Why phenolphthalein is colourless in acids?

Phenolphthalein behaves differently in acidic and basic solutions because of how its structure changes.

In Acidic Solution:

• Phenolphthalein stays colorless in acids.
• This happens because it remains in a neutral (unionized) form that does not break apart when many (H^{+}) ions are present.
• The neutral form lacks the extended conjugation (the linked bonds) needed to absorb visible light, so no color is seen.

In Basic Solution:

• In basic conditions, hydroxide ions oh-2 remove hydrogen ions from phenolphthalein.
• This causes a structural change in the molecule—it opens up a part of the structure called the lactone ring.
• The new form is a pink-colored anion (a negatively charged ion).

Reason for the Color Change:

• The pink form has an extended conjugated system, which allows it to absorb light in the visible range.
• This is why phenolphthalein turns pink in basic solutions and remains colorless in acidic solutions.