Final answer:
The melting point of hydrocarbons increases with the number of carbon atoms because larger molecules have a higher surface area and stronger London dispersion forces. Alkanes, as saturated hydrocarbons, display this trend with their straight-chain homologs' boiling points increasing alongside molar mass.
Step-by-step explanation:
The melting point of hydrocarbons increases as the number of carbon atoms per molecule increases because larger hydrocarbon molecules have greater surface area and a larger electron cloud, which enhances the strength of the London dispersion forces between them. These forces are a form of van der Waals force which are the only intermolecular forces present in nonpolar molecules like hydrocarbons. As the surface area and number of electrons increase, these London dispersion forces become stronger and thus require more energy to overcome when melting the substance.
Furthermore, as mentioned in Table 12.7.1, the boiling points of the straight-chain alkanes, which are saturated hydrocarbons, increase with increasing molar mass. In saturated hydrocarbons, such as alkanes, each carbon atom is bonded to the maximum number of hydrogen atoms through covalent bonds making them fully ‘saturated’ with hydrogen.
Lastly, the concept of catenation highlights the unique ability of carbon atoms to bond strongly to each other, forming longer chains and complex molecules with varying properties including melting and boiling points. This characteristic is rooted in the non-polar bond character between carbon atoms, which share electrons evenly due to their similar electronegativities.