Why do polar liquids have low vapour pressure and non polar liquds have high vapour pressure

Dear student,

Nonpolar molecules have less intermolecular forces between them due to less dipole-dipole interactions. This means that when heat is added to a non polar substance it is more likely to evaporate than a polar substance.This means that non polar substances tend to evaporate at lower temperatures and tend to have higher vapour pressures since all the heat added to the nonpolar substance goes to raising the temperature making the substance evaporate more easily.
Polar molecules can attract each other through dipole-dipole and dipole-induced dipole interactions in addition to the induced dipole-induced dipole intermolecular interactions that all molecules experience. Thus, all else (molar mass, for example) equal, a polar molecule will generally have a lower vapour pressure than a non-polar molecule.


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