Chemistry Question: For the ignition test, to determine whether unknown chemical has a benzene ring, how would you explain the chemistry?

One of the definitions of "unsaturated hydrocarbon" is that the hydrocarbon does not contain the maximum number of hydrogen atoms, i.e. unsaturated with hydrogen atoms.

A cyclic molecule with 6 C atoms would have at most 12 H atoms, but benzene has only 6 H atoms. Hence, benzene is considered as an unsaturated compound.

Also, the presence C=C double bonds and C≡C triple bonds would lead to the unsaturation of hydrocarbons. Benzene is considered as an unsaturated compound because its molecule contain 3 delocalized C=C bonds.

Actually, all aromatic hydrocarbons are unsaturated.