n-Heptane is the systematic (IUPAC) name for heptane. It is used to distinguish heptane from isomers of heptane, which are other compounds that have the same molecular formula but different structures.
For example, one of the isomers of heptane is methylcyclohexane, which has the same molecular formula (C7H16) as heptane, but a different structure. Methylcyclohexane is formed by a single ring of six carbon atoms with a single carbon attached to one of the carbons in the ring, while heptane is formed by a straight chain of seven carbon atoms.
