Question

How do intermolecular forces help explain differences in phase changes (boiling point, melting point)

Answer 1

Do you mean differences among various substances eg: H2O vs CO2? IMFs determine boiling and melting points by deciding how strongly molecules want to hold on to each other. The strength of IMFs depend on the type of IMFs, eg: dipole-dipole, ion-dipole, induced dipole (London dispersion), etc. Weak IMFs= low boiling point. IMFs also determine many other properties including vapor pressure, etc.