Question

why phenolphthalein colourless in acidic media

Answer 1

phenolphthelene exist in two different structural forms in acidic and basic media.Enol form which is present in acidic solution is colourless so u will not find any colour in this medium while Keto form which is preseent in basic medium appears pink so it is pink in basic medium.
often used as an indicator in acid–base titrations. For this application, it turns colorless in acidic solutions and pink in basic solutions. It is a weak acid, which can lose H+ ions in solution