Which indicator shows pink in alkaline solution and colorless in acidic solution?

• A. Litmus
• B. Methyl Orange
• C. Phenolphthalein
• D. Bromothymol Blue

Answer: (C)

Phenolphthalein is an indicator whose color changes with pH because its molecular structure shifts between forms as it gains or loses protons. In acidic solutions it stays colorless because it remains in the protonated form that absorbs light differently. When the solution becomes alkaline, it loses protons and converts to a deprotonated form, which appears pink. The noticeable transition happens around pH 8.2 to 10, so it stays colorless in acidic conditions and turns pink in basic conditions. This makes it the right choice for a scenario asking for pink in alkaline and colorless in acidic. Other indicators change to different colors at their respective pH ranges (for example, litmus shifts red to blue, methyl orange from red to yellow, bromothymol blue from yellow to blue), so they don’t fit the described color change.