What color is phenolphthalein in a basic solution?

• A. Blue
• B. Pink
• C. Yellow
• D. Colorless

Answer: (B)

Phenolphthalein is a pH indicator whose color depends on its protonation state. In acidic solutions it stays colorless because the molecule is in its protonated form. As the solution becomes basic, phenolphthalein loses protons and shifts to a deprotonated form that absorbs light differently, giving a pink color. This pink appearance typically starts once the pH rises above about 8.2 and can deepen toward a fuchsia at higher pH. So in a basic solution you would observe pink. The other color options don’t match phenolphthalein’s behavior under normal aqueous pH ranges.