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

The white solid is sodium carbonate (Na_2CO_3), or option (c).

Step 1: Understand Hygroscopic Nature of NaOH

NaOH is known to be hygroscopic, which means it can absorb moisture from the air. When NaOH is left exposed, it attracts and absorbs water vapor.

Step 2: Reaction with Carbon Dioxide

As NaOH absorbs moisture, it also reacts with carbon dioxide (CO_2) in the atmosphere. This reaction leads to the formation of sodium carbonate (Na_2CO_3).

Step 3: Formation of the White Solid

When NaOH reacts with CO_2, the resultant product, Na_2CO_3, appears as a white solid. This is due to the widespread reaction which takes place in everyday environments where CO_2 is present.

Step 4: Identify the White Solid

The white solid observed on the NaOH pellet after exposure to air is sodium carbonate (Na_2CO_3), corresponding to option (c).

Key Concepts

Hygroscopic Nature

Sodium hydroxide (NaOH) is a substance with a fascinating property known as hygroscopic nature. This means it has the ability to attract and hold water molecules from the surrounding environment. When you leave NaOH exposed to the air, it doesn't just sit there. Instead, it is actively absorbing moisture from the atmosphere. You might wonder why it even matters if a substance absorbs water vapor. Having this property is important because it alters the behavior of substances like NaOH.
In practical terms, the moisture absorption can lead to changes in the physical and chemical properties of the substance. For students observing a simple experiment, such as leaving a NaOH pellet on a watch glass, this hygroscopic nature is the first step in a chain reaction that leads to interesting transformations. If you've ever wondered why substances like NaOH or common table salts change when left out, understanding their hygroscopic nature is key.

Sodium Carbonate Formation

Once sodium hydroxide absorbs moisture, it doesn't just stop there. The absorbed water facilitates further reactions between NaOH and carbon dioxide (CO_2) present in the air.When NaOH comes into contact with moisture from the atmosphere along with carbon dioxide, a chemical reaction occurs:\[ 2 \, \text{NaOH} (s) + \text{CO}_2 (g) \rightarrow \text{Na}_2\text{CO}_3 (s) + \text{H}_2\text{O} (l) \]This reaction results in the formation of sodium carbonate (Na_2CO_3), which is a new compound with completely different properties than the original NaOH.Why Does Formation Matter?
The significance of this chemical reaction is illustrated in everyday scenarios. Over time, the sodium carbonate forms a solid layer on the NaOH pellet, often observed as a white solid.
Understanding this process and outcome helps students make sense of why a simple NaOH pellet can transform after being left in open air. From a simple aspect of observing, to understanding, it becomes a vital part of learning chemistry fundamentals.

Reaction with Carbon Dioxide

The interaction between sodium hydroxide and the carbon dioxide in the air is integral to the observed changes in NaOH. This reaction between NaOH and CO₂ has broad implications, not only in laboratory settings but also in industrial applications. How Does the Reaction Work?

• As soon as sodium hydroxide absorbs moisture, it becomes more susceptible to picking up carbon dioxide from the surrounding atmosphere.
• The presence of CO₂ triggers the transformation of NaOH into sodium carbonate.
• This transformation is manifested as the appearance of a white solid layer over the original NaOH.

The Broader Impact
This ability to react with carbon dioxide is utilized in various fields. It serves not only in understanding environmental interactions but also in processes like CO₂ scrubbing from gases.
In the exercise context, realizing this reaction helps students understand how atmospheric interactions can lead to new substances forming from seemingly simple starting materials.