Question

What do you observe when you add a few drops of phenolphthalein to a test tube containing acetic acid?
$(A)$ Blue colour
$(B)$ Red colour
$(C)$ Colourless
$(D)$ None of these

Answer 1

Hint: Acetic acid is also called Ethanoic Acid. It is a colorless organic Compound that is a weak acid since it does not completely dissociate into ions in water as compared to strong acids that show 100% dissociation in water. We know that in order to differentiate acids and basic nature of organic compounds we use certain compounds termed as Indicators. These can be either natural like turmeric, onion etc. or synthetic like Phenolphthalein, litmus etc.

Complete answer:
Phenolphthalein $({C_{20}}{H_{14}}{O_4})$ is an example of synthetic indicator that belongs to the phthalein family. It is a very common acid-base indicator used in chemistry. It is slightly soluble in water and dissolves easily in alcohol. It appears as white or yellowish-white to little orange crystalline powder. It is a weak acid and is colorless however when it loses proton it turns out to be pink in color. When we add it to acidic medium the Equilibrium is shifted to the left and solution remains colorless however when we add it to alkaline medium the equilibrium shifts to the right producing pink color. Or we can say that it is observed that Phenolphthalein is colorless below pH 8.5 and turns pink to red above pH 9.
Here in the question we add a few drops of Phenolphthalein indicator to the test tube containing acetic acid, since we know that Acetic acid is a weak acid so the indicator will remain colorless indicating acidity of medium.

Therefore Option(C) is correct.

Note:
Phenolphthalein was first discovered by scientist Adolf von Baeyer. It is manufactured by the reaction of phenol and phthalic anhydride in the presence of Sulphuric acid or zinc chloride. Phenolphthalein finds Applications in Acid-Base titrations in Laboratory. By this way we can easily calculate the concentration of unknown solutions using the known solution.