further reactions.

Decomposition Reaction

A decomposition reaction is a type of chemical reaction in which a single compound breaks down into two or more products. These reactions often require heat, light, or electricity to occur. In our exercise, the decomposition of sodium bicarbonate (NaHCO_{3}) is triggered by heating.

When sodium bicarbonate is heated, it undergoes a breakdown that releases carbon dioxide (CO_{2}) gas. The chemical equation for this reaction is:
\[ 2 \text{NaHCO}_3 \rightarrow \text{Na}_2\text{CO}_3 + \text{CO}_2 + \text{H}_2 \text{O} \]

This decomposition produces three products: carbon dioxide gas, water, and sodium carbonate (Na_{2}CO_{3}). Sodium carbonate is a highly alkaline substance with a pH range of 10 to 11, unlike sodium bicarbonate's mild alkalinity.

Understanding the decomposition reaction is vital because it relates to the transformation process from one type of chemical property (mildly alkaline) to another (highly alkaline). This forms a basis for recognizing how compounds can change under different conditions.

Gas Identification

Gas identification involves recognizing gases based on their chemical behavior and interactions with other substances. In this context, when sodium bicarbonate decomposes, it releases carbon dioxide (CO_{2}) as one of the products.

Carbon dioxide is a colorless gas that is known for its distinct interaction with lime water, a solution of calcium hydroxide (Ca(OH)_{2}). When carbon dioxide is bubbled through lime water, a white precipitate of calcium carbonate (CaCO_{3}) forms, causing the solution to appear milky:
\[ \text{CO}_2 + \text{Ca(OH)}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O} \]

This milkiness is a clear indicator of the presence of carbon dioxide. Identifying gases through such reactions assists in confirming the specific derivate or byproduct of a decomposition reaction, as in our sodium bicarbonate example. Gas identification is a practical skill in chemistry that aids in understanding reaction products and analyzing substances.