In chemistry, what does pH measure?

pH is a scale used to quantify the acidity or alkalinity of a solution. It specifically measures the concentration of hydrogen ions (H⁺) present in that solution. A lower pH indicates higher acidity, while a higher pH indicates alkalinity. Pure water has a neutral pH of 7, with values below 7 being acidic and values above 7 being basic (or alkaline). Understanding pH is crucial in various fields such as chemistry, biology, environmental science, and agriculture, as it affects chemical reactions, biological processes, and the solubility and availability of nutrients.

The other options pertain to different aspects of chemistry. For instance, concentration of salt relates to solutions' ionic strength and solubility, while temperature measures thermal conditions that can affect reaction rates. Dissolved oxygen levels are important in aquatic chemistry but do not relate to the acidity or alkalinity of a solution. Each of these factors plays a significant role in chemistry, but they do not directly measure pH.