The core pattern of increasing carbon number (homologues, similar structures): Generally speaking, as the number of carbon atoms in an aliphatic hydrocarbon molecule increases, the melting point generally shows a gradual upward trend. The core reason is that the relative molecular mass increases, and the van der Waals forces between molecules gradually strengthen. Examples to verify this:
Table: Name | Number of Carbon Atoms | Melting Point (°C)
Ethane | 2 | -183.3
Propane | 3 | -187.7
Butane | 4 | -138.4
Pentane | 5 | -129.7
Propane shows slight fluctuations due to crystal arrangement, but the overall trend still conforms to the increasing pattern.
The pattern of the same number of carbon atoms (isomers, same relative molecular mass): When aliphatic hydrocarbons have the same number of carbon atoms (i.e., are isomers), the pattern is: the more branches, the lower the melting point. This is because more branches lead to less compact molecular arrangement, weaker intermolecular forces, and therefore a lower melting point.
Special Notes
In addition to the above rules, two other points require special attention:
Although neopentane has 5 carbon atoms (within the typical liquid range), it is gaseous at room temperature, and its melting and boiling points are lower than those of straight-chain pentane, conforming to the rule that the more branched the isomer, the lower the boiling and melting points;
All aliphatic hydrocarbons have a density less than water, a property that does not change with the number of carbon atoms.