Question

why phenolphthalein gives pink colour in basic medium but no colour in acidic medium ???​

Answer 1

Ionized molecules attract other molecules with the opposite charge and repel those with the same charge. Phenolphthalein is a weak acid and is colorless in solution although its ion is pink. ... Adding hydroxide ions (OH-, as found in bases) will change the phenolphthalein into its ion and turn the solution pink.

Answer 2

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Because phenolphthalein is weak acid and it is colourless in acidic medium although it's ion are pink. While adding OH ions it changes into pink (OH ions are found in bases )