How can a compound joined by a covalent bond form ions when it dissolves in water?

Answer 1

If you are referring to acids and bases it mainly has to do with something called and acid-base equilibrium.

When an acid dissociates in water the reaction is something like this (using HCl as an example)

HCl + H2O --> Cl- + H3O+

The hydrogen ion (H+) is pulled out of the HCl molecule and bonds to a lone pair of electrons to form a hydronium ion (H3O+) . The new bond between the hydrogen and oxygen is essentially the same as a normal covalent bond, only the oxygen contributes both electrons rather than each atom contributing one.

When a weak covalent base (such as ammonia, NH3) dissociates in water a somewhat similar reaction occurs.

NH3 + H2O --> NH4+ + OH-

In this case it is the water molecule that donates a hydrogen ion, which bonds to the lone pair of electrons on the ammonia molecule to turn it into an ammonium ion (NH4+).

In reactions such as these the acid is the substance that gives up a hydrogen ion (proton donor) and the base is the one that gains a hydrogen ion (proton acceptor).

Answer 2

When a covalent compound dissolves in water, it can react with water molecules to form ions through a process called ionization or dissociation. This can happen if the compound contains polar covalent bonds that can break apart into ions when exposed to water. The water molecules surround the ions, stabilizing them and allowing them to stay in solution.