pH is defined as which of the following?

• A. Energy used at rest
• B. Measure of acidity or alkalinity
• C. Normal Blood pH
• D. Elevated CO2

Answer: (B)

pH is a measure of how acidic or basic a solution is. It reflects the hydrogen ion concentration: more H+ means more acidity and a lower pH, while fewer H+ means more basicity and a higher pH. The pH scale is the negative logarithm of [H+], so small changes in hydrogen ion concentration produce larger changes in pH. Neutral on the usual scale is around pH 7, with values below 7 being acidic and above 7 being basic. This concept underpins why physiological systems regulate pH tightly, especially in blood where buffering keeps it roughly in the narrow range of about 7.35–7.45. The other statements describe different ideas—energy use at rest is a metabolic rate concept, elevated CO2 can influence pH but isn’t the definition, and normal blood pH is a reference value, not the definition of pH itself.