The measure of a substance's acidity or alkalinity is called the pH scale. This scale ranges from 0 to 14, with lower values indicating more acidic solutions, higher values indicating more alkaline (or basic) solutions, and a pH of 7 being neutral. Monitoring pH is essential in various fields, including chemistry, biology, and environmental science, as it affects chemical reactions, biological processes, and the health of ecosystems, such as soil and aquatic environments.