Each bond absorbs its own wavenumbers, so the infrared light a molecule absorbs shows which bonds it has. Chemists can tell an alcohol from a ketone or an acid at a glance.
An infrared spectrometer shines infrared light of every wavenumber through a drop of sample and measures how much of each gets through. It draws the result as an infrared spectrum. The line runs along the top where the light gets through, and each dip marks light the bonds absorbed. Chemists print the wavenumbers from high on the left to low on the right.
Three molecules, three patterns
Ethanol, an alcohol, has a wide, rounded O–H dip centred near 3340, and a deep C–O dip near 1050. Propanone, the ketone in nail polish remover, has no O–H, and a deep, sharp C=O dip at 1715. Ethanoic acid, with its -COOH group, shows both: a C=O dip at 1710 and an O–H dip so wide that it runs from about 2500 to 3300. All three have C–H dips just below 3000.
Why the O–H dip is wide
The δ+ hydrogen of an O–H group makes hydrogen bonds with the oxygen atoms of neighbouring molecules, as water does. Each hydrogen bond tugs on its O–H a little differently, so each O–H vibrates at a slightly different rate, and the dips merge into one broad one. Ethanoic acid molecules pair up, joined by two strong hydrogen bonds, one from each -OH to the other molecule's C=O, so their O–H dip is wider still and sits at lower wavenumbers.
Below about 1500, each molecule has its own crowded pattern of dips from many bends and stretches together. Chemists call this the fingerprint region. Each molecule's pattern there is its own, so a chemist can name an unknown liquid by matching its spectrum against a library of known ones.
Propanone's deep dips near 1360 and 1220 belong to this region: they come from its CH₃ groups and its carbon skeleton bending and stretching together.