Final answer:
Octane has a higher melting point than butane due to its greater molecular mass, leading to stronger London dispersion forces and increased intermolecular interactions.
Step-by-step explanation:
The melting point of octane (C8H18) is higher than that of butane (C4H10) because octane has a greater molecular mass and, consequently, stronger London dispersion forces. These intermolecular forces are the dominant interactions in nonpolar alkanes. The larger the molecule, the more pronounced these forces typically are, leading to a higher melting and boiling points for longer-chain alkanes. In the case of butane and octane, the straight-chain structures (known as normal alkanes, represented as n-butane and n-octane) allow for maximum surface contact between molecules, further increasing the strength of the London dispersion forces compared to their branched isomers.