Question

What will be the color of phenolphthalein solution on adding oxalic acid?
A. Colorless
B. Pink
C. Blue
D. Orange

Answer 1

We need to know that the indicator is a substance which used to appear visually by change in colour when adding the solution according with the presence of a specific materials. And that material may be alkali or free acid. The litmus and the phenolphthalein are acid indicators. The methyl orange is a pH indicator and it is widely used in titration due to its distinct colour variations in each pH values. The methyl orange appear red colour in acidic medium and in basic medium it shows yellow colour.

Complete answer:
As we know that he phenolphthalein is an organic compound having the molecular formula, ${C_{20}}{H_{14}}{O_4}$. And this is an acid – base indicator. The phenolphthalein appear colorless when the pH attain below $8.5$ and it becomes deep pink color at the pH above $9.0$.
Here the colour of phenolphthalein solution on adding oxalic acid become colorless. Because, the oxalic acid is acidic in nature. The colour of phenolphthalein remains colorless in acidic solutions. And the colour of phenolphthalein in basic solution become pink in basic solutions. Generally the phenolphthalein is weakly acidic in nature. In aqueous solutions, the phenolphthalein is dissociates into ions. The concentration of the ions is responsible for the color of phenolphthalein. When it is contact with acidic solutions, the concentration of these ions will decreases and it becomes very low. Hence, the solution remains colorless. Hence, option (A) is correct.
The phenolphthalein show pink color in basic solutions. Hence, option (B) is incorrect.
The phenolphthalein indicators does not show blue color in any of the solution. Hence, option (C) is incorrect.
The colour of phenolphthalein is not orange color in acidic solutions. Hence, option (D) is incorrect.

So, the correct answer is “Option A”.

Note:
We need to know that in acidic solution, the phenolphthalein remains colorless and under basic conditions, it becomes a deep pin in colour. Hence, this is widely used as an acid – base indicator. The color change of the indicator is due to ionization and this will also change the charge and shape of the phenolphthalein. Some indicators are prepared from natural sources and these are called natural indicators.