Infrared spectroscopy — Chemistry, 14–17
Using infrared light to detect bonds in molecules. Chemistry, 14–17 years.
Molecules have moving bonds
In infrared spectroscopy, a sample is exposed to infrared light and the wavelengths it absorbs are recorded. Different bonds vibrate in different ways, so the pattern acts like a molecular fingerprint and can reveal groups such as O–H, C=O or C–H.
The problem of invisible structure
A clear liquid can look exactly like another clear liquid, even though its molecules are different. Chemists needed a quick, non-destructive clue about those molecules; infrared absorption provides one by linking measured peaks to particular bond vibrations.
Recognising ethanol
Suppose an unknown liquid has a broad absorption near 3300 cm⁻¹ and a strong one near 1050 cm⁻¹. The broad peak fits an O–H bond, while the lower peak fits a C–O bond. Together they support an alcohol such as ethanol, but you should compare the whole spectrum before claiming an exact identity.
One peak is not a whole answer
It is reasonable to match one striking peak and stop, because the graph seems to offer a clear label. But several molecules can contain the same bond, and peaks can overlap; use the whole pattern, especially the fingerprint region, and compare it with a reference.
Checking products and pollution
Pharmaceutical companies use infrared spectra to check whether a material contains the expected functional groups. Forensic and environmental laboratories can compare an unknown powder, paint or pollutant with reference spectra, often without using up the entire sample.
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