Which of the following is shown by benzoic acid but NOT by phenol?

The correct answer is Option (2) → It evolves carbon dioxide with aqueous sodium hydrogencarbonate.

Benzoic acid and phenol are both acidic, so they share several reactions. Sodium hydrogencarbonate is a weak base, and only a carboxylic acid is acidic enough to react with it and set free carbon dioxide. Phenol is a weaker acid and gives no CO₂. This reaction is the test for a carboxyl group:

    \( \displaystyle \ce{C6H5COOH + NaHCO3 -> C6H5COONa + H2O + CO2 ^} \)

Why the other options are incorrect

Option (1) It evolves hydrogen gas with sodium metal. — Incorrect. Carboxylic acids, like alcohols, evolve hydrogen with electropositive metals, and phenol also reacts with sodium to give sodium phenoxide and hydrogen.

Option (3) It forms a salt with aqueous sodium hydroxide. — Incorrect. Carboxylic acids form salts with alkalies similar to phenols; phenol gives sodium phenoxide with NaOH.

Option (4) It is a stronger acid than ethanol. — Incorrect. Phenol (pK_a 10) is also a stronger acid than ethanol (pK_a about 16).

NCERT (2026-27 edition):
8.9.1 Reactions Involving Cleavage of O–H Bond – “Reactions with metals and alkalies
The carboxylic acids like alcohols evolve hydrogen with electropositive metals and form salts with alkalies similar to phenols. However, unlike phenols they react with weaker bases such as carbonates and hydrogencarbonates to evolve carbon dioxide. This reaction is used to detect the presence of carboxyl group in an organic compound.”