pH indicator strips are small pieces of paper or plastic that change color when dipped into a liquid. They are used to measure the acidity or alkalinity of a solution, which is indicated by a pH scale ranging from 0 to 14. A pH of 7 is neutral, below 7 is acidic, and above 7 is alkaline. These strips are coated with a mixture of dyes that react to hydrogen ion concentration in the solution. When the strip is immersed, it changes color, allowing users to compare the result against a color chart to determine the pH level of the liquid being tested.