What is pH?

• A. A measure of temperature effect on reaction rate
• B. A unit of concentration
• C. A property of light
• D. A measurement of how acidic or basic a solution is

Answer: (D)

pH is a measure of how acidic or basic a solution is, based on hydrogen ion concentration. It’s defined as pH = −log10([H+]), so more hydrogen ions (lower pH) mean a more acidic solution, while fewer hydrogen ions (higher pH) mean a more basic or alkaline solution. Neutral water at room temperature sits around 7, and the scale generally runs from 0 to 14, though extremes can occur in nonstandard conditions. Because the scale is logarithmic, each unit change reflects a tenfold change in hydrogen ion concentration, which is why small pH shifts can have big effects on chemical reactions and biological processes. This concept helps explain why pH matters in biology, chemistry, medicine, and everyday contexts, and it isn’t describing temperature effects on reaction rates, a unit of concentration, or a property of light.