VCE · Topic explainer

What is a carboxylic acid?

Carboxylic acids contain the carboxyl group, –COOH, with a carbonyl and hydroxyl group on the same carbon. In water, simple carboxylic acids donate a proton and partially ionise. They can also form hydrogen bonds, which helps explain their relatively high boiling points.

Worked example

Ethanoic acid has the condensed formula CH₃COOH. In water, CH₃COOH + H₂O ⇌ CH₃COO⁻ + H₃O⁺. The equilibrium arrow matters because ethanoic acid only partially ionises.

To identify a carboxylic acid on an infrared spectrum, look for the very broad acid O–H absorption together with a strong C=O absorption. An O–H signal alone could come from an alcohol. Check both features before deciding.

Ethanoic acid: CH₃–C(=O)–OH. Identify C=O and O–H. on the same carbon. That group is –COOH.

Official exam practice

2025 VCE Chemistry, Section B, Question 4(b)

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