Video Transcript
Acid–base indicators, such as phenolphthalein, contain compounds that react with ions
from acids and alkalis in reversible reactions. Phenolphthalein is pink when added to an alkaline solution and clear when added to an
acidic solution. What color would the solution be at stage III? (A) Clear as stage I. (B) Pink as stage II. (C) Darker pink than stage II. Or (D) lighter pink than stage II.
In this question, we want to know the color of the solution in the third conical
flask. Phenolphthalein is a common acid–base indicator. Acid–base indicators work through reversible reactions. A reversible reaction is a reaction that proceeds in both directions; reactants react
to form products, and the products react to reform the reactants.
When an acid or alkali is added to an indicator in sufficient quantity, a color
change can be observed. The question tells us that when an alkali is added to phenolphthalein, it turns pink,
and when an acid is added, it turns clear.
If we look at the first stage of this question, we are adding an alkali, and we see a
change from colorless to pink. Then at stage II, we are adding a greater quantity of an acid of the same molarity,
which will reverse the reaction and cause the pink color to vanish, leaving a
colorless solution. As such, the color that the solution would be at stage III is (A), clear as stage
I.
