What color does phenolphthalein indicate in an alkaline solution?

• A. Purple
• B. Red
• C. Colorless
• D. Green

Answer: (A)

Phenolphthalein acts as a pH indicator by changing color as the solution’s acidity or basicity changes. In acidic solutions it stays colorless, but when the environment becomes alkaline, phenolphthalein loses protons and its structure changes so it absorbs light differently, showing a pink color that can look purple in stronger bases. This color shift happens around pH 8.2 to 10, so in alkaline conditions it appears purple.