Spectroscopy

Nobody can look at a molecule, yet chemists routinely work out structures of compounds they have never seen. They shine light of every kind through them and read the clues. Measure concentrations with the Beer–Lambert law, treat bonds as springs that absorb infrared, weigh molecules and read chlorine and bromine from their isotope patterns, then master NMR: colour equivalent hydrogens, predict splitting patterns, and combine all the evidence to solve unknown structures.