Question

Sodium hydroxide is hygroscopic - that is, it absorbs moisture when exposed to the atmosphere. A student placed a pellet of \(\mathrm{NaOH}\) on a watch glass. A few days later, she noticed that the pellet was covered with a white solid. What is the identity of this solid? (Hint: Air contains \(\mathrm{CO}_{2}\).)

Short answer

The white solid is sodium carbonate (\(\text{Na}_2\text{CO}_3\)).

Step-by-step solution

Understanding Hygroscopy

Sodium hydroxide (\(\text{NaOH}\)) is hygroscopic, which means it absorbs water from the air. Initially, the solid sodium hydroxide attracts water molecules.

Reaction with Carbon Dioxide

Air contains carbon dioxide (\(\text{CO}_2\)). When sodium hydroxide absorbs moisture, it can also react with carbon dioxide present in the atmosphere.

Formation of Sodium Carbonate

The reaction between sodium hydroxide and carbon dioxide, in the presence of the absorbed moisture, forms sodium carbonate via the chemical reaction: \[2\text{NaOH (s)} + \text{CO}_2 \text{ (g)} \rightarrow \text{Na}_2\text{CO}_3 \text{ (s)} + \text{H}_2\text{O (l)}\]This white solid is sodium carbonate (\(\text{Na}_2\text{CO}_3\)).

Conclusion: Identify the White Solid

The white solid formed on the sodium hydroxide pellet is sodium carbonate (\(\text{Na}_2\text{CO}_3\)).

Key concepts

Sodium Hydroxide (NaOH)

Sodium hydroxide, often known as lye or caustic soda, is a powerful inorganic compound with the chemical formula \( ext{NaOH}\). It is widely used in various industries, including paper, textiles, and chemical manufacturing.
One defining characteristic of sodium hydroxide is its hygroscopic nature. Hygroscopic substances readily absorb moisture from their surroundings. This quality means that when exposed to air, \( ext{NaOH}\) will pull in water vapor, turning from a solid pellet into a sticky liquid.
Sodium hydroxide is also a strong base, meaning it can react with acids and carbon compounds. Its strength and reactivity make it superb for applications such as pH regulation, chemical synthesis, and even in household drain cleaners.
Because it can absorb moisture and react with air components, \( ext{NaOH}\) needs to be stored in airtight containers to maintain its stability and effectiveness.

Sodium Carbonate (Na₂CO₃)

Sodium carbonate, also known as washing soda or soda ash, appears as a white crystalline solid. With the chemical formula \( ext{Na}_2 ext{CO}_3\), it is often used in glass making, detergent manufacturing, and water softening.
In the context of sodium hydroxide's hygroscopic behavior, sodium carbonate is formed when \( ext{NaOH}\) absorbs water and reacts with carbon dioxide. The chemical reaction involved is: \[2\text{NaOH (s)} + \text{CO}_2 \text{ (g)} \rightarrow \text{Na}_2\text{CO}_3 \text{ (s)} + \text{H}_2\text{O (l)}\]
This process turns \( ext{NaOH}\) from a solid into a white solid coating on the pellet, identifiable as \( ext{Na}_2 ext{CO}_3\).
Sodium carbonate is less reactive than sodium hydroxide, yet it is highly beneficial due to its chemical properties and role in neutralizing acids. Its relative stability makes \( ext{Na}_2 ext{CO}_3\) easy to handle and store.

Carbon Dioxide (CO₂)

Carbon dioxide, represented chemically as \( ext{CO}_2\), is a colorless gas prevalent in the Earth's atmosphere. It is a byproduct of both natural processes like respiration and man-made activities such as burning fossil fuels.
Remarkably significant in environmental science, \( ext{CO}_2\) plays a crucial role in plant photosynthesis and the Earth's carbon cycle. However, in chemical contexts, it can also act as a reactive agent, especially with bases like sodium hydroxide.
When \( ext{NaOH}\) absorbs moisture and \( ext{CO}_2\), it undergoes a reaction producing sodium carbonate. This reaction exemplifies how ubiquitous gases can affect the composition of exposed chemicals. Thus, \( ext{CO}_2\) not only influences biological processes but also impacts various inorganic compounds when conditions permit a reaction.