What is pH and what does it measure in aqueous solutions?

• A. pH measures the temperature of a solution.
• B. pH is a measure of how acidic or basic a solution is, defined as the negative logarithm of the hydrogen ion concentration.
• C. pH is the ratio of base to acid in a solution.
• D. pH is the concentration of hydroxide ions only.

Answer: (B)

pH tells you how acidic or basic a solution is by describing the hydrogen ion concentration on a logarithmic scale. It is defined as pH = -log10[H+], so a higher hydrogen ion concentration makes the pH value lower (more acidic) and a lower hydrogen ion concentration makes the pH value higher (more basic). The familiar scale runs roughly from 0 to 14 at room temperature, with acidic below 7, neutral around 7, and basic above 7. This measure centers on hydrogen ions, not temperature, and it isn’t simply the ratio of base to acid nor the concentration of hydroxide ions alone (that would be pOH). In environmental contexts, pH affects chemical processes in water and the well-being of aquatic life.