Explain why the boiling point and melting point of alkanes increase with an increase in molecular weight.

(N/A) As the molecular weight of an alkane increases,both its boiling point and melting point generally increase. This is due to the following reasons:
$1$. Alkanes contain $C-C$ and $C-H$ covalent bonds. Due to the small difference in electronegativity between carbon $(2.5)$ and hydrogen $(2.1)$,alkanes are essentially non-polar molecules.
$2$. Consequently,the intermolecular forces present are weak van der Waals forces.
$3$. As the molecular weight increases,the molecular size and surface area of the alkane increase. $A$ larger surface area leads to stronger van der Waals forces of attraction between the molecules.
$4$. Stronger intermolecular forces require more energy to overcome,resulting in higher boiling and melting points.

Molecular formula	Name	Molecular weight	$B.P. (K)$	$M.P. (K)$
$CH_4$	Methane	$16$	$111.0$	$90.5$
$C_2H_6$	Ethane	$30$	$184.4$	$101.0$
$C_3H_8$	Propane	$44$	$230.9$	$85.3$
$C_4H_{10}$	$n$-Butane	$58$	$272.4$	$134.6$
$C_5H_{12}$	$n$-Pentane	$72$	$309.1$	$143.3$
$C_6H_{14}$	Hexane	$86$	$341.9$	$178.5$
$C_7H_{16}$	Heptane	$100$	$371.4$	$182.4$
$C_8H_{18}$	Octane	$114$	$398.7$	$216.2$
$C_9H_{20}$	Nonane	$128$	$423.8$	$222.0$
$C_{10}H_{22}$	Decane	$142$	$447.1$	$243.3$
$C_{20}H_{42}$	Icosane	$282$	$615.0$	$236.2$