The crystals on the filter paper are mostly aspirin. They also hold some salicylic acid that never reacted, a little ethanoic acid, and traces of side products. Chemists clean them up, then test them.
Recrystallising
To clean up crystals, chemists recrystallise them. The crude crystals go into the smallest amount of a hot solvent that will dissolve them, for aspirin a warm mix of ethanol and water. Aspirin dissolves well when the solvent is hot and poorly when it is cold. As the solution cools slowly, aspirin molecules settle onto growing crystals one at a time, each in its place in a neat, repeating stack. Molecules of other substances fit the stack badly, and there are only a few of them, so they stay dissolved in the liquid. A second filtering collects the cleaner crystals.
A sharp melting point
A pure solid melts sharply, within about a degree, and other molecules mixed in make it start melting lower and over a range, the melting-point test you met when separating mixtures. Chemists heat a few crystals in a thin glass tube and watch for the moment they turn to liquid.
| Sample | Melts at |
|---|---|
| pure aspirin | 135 to 136 °C |
| pure salicylic acid | about 159 °C |
| aspirin with salicylic acid mixed in | lower, and over several degrees |
A spot test
Chromatography gives a second check. It works the way it did with black ink on paper, here on a plate coated with a thin layer of silica: a dot of each sample goes on a pencil start line, and the solvent carries each kind of molecule its own distance, so each has its own Rf. Under an ultraviolet lamp the spots show up dark.
Chemists run their product beside pure aspirin and pure salicylic acid on the same plate. A pure product gives a single spot, level with the pure aspirin. Salicylic acid moves its own distance, so any left in the product shows up as a second spot level with it.