Why is it important to stopper a flask containing NaOH in the laboratory?

• A. To avoid contamination of NaOH by dust and other particles.
• B. To prevent NaOH from evaporating rapidly at room temperature.
• C. To prevent NaOH from absorbing carbon dioxide from the air, which can alter its concentration.
• D. To protect the flask from breaking due to pressure buildup.

Answer: (C)

Verified step by step guidance

1. Understand that solid sodium hydroxide (NaOH) is highly hygroscopic, meaning it readily absorbs moisture and gases from the air.
2. Recognize that when NaOH is exposed to air, it can absorb carbon dioxide (CO\_2), which reacts with NaOH to form sodium carbonate (Na\_2CO\_3), altering the chemical composition and concentration of the original NaOH.
3. Know that this reaction can be represented as: \$2\ \mathrm{NaOH} + \mathrm{CO}_2 \rightarrow \mathrm{Na}_2\mathrm{CO}_3 + \mathrm{H}_2\mathrm{O}$, which reduces the effectiveness of NaOH as a strong base.
4. Realize that stopping the flask tightly prevents air (and thus CO\_2) from coming into contact with the NaOH, maintaining its purity and concentration for accurate experimental results.
5. Conclude that the primary reason to stopper the flask is to prevent contamination by CO\_2 absorption, rather than to prevent evaporation or physical damage.