* Phenolphthalein in neutral or acidic solutions: Phenolphthalein is colorless in neutral or acidic solutions (pH below 8.2).
* Phenolphthalein in basic solutions: When phenolphthalein is added to a basic solution (pH above 8.2), it turns a vibrant pink color.
Caustic soda (NaOH) is a strong base and will significantly increase the pH of a solution.
Therefore, when caustic soda is added to phenolphthalein, the solution will turn pink due to the increase in pH. This is a classic demonstration of how phenolphthalein changes color in response to changes in acidity/alkalinity.
