Final answer:
The higher the H⁺ (hydronium ion) concentration in a solution, the lower the OH⁻ (hydroxide ion) concentration due to the water autoionization equilibrium, which maintains a constant product of their concentrations.
Step-by-step explanation:
In a liquid solution, the higher the H⁺ concentration, the lower the OH⁻ concentration. This inverse relationship is due to the water autoionization equilibrium, which states that the product of the concentration of hydronium ions [H3O⁺] and hydroxide ions [OH⁻] is always 1.0×10⁻¹⁴ at 25 °C. Thus, if the concentration of hydronium ions is greater than 1.0×10⁻⁷ M, the solution is considered acidic, and the hydroxide ion concentration must be proportionally lower to maintain the constant product.