Phenolphthalein is a commonly used synthetic indicator to test the acidic or basic nature of a substance. Here is an explanation of why phenolphthalein is used as an indicator and how it works:

Indicators:
Indicators are substances that change their color in the presence of acids or bases. They are used to detect the presence and determine the nature of these substances in a solution. Indicators exist in both natural and synthetic forms. Synthetic indicators are often preferred due to their stability and reliability.

Phenolphthalein:
Phenolphthalein is a synthetic indicator that is frequently used in acid-base titrations and to test the pH of a substance. It is a colorless compound in its acidic form and turns pink or purple in the presence of a base. Phenolphthalein can be used to detect both strong and weak bases.

Working of Phenolphthalein:
When phenolphthalein is added to a solution, it undergoes a chemical reaction with the base present in the solution. In an acidic solution, the concentration of H+ ions is high, and these ions react with phenolphthalein to keep it colorless. However, in a basic solution, the concentration of OH- ions is high, and they react with phenolphthalein to produce an ion that is pink or purple in color.

Color Change:
The color change of phenolphthalein from colorless to pink or purple occurs at a specific pH range, known as the indicator's transition range. For phenolphthalein, the transition range is typically between pH 8.2 and 10.0. This means that phenolphthalein will remain colorless in acidic solutions with a pH below 8.2, but as the pH increases and becomes more basic, the indicator will gradually turn pink or purple.

Conclusion:
Phenolphthalein is a commonly used synthetic indicator to test the acidic or basic nature of a substance. Its colorless to pink/purple color change in the presence of a base makes it a reliable indicator for determining the pH of a solution.