The effect of increasing the pressure is to raise the boiling point. Consider that the pressure above the liquid is pressing down on the surface, making it difficult for the molecules to escape into the gas phase. The more pressure, the more energy is required, so the boiling point is higher at higher pressures.
In general, larger molecules have higher boiling points than smaller molecules of the same kind, indicating that dispersion forces increase with mass, number of electrons, number of atoms or some combination thereof.
Intermolecular forces (IMFs) can be used to predict relative boiling points. The stronger the IMFs, the lower the vapor pressure of the substance and the higher the boiling point. Therefore, we can compare the relative strengths of the IMFs of the compounds to predict their relative boiling points.
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