The three figures on this page show the infra red absorption spectra of the carboxylic acids methanoic acid, ethanoic acid and propanoic acid.

Note the broad feature between 2500 and 3300 cm-1 (marked by the black arrows) which is characteristic of the -OH group in carboxylic acids.

This feature is considerably broader than most of the other features in infra red spectra due to strong hydrogen bonding.

Also note the feature near 1700 cm-1 (marked by the red arrows) which is characteristic of the C=O group in aldehydes, ketones, carboxylic acids and esters.

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ethanoic acid

methanoic acid

propanoic acid