The instrument utilized to measure the acidity of substances is called a pH meter or an acid-base indicator. A pH meter provides a precise measurement of the hydrogen ion concentration in a solution, indicating its acidity or alkalinity on a scale from 0 to 14. Acid-base indicators change color at specific pH levels, offering a visual method for determining acidity. These tools are essential in various fields including chemistry, biology, and environmental science for monitoring chemical reactions and ecological health.