What is the colour of phenolphthalein indicator in a basic medium?

Step 1: Analyze the question You are asked for the colour of phenolphthalein in a basic (alkaline) medium.

Step 2: The concept Phenolphthalein is a weak acid indicator that changes colour over a pH range of about 8.3 to 10.0.

• In acidic and neutral solutions (pH below about 8.3), it is colourless.
• In basic solutions (pH above about 10), it turns pink (sometimes described as pinkish-red or magenta).

The change happens because the indicator's molecular structure changes when it loses hydrogen ions in alkaline conditions. The new structure absorbs light differently and appears pink.

Step 3: Check the options

• A. Blue: this is the colour of litmus in base, or of bromothymol blue in base. It is not phenolphthalein.
• B. Colourless: this is phenolphthalein in acid, not in base.
• C. Orange: this is methyl orange in neutral or weakly acidic conditions.
• D. Pink: correct.
• E. Yellow: this is methyl orange in base.

Step 4: Answer Phenolphthalein is pink in a basic medium, which is option D.

Exam tip: Remember "Phenolphthalein: Pink in Pase", or simply "colourless in acid, pink in base". It is commonly used in titrations of strong base with strong acid or weak acid with strong base. For methyl orange, remember "red in acid, yellow in base".