The relative strengths of covalent bonds and van der Waals interactions remain the same when tested in a vacuum or in water. However, this is not true of hydrogen bonds or ionic bonds, whose bond strength is lowered considerably in the presence of water in comparison with the bond strength observed in a vacuum. A logical explanation to these observations is that the estimated bond strengths measure the amount of energy needed to break them. Covalent and Van der Waals attractions have intrinsic value independent of the environment while hydrogen bonds depend on any charged or polar molecule with reducing strength of the interaction they would otherwise have in the absence of water (in a vacuum).

Van der waals interactions are a type of hydrophobic interaction, in which they do not interact with the polar water molecule. Covalent bonds involve the sharing of electrons between atoms of relatively similar electronegativities, and are most often too strong to disrupt by polar molecules of water. Therefore, covalent bonds and van der waals forces have an Intrinsic bond strength value that is independent of the environment.

However, either the partial negative oxygen atom or the partial positive hydrogen atoms in water molecules disrupt hydrogen or ionic bonds. Water is known to form hydrogen bonds with other polar or charged molecules, thus reducing the strength of interaction these molecules would normally have in the absence of water. Basically, these compounds with Hydrogen or Ionic bonds ionize, whether partially or fully in water, thereby leading to a decrease in bond strength in water.

QED!