When the concentration of hydronium ions (H3O+) increases in an aqueous solution, the concentration of hydroxide ions (OH-) decreases. This is due to the ionic product of water constant (Kw).

Here's why:

* Kw is a constant: The ionic product of water (Kw) is a constant at a given temperature. It represents the product of the concentrations of H3O+ and OH- ions in water. At 25°C, Kw = 1.0 x 10^-14.

* Inverse relationship: The equation for Kw is:

Kw = [H3O+][OH-]

* Shift in equilibrium: If the concentration of H3O+ increases, the equilibrium shifts to the left to maintain the constant value of Kw. This means the concentration of OH- must decrease to compensate for the increase in H3O+.

In simpler terms:

Imagine a seesaw. H3O+ and OH- are on opposite ends. When one side goes up (H3O+ concentration increases), the other side must go down (OH- concentration decreases) to maintain the balance (Kw constant).

Therefore, increasing H3O+ in an aqueous solution leads to a decrease in OH- concentration. This makes the solution more acidic.