Answer:
Phenolphthalein is an indicator that turns pink in alkaline or basic solutions (pH above 8.2) and colorless in acidic solutions (pH below 6.8).
Step-by-step explanation:
In alkaline solutions (basic), phenolphthalein turns pink due to the presence of hydroxide ions (OH-) which react with the colorless form of phenolphthalein to form a pink ionized species. This change occurs because phenolphthalein itself acts as a weak acid, and in the presence of hydroxide ions, it undergoes deprotonation, leading to the formation of the pink color.
In acidic solutions, where the concentration of hydroxide ions is low, the colorless form of phenolphthalein predominates. This is because the weak acidic nature of phenolphthalein allows it to remain largely in its non-ionized, colorless form in the presence of excess H+ ions.
So, the color change of phenolphthalein is a result of its ability to undergo a reversible reaction with different concentrations of hydroxide and hydrogen ions, making it a suitable indicator for distinguishing between acidic and alkaline solutions.