Problem 87 - A Complete Resource Book in Chemistry for JEE Main

Question

Sodium hydroxide being hygroscopic absorbs moisture when exposed to the atmosphere. A student placed a pellet of \(\mathrm{NaOH}\) on a watch glass. A few days later, he noticed that the pellet was covered with a white solid. Identify this white solid? (a) Na metal (b) \(\mathrm{Na}_{2} \mathrm{SO}_{3}\) (c) \(\mathrm{Na}_{2} \mathrm{CO}_{3}\) (d) \(\mathrm{Na}_{2} \mathrm{~S}\)

Step-by-Step Solution

Answer

(c) Na₂CO₃

Step 1: Understand Hygroscopic Property of NaOH

Sodium hydroxide (NaOH) is highly hygroscopic, which means it readily absorbs moisture from the air. This property causes it to react with moisture and substances in the air.

Step 2: Reaction with Atmospheric CO2

When sodium hydroxide absorbs moisture from the air, it can also react with carbon dioxide (CO_2) present in the atmosphere according to the reaction: 2 ext{NaOH} + ext{CO}_2 ightarrow ext{Na}_2 ext{CO}_3 + ext{H}_2 ext{O}. This results in the formation of sodium carbonate ( ext{Na}_2 ext{CO}_3).

Step 3: Identify the White Solid

The white solid observed on the pellet after moisture absorption and reaction with carbon dioxide is sodium carbonate ( ext{Na}_2 ext{CO}_3). This compound is a white solid, hence matching the observation.

Step 4: Answer the Question

Based on the reaction and properties described, the white solid formed on the pellet of sodium hydroxide is identified as sodium carbonate. Thus, the correct option is (c)  ext{Na}_2 ext{CO}_3.

Key Concepts

Sodium Hydroxide Reactions

Sodium hydroxide, commonly known as NaOH, is a chemical compound that showcases significant reactivity due to its strong base properties. When exposed to its environment, NaOH has the unique characteristic of absorbing moisture from the air.
This is attributed to its hygroscopic nature. As NaOH gathers moisture, it participates in reactions with environmental gases. This reactive behavior is crucial, especially when in contact with substances like carbon dioxide. Understanding how NaOH reacts with these substances underpins many applications and safety protocols in chemistry.

Atmospheric CO2 Reaction

In an atmosphere teeming with carbon dioxide, NaOH does not merely remain a passive absorber of moisture. It actively reacts with CO2 from the atmosphere. This reaction is represented by the equation: \[ 2 \text{NaOH} + \text{CO}_2 \rightarrow \text{Na}_2\text{CO}_3 + \text{H}_2\text{O} \]In this reaction, sodium hydroxide combines with carbon dioxide to form sodium carbonate. This new compound is accompanied by the production of water.
This interaction is significant because CO2 is abundantly present in the air, making NaOH highly susceptible to this transformation when exposed.

Sodium Carbonate Formation

When sodium hydroxide reacts with carbon dioxide, the end product is sodium carbonate (\(\text{Na}_2\text{CO}_3\)). This compound appears as a white solid, which is often the result when NaOH is left exposed.
Sodium carbonate itself is a well-known compound used in a variety of applications, such as laundry detergents and glass manufacturing. It is crucial to identify sodium carbonate correctly, as its formation signifies the interaction between sodium hydroxide and atmospheric elements. Identifying the resultant compound as sodium carbonate is key when observing such chemical transformations.

• Solid white appearance
• Widespread usage in industry
• Byproduct of NaOH and CO2 reaction

This understanding can assist in preventing unintended chemical transformations or in effectively leveraging NaOH's reactivity in practical applications.