How is the concentration of hydronium ions \( H_3 O^+ \) affected when a solution of an acid is diluted?

The concentration of hydronium ions \( H_3 O^+ \) decreases when an acid is diluted because, on adding water the \( H^+ \) ions of the acid and hydroxyl ions of water react to form water molecules and this in-turn decreases Hydronium concentration in the solution.