The tool used to measure the acidity or alkalinity of a solution is a pH meter or litmus paper. A pH meter provides precise measurements of the hydrogen ion concentration in a solution, enabling accurate assessments of its acidity or alkalinity on a scale from 0 to 14. Litmus paper, on the other hand, is a simple qualitative method that changes color based on the pH level of the solution. Both tools are essential in various scientific fields, including chemistry, biology, and environmental science, for understanding the properties of solutions.