pH is defined as what?

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

Answer: (A)

pH is a measure of how acidic or alkaline a solution is. It is defined as the negative logarithm of the hydrogen ion concentration in the solution, expressed as pH = -log10[H+]. Because the scale is logarithmic, a one-unit change represents a tenfold change in hydrogen ion concentration. In biological systems, pH indicates whether the environment is more acidic (lower pH) or more basic (higher pH), with blood normally kept around 7.35–7.45. This helps enzymes and metabolic processes function properly. The other statements refer to related concepts—energy use at rest, a specific normal blood pH value, or elevated CO2 that can shift pH via carbonic acid—but they do not define what pH itself represents.