heptane - liquid paraffinic hydrocarbon containing seven carbon atoms in the molecule, which may be straight-chain (normal) or branched-chain (iso). Heptane can be used in place of hexane where a less volatile solvent is desired, as in the manufacture of certain adhesives and lacquers, and in extraction of edible and commercial oils. Heptane is blended with isooctane to create a standard reference fuel in laboratory determinations of octane number.