Chapter 4, Problem 4.76E

The covalent compounds ethyl alcohol and dimethyl ether both have the formula ${\text{C}}_{\text{2}}{\text{H}}_{\text{6}}\text{O}$. However, the alcohol melts at $-117.3°\text{C}$ and boils at $78.5°\text{C}$, whereas the ether melts at $-138.5°\text{C}$ and boils at $-23.7°\text{C}$. How could differences in forces between molecules be used to explain these observations?