A chemist is a scientist trained in the science of chemistry. Chemists study the composition of matter and its small-scale properties such as density and acidity instead of large-scale properties like size and shape.
Arik Soong's mother was a chemist. (ENT: "Cold Station 12")
During the mid-2360s, Vaatrik, a Bajoran chemist, ran the Chemist's shop on Terok Nor. (DS9: "Necessary Evil")