In chemistry experiments, millions of identical molecules mix and react at the same time. Scientists cannot easily tell one specific molecule apart from another in a complex blend. A chemical label acts like a tag that stands out from the rest of the mixture.
This tag is usually a radioactive isotope or a glowing fluorescent dye. For many years, scientists preferred radioactive isotopes because they behave exactly like normal atoms. For example, replacing a normal carbon atom with a carbon-14 atom does not change the molecular reactions.
The radioactive emissions are easy to detect even in very small amounts. Today, many labs prefer fluorescent dyes because they do not emit dangerous radiation. These modern dyes glow brightly under light of a specific wavelength.
Another choice is using stable isotopes that change the mass of the molecule. Researchers can measure this weight change using a mass spectrometer instrument. The main rule is that the label must not make the target molecule too heavy or bulky.
A large tag might block the molecule from entering a cell or reacting with other substances. Common student mistakes include thinking that labels change the chemical properties or make the molecule react faster.