In an aqueous solution, hydrogen ions (H+) are always joined to water molecules to form hydronium ions (H3O+).

Here's why:

* Hydrogen ions are highly reactive: They have a strong tendency to gain an electron to become neutral.

* Water molecules are polar: They have a partial positive charge on the hydrogen atoms and a partial negative charge on the oxygen atom.

* Hydronium ion formation: The positive charge on the hydrogen ion attracts the lone pair of electrons on the oxygen atom of a water molecule, forming a covalent bond. This results in a hydronium ion (H3O+).

Therefore, while we often write H+ to represent hydrogen ions in solution, it's important to remember that they exist as hydronium ions in the presence of water.