Question

Which colour appears when a few drops of phenolphthalein put into the test tube contains lime water?
(A) Yellow
(B) Orange
(C) Pink
(D) Colourless

Answer 1

Hint: To answer this question we should know about the indicator properties of phenolphthalein. We should also know about the acidic or basic nature of lime water. Lime water is the common name of dilute solution of $Ca{{(OH)}_{2}}$. Lime water is basic in nature.

Complete answer:
Let’s try to understand the acidic or basic nature of lime water.
> Lime water is known as a very dilute solution of aqueous calcium hydroxide. It is used to relieve acidity. This is because lime water has basic properties as it is a strong base. It furnishes OH ions in aqueous solution.
> Now, phenolphthalein is an indicator. It is an organic compound of the phthalien family. It shows different colours in acidic and basic solutions. It shows pink colour in basic medium while remaining colourless in acidic medium. This happens as the colour of ionized and unionized forms of phenolphthalein have different colours.
> In the acidic medium it remains in its unionized form which is colourless. While in basic medium the OH ions ionizes it and converts it into ionized form which is pink in colour.
Since, lime water is basic in nature, so, it will convert phenolphthalein in its ionized form and its colour will change to pink.

Hence, the answer for the given question is option (C).

Note: Phenolphthalein is a synthetic indicator. It remains colourless for solutions having pH less than 8.5 and shows pink to deep red colour for solutions having pH greater than 9.0.