What does pH measure?

pH measures the acidity or alkalinity of a substance, which is crucial for understanding how different products will interact with the skin. The pH scale ranges from 0 to 14, where values below 7 indicate acidity, values above 7 indicate alkalinity, and a value of 7 is considered neutral. For skin care professionals, knowledge of pH is vital because the skin has a natural pH level that typically ranges from 4.5 to 5.5, which means it is slightly acidic. Using products that are not compatible with the skin's natural pH can disrupt its barrier function and lead to issues such as irritation, dryness, or breakouts. This understanding helps estheticians select appropriate products to maintain or restore the skin's balance and health.

The other options do not accurately reflect what pH measures. Total volume pertains to the amount of substance in a solution rather than its acidity or alkalinity. A specific temperature of a chemical reaction is unrelated to pH, though temperature can affect reactions in general. Lastly, the color scale of liquid substances refers to visual characteristics and is not indicative of pH levels. Thus, option B clearly and accurately conveys the definition and significance of pH in esthetics.