Quinoline appears as a colorless liquid with a peculiar odor. Slightly denser than water. Contact may irritate to skin, eyes, and mucous membranes. May be toxic by ingestion. Used to make other chemicals. Quinoline is the simplest member of the quinoline class of compounds, comprising a benzene ring ortho fused to C-2 and C-3 of a pyridine ring. It is a mancude organic heterobicyclic parent, a member of quinolines, an azaarene and an ortho-fused heteroarene.