H3O is called a hydronium ion.
The hydronium ion is formed when a water molecule (H2O) accepts a proton (H+), which often happens when an acid is dissolved in water. The H+ ion bonds to one of the lone pairs of electrons on the oxygen atom in the water molecule, creating H3O+.
Because free protons (H+) do not exist in appreciable amounts in water, hydronium is often used to represent the concentration of H+ ions in aqueous solutions. It is a more accurate representation of the actual species present. The terms "proton" and "hydronium ion" are often used interchangeably in the context of acid-base chemistry in aqueous solutions, though hydronium is technically the more correct term.
The presence of hydronium ions is what makes a solution acidic. The higher the concentration of hydronium ions, the lower the pH and the more acidic the solution.
For example, when hydrochloric acid (HCl) dissolves in water, it dissociates into H+ and Cl- ions. The H+ ions immediately react with water molecules to form hydronium ions (H3O+).
HCl + H2O → H3O+ + Cl-
This reaction explains why hydrochloric acid is a strong acid; it completely dissociates in water to form hydronium ions.
