Question

In an alkaline solution, what is the change in color of phenolphthalein indicator?
A. Pink to colorless
B. Pink to red
C. Red to colorless
D. Colorless to pink

Answer 1

Hint: We know that there are different types of indicators to test whether the given substance is an acid or a base. These indicators include olfactory indicators, natural indicators, and synthetic indicators. Phenolphthalein is a synthetic indicator and is naturally colorless.

Complete answer:
There are different types of indicators in chemistry such as olfactory indicators, natural or synthetic indicators, etc. Indicators are substances that are used to test whether the given substance is an acid or a base. An acid is sour, turns blue litmus red, and gives ${H^ + }$ ions when dissolved in water. Examples of acids include vinegar $\left( {C{H_3}COOH} \right)$ , hydrochloric acid $\left( {HCl} \right)$, citric acids $\left( {{C_6}{H_8}{O_7}} \right)$ found in citrus fruits like lemon, etc. On the other hand, bases are bitter, turn red litmus blue, and give $O{H^ - }$ ions when dissolved in water. Examples of bases are sodium hydroxide $\left( {NaOH} \right)$ , potassium hydroxide $\left( {KOH} \right)$ , etc. A base that dissolves in water is called an alkali and such solutions are known as alkaline solutions.
Phenolphthalein is a synthetic indicator and is a weak acid. It is naturally colorless. It is represented by ${C_{20}}{H_{14}}{O_4}$

Phenolphthalein is naturally colorless and remains colorless when added to acid and turns pink in an alkaline solution.

Therefore the correct option is $D$.

Note:
We must know that indicators are themselves weak acids or bases. Phenolphthalein is a weak acid that dissociates very slightly and forms ions. The choice of an indicator for a particular reaction depends on $pH$ of the reaction.