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 \(\mathrm{Na}_2\mathrm{CO}_3\) (sodium carbonate).

Step 1: Understand the Reaction

Sodium hydroxide (NaOH) is hygroscopic, meaning it absorbs moisture from the air. It can also react with carbon dioxide (CO_2) in the atmosphere.

Step 2: Chemical Reaction Process

When NaOH absorbs moisture, it dissolves in the water it absorbs. Subsequently, it reacts with CO_2 in the air, forming sodium carbonate.

Step 3: Formation of Sodium Carbonate

The chemical reaction occurring is : 2NaOH + CO_2  2... Na_2CO_3 + H2O. This results in the formation of white solid Na2CO3 (sodium carbonate).

Step 4: Identify the White Solid

Based on the chemical reaction, the white solid formed on the surface of the NaOH pellet is sodium carbonate (Na2CO3).

Key Concepts

Hygroscopic Properties

Sodium hydroxide, also known as NaOH, has a special ability to attract and hold water molecules from its surroundings. This is what makes it hygroscopic. When sodium hydroxide is left exposed to the air, it absorbs moisture. This can be easily observed when pieces of sodium hydroxide, often in pellet form, are placed on a surface like a watch glass. Over a period of time, you might notice it becoming wet as it pulls in water from the atmosphere.

This hygroscopic property makes sodium hydroxide able to dissolve itself using the water it has absorbed from the air, forming a solution in the process. In everyday life, chemicals that are hygroscopic are essential for various applications, including keeping things dry or controlling humidity levels. For example, silica gel packs are often used for this purpose. Sodium hydroxide's hygroscopic nature is crucial in its industry uses, as it needs to be handled with care to prevent it from absorbing too much moisture and potentially undergoing unwanted reactions.

Reaction with Carbon Dioxide

When sodium hydroxide absorbs moisture from the air, it does more than just become a liquid. It also opens itself to new chemical reactions, particularly with carbon dioxide. Carbon dioxide, or CO extsubscript{2}, is a gas commonly present in our atmosphere.

Here’s what happens:

• Once NaOH has absorbed enough water and is in a dissolved form, it reacts with carbon dioxide.
• This reaction is quite common because CO extsubscript{2} is everywhere in the air we breathe.
• As the CO extsubscript{2} molecules mix with the dissolved NaOH, a chemical reaction takes place.

This interaction alters the composition of sodium hydroxide, leading to the creation of a different compound. This reaction showcases the dynamic nature of chemicals and how they can often change under specific conditions. This reaction plays a significant role in various industrial processes, especially in removing CO extsubscript{2} from gases.

Formation of Sodium Carbonate

The reaction between dissolved sodium hydroxide and carbon dioxide results in the formation of a new chemical compound called sodium carbonate. To better understand this transformation, let's look at the chemical equation: \[ 2 \text{NaOH} + \text{CO}_2 \rightarrow \text{Na}_2\text{CO}_3 + \text{H}_2\text{O} \] Here's what happens in this reaction:

• Two molecules of sodium hydroxide (\text{NaOH}) react with one molecule of carbon dioxide (\text{CO}_2).
• As a result, sodium carbonate (\text{Na}_2 ext{CO}_3) is produced along with water (\text{H}_2 ext{O}).
• Sodium carbonate is the white solid that forms on the surface of the sodium hydroxide pellet.

Sodium carbonate is commonly known as washing soda. It is used in a variety of household and industrial applications, such as cooking, cleaning, and even in glass-making. Recognizing how sodium hydroxide transforms into sodium carbonate provides key insights into chemical processes, illustrating how substances can change form and function through reactions with other elements or compounds.