Question

What do you observe in the titration setup when \[15ml\] of the \[NaOH\] with an unknown molarity is poured into the flask and then \[5\] drops of phenolphthalein indicator are added?
A. A pinkish colour will appear throughout the solution.
B. A blue colour will appear throughout the solution.
C. There will be a temporary pinkish colour that will dissipate.
D. There will be a temporary blue colour that will dissipate.
E. There will be no colour change.

Answer 1

Hint: Phenolphthalein is an acid-base indicator which is added to titration flasks as to observe a visible change in colour of the solution when a significant change in the pH (a measure of acidity or basicity) takes place.

Complete answer:
Phenolphthalein is an organic dye that has a highly conjugated system capable of absorbing the UV radiations and imparting a complimentary colour to its solution. This acidic dye is an indicator that gets transformed into a quinonoid resonance stabilized dianion upon addition of a basic solution to it. Therefore phenolphthalein solutions turn reddish-pinkish in the presence of a base.
Like each acid-base indicator phenolphthalein works effectively in a limited range of pH which is such that it remains colourless below a pH of \[8.5\] and attains a pinkish colour above it. If the pH values are extremely high i.e. above \[10\] then the solution turns deep red.
At no point the phenolphthalein indicator shows blue colour in solution.
Sodium hydroxide is a very strong Arrhenius base that completely dissociates in water to give hydroxide ions. Thus even dilute solutions of sodium hydroxide are basic enough in water to give a pinkish colour in presence of the phenolphthalein indicator.

So, the correct answer is “Option A”.

Note:
 The pinkish colour observed in addition to sodium hydroxide is due to its basic character but if any acid is poured into the container containing the basic solution, then a neutralization reaction will take place and the base will get consumed and there will come a point when the solution becomes colourless.