International General Certificate of Secondary Education (IGCSE) Chemistry Practice Exam

Why does the solution formed after electrolysis of sodium chloride turn pink when tested with phenolphthalein?

1. It contains hydrochloric acid

2. It contains sodium hydroxide

3. It contains chlorine gas

4. It is neutral

The correct answer is: It contains sodium hydroxide

The solution formed after the electrolysis of sodium chloride turns pink when tested with phenolphthalein due to the presence of sodium hydroxide. During electrolysis, water and sodium chloride are broken down into their constituent ions. At the anode, chlorine gas is produced, and at the cathode, hydrogen gas is released. The reduction of water at the cathode leads to the formation of hydroxide ions (OH⁻), which increase the pH of the solution, making it basic. Phenolphthalein is a pH indicator that turns pink in basic solutions. When sodium hydroxide is present in sufficient quantities in the solution, it raises the pH above 8.2, and phenolphthalein changes color from colorless to pink, indicating the basic nature of the solution. The presence of sodium hydroxide is the key factor that causes this color change during the testing process.