Question

In bases, phenolphthalein turns:
A. Pink
B. Yellow
C. Black
D. Red

Answer 1

Phenolphthalein is a colourless acid-base indicator, which shows a colour change only in one solution, either acid or base. On studying the reason for its colour change, we will be able to determine in which solution- acidic or basic, will phenolphthalein show colour change.

Complete step by step answer:
Let us get a brief idea about how phenolphthalein reacts with acid and base to know the colour change in it.
Phenolphthalein is a colourless organic compound. It dissolves in water and dissociates slightly to form ${{\text{H}}^{+}}$ ions (colourless) and $\text{P}{{\text{h}}^{-}}$ ions (pink). The indicator first undergoes a tautomeric reversible change into a substance $\text{HPh}$ of different colour. This substance then ionises into ${{\text{H}}^{+}}$ and $\text{P}{{\text{h}}^{-}}$ ions.
If the solution is made acidic by adding an acid, say HCl, then the increase in ${{\text{H}}^{+}}$ ion concentration suppresses the dissociation of phenolphthalein. It is due to the common ion effect. The equilibrium shifts towards the left and the solution remains colourless.
If the solution is made basic by adding a few drops of any base such as $NaOH$, the $\text{O}{{\text{H}}^{-}}$ ions react with ${{\text{H}}^{+}}$ ions to form unionised water molecules. The decrease in ${{\text{H}}^{+}}$ ion concentration shifts the equilibrium to the right. Thus, more phenolphthalein ionises and the solution becomes pink (due to $\text{P}{{\text{h}}^{-}}$ ion).
Hence, phenolphthalein is colourless in acidic solution and pink in the basic solution.

Thus, option A is the correct answer.

Note: Phenolphthalein is either colourless (normally and in acids) or pink (in bases). It shows no other colour change.
In bases, methyl orange and methyl red turns yellow, and not phenolphthalein, hence, option B is incorrect.
No indicator turns black in bases; thus, option C is incorrect.
Cresol red and phenol red turns red in bases, and not phenolphthalein. Hence, option D is incorrect.