Question

Which of the following changes occurs when phenolphthalein is added in acidic and basic solutions respectively:
  (A) Colourless and pink
  (B) Colourless and blue
  (C) Pink and colourless
  (D) Blue and colourless

Answer 1

Hint: Phenolphthalein is an example of acid-base indicator because of its ability to change color in different pH. Phenolphthalein remains colorless throughout the range of pH levels below seven and begins to turn pink at a pH level of 8.2 and at a pH above 10, it becomes magenta in colour.


Complete step by step solution:
   -As we know the term acid and base have been defined in different ways in chemistry. Generally, we can say that an acid is any hydrogen-containing substance which is capable of donating hydrogen ion(proton) to another substance and base is capable of donating $O{{H}^{-}}$ ion.
  - Solutions can be classified as acidic or basic on their hydrogen ion concentration relative to pure water. If the pH of the solution is above seven then the solution is said to be alkaline and if below seven, then the solution is acidic.
 - Phenolphthalein is an example of a synthetic indicator which is used in acid - base titrations. When the pH of a solution varies, the colour of phenolphthalein also varies accordingly.
 - Phenolphthalein is a weak acid and has a white to yellow crystalline solid nature. It's a large organic molecule having the molecular formula ${{C}_{20}}{{H}_{14}}{{O}_{4}}$.
 - We can write the general equation for an indicator which is a weak acid as follows

 ${ HIn }_{ (aq) }+{ H }_{ 2 }O_{ (l) }\rightleftharpoons { In }^{ - }+H_{ 3 }{ O }^{ + }_{ (aq) }$
 Acid base
Colour A Colour B
   - As we mentioned, phenolphthalein is a weak base and its colourless in acidic solution and has a pink colour in alkaline solution.
  - When the solution is acidic, the equilibrium in above reaction is to the left hand side, and the concentration of the anions is too low for the pink colour to be observed and the equilibrium is to the right when the solution is basic, , and the concentration of the anion becomes sufficient for the pink colour to be seen.

Therefore, the answer is option (A). That is phenolphthalein is colourless in acidic solution and pink in basic solution.

Note: It should be noted that, based on whether the phenolphthalein indicator is in acid(colourless) or alkali (basic) solution, it has two different structures also. Even though both of these structures absorb light in the ultraviolet region which is not accessible for the human eye, the pink form or phenolphthalein indicator in alkaline solution which absorbs light in the visible region(pink) and thus it appears to be pink in the human eye.