Which of the following changes occurs when phenolphthalein is added in acidic and basic solutions respectively:

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Colourless and pink

Colourless and blue

Pink and colourless

Blue and colourless

answer is A.

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Detailed Solution

Concept- Since phenolphthalein may change colour at various pH levels, it serves as an example of an acid-base indicator. Phenolphthalein is colourless from pH levels below 7 to 8.2, when it starts to turn pink, and to pH levels above 10, when it turns magenta.

The weak basic phenolphthalein is colourless in an acidic solution and becomes pink in an alkaline solution. When the solution is acidic, the equilibrium in the above reaction is to the left, and the concentration of the anions is too low to be able to see the pink colour.
When the solution is basic, however, the equilibrium is to the right and the concentration of the anion is high enough to see the pink colour.
Hence, the correct answer is option 1.

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