What does pH measure in water?

pH is a measure of the hydrogen ion concentration in a solution, which directly indicates the acidity or alkalinity of that solution. When the pH scale is used, values lower than 7 indicate acidity (with more hydrogen ions), whereas values higher than 7 indicate alkalinity (with fewer hydrogen ions). A pH of exactly 7 is considered neutral, indicating a balance between hydrogen ions and hydroxide ions. This scale is critical in water quality testing because it affects chemical reactions, biological processes, and the overall health of aquatic ecosystems.

In contrast, the other options do not measure the same properties. Color intensity pertains to the visual appearance of the water, temperature measures the thermal condition of the water, and salinity quantifies the salt concentration. While these factors are also important for assessing water quality, they do not provide information about the acidity or alkalinity, which is specifically what pH measures.