Substances with non-polar molecules are usually insoluble in water. How is this behavior on the basis of intermolecular forces?

Due to the high polarity of water, and the presence of partially charged oxygen and hydrogen atoms in water molecules, it readily forms hydrogen bonds between molecules. In order for another substance to dissolve in water, it needs to be able to form solute-water interactions that are more energetically favourable than water-water interactions (i.e. hydrogen bonds).

Things like ionic substances can easily do this, as they consist of fully charged ions. Non-polar substances, however, do not exhibit hydrogen bonds nor ionic interactions, and the only intermolecular forces are van der Waals forces, which compared to hydrogen bonds are very weak - they are basically temporary dipoles that constantly fluctuate. They are far too weak to have any effect on hydrogen bonds. As a result, there is little or no "water solute" interaction, and the substance does not dissolve in the water.

This behavior is based on the fact that water is a polar molecule due to its unequal distribution of charge. Non-polar molecules lack a significant dipole moment, so they are unable to form strong attractions with water molecules (hydrogen bonding). Instead, non-polar molecules tend to interact with each other via weak van der Waals forces, which are not strong enough to overcome the strong hydrogen bonding present in water. As a result, non-polar molecules are usually insoluble in water.