Acids are substances that produce hydronium ions, H3O+,
when dissolved in water.
"Strong" acids ionize completely
in water. For example,
The above reaction indicates that when hydrogen chloride gas reacts with water,
hydronium ions are produced in aqueous solution. Note that the arrow points
to the products, indicating that virtually ALL the HCl becomes H3O+
and Cl- ions in aqueous solution. HCl is therefore a strong
"Weak" acids ionize partially in
Acetic acid, CH3COOH, does not completely ionize
in water. Most of the acetic acid in solution remains un-ionized and a small
fraction ionizes to form CH3COO- and H3O+
ions. To represent this in a chemical equation, a double arrow is used. The
longer arrow pointing toward the reactants indicates that acetic acid in solution
remains un-ionized. CH3COOH is a weak acid.