Final answer:
In a liquid solution, the lower the H+ concentration, the higher the OH- concentration. This is due to the constant product of these ions in water which must always be maintained.
Step-by-step explanation:
In a liquid solution, the lower the H+ concentration, the higher the OH− concentration. This relationship is due to the constant product of the concentrations of these ions in water, known as the water ion-product or the autoionization constant of water (Kw), which is always equal to 1.0×10−14 at 25°C. When the concentration of hydronium ions (H3O+ or equivalently H+) decreases, the concentration of hydroxide ions (OH−) must increase to maintain this constant product.
For example, in pure water, the concentrations of H+ and OH− are both 1.0×10−7 M. If the H+ concentration drops below this value, the solution becomes basic, and the OH− concentration rises above 1.0×10−7 M, resulting in a pH greater than 7. The acidity of a solution is directly related to the hydronium ion concentration: the higher the H3O+ concentration, the more acidic the solution is and vice versa.