In an aqueous solution, solute molecules or ions require a certain amount of time to migrate through the solution. The rate of this migration sets an upper limit on how fast reactions can take place, because no reaction can take place faster than the ions can be supplied. This limit is known as the diffusion-controlled rate. It has been found that in aqueous solution the diffusion-controlled rate for hydronium ions (H₃O⁺, Fundamentals J) is about three times as fast as that for other ions. Explain why this is so.