Which of the following is an amphoteric acid?

Glycine

The correct answer is option 1. glycine.

Glycine is the simplest amino acid, with the chemical formula \(NH_2CH_2COOH\). It contains both an amino group \((-NH_2)\) and a carboxyl group \((-COOH)\), making it amphiprotic, meaning it can donate or accept a proton depending on the \(pH\) of the solution. In acidic conditions, glycine can donate a proton from its carboxyl group, behaving as an acid

\(NH_2CH_2COOH ⇌ NH_3^+CH_2COO^-\)

In basic conditions, glycine can accept a proton at its amino group, behaving as a base

\(NH_2CH_2COOH + H^+ ⇌ NH_3^+CH_2COOH\)

Due to its ability to act as both an acid and a base, glycine is considered an amphoteric compound.

Salicylic acid (2-hydroxybenzoic acid), benzoic acid, and citric acid are all monoprotic acids, meaning they can donate only one proton \((H^+)\) in an aqueous solution. They exhibit acidic properties by donating a proton to water, forming the corresponding conjugate base. However, they do not have the ability to accept a proton and behave as bases. Therefore, they are not considered amphoteric compounds.

In summary, glycine is classified as an amphoteric compound because it contains both acidic and basic functional groups, allowing it to act as both an acid and a base depending on the pH of the solution. Salicylic acid, benzoic acid, and citric acid are not amphoteric because they can only donate protons and do not have the ability to accept protons.