Final answer:
The concentration of hydroxyl ions (OH-) in an aqueous solution increases as the concentration of hydronium ions (H3O+) decreases due to the ion product constant of water (Kw). This inverse relationship helps maintain a constant Kw value and is key to understanding the pH scale.
Step-by-step explanation:
In aqueous solutions, the relationship between the concentration of hydroxyl ions (OH−) and hydronium ions (H3O+) is described by the concept of chemical equilibrium and the dissociation of water. According to the principles of acid-base chemistry, water (H2O) partially dissociates into H3O+ and OH− ions. This dissociation is represented by the following reversible reaction:
2 H2O (l) ⇋ H3O+ (aq) + OH− (aq)
The concentrations of H3O+ and OH− ions are inversely related due to the constant product of ion concentrations at a given temperature, known as the ion product constant of water (Kw). The value of Kw at 25°C is 1.0 x 10⁻¹ M². Therefore, as the concentration of H3O+ ions decreases, the concentration of OH− ions increases, and vice versa, to maintain the constant value of Kw.
If the solution is becoming more basic, or alkaline, it means the concentration of H3O+ ions is decreasing, allowing the OH− ion concentration to increase. Conversely, if the solution is becoming more acidic, the concentration of H3O+ ions is increasing, which causes a decrease in OH− ion concentration. This is a fundamental aspect of the pH scale, which measures the acidity or basicity of a solution.