What does red litmus paper changing to blue indicate?

When red litmus paper changes to blue, it indicates that the solution being tested is basic or alkaline. Litmus paper is a common pH indicator used in various scientific fields, including chemistry and biology, to determine the acidity or alkalinity of a solution. The change from red to blue occurs because the pH of the solution is above 7, which is considered to be basic.

In basic solutions, the concentration of hydroxide ions (OH⁻) is higher than that of hydrogen ions (H⁺), contrasting with acidic solutions where the reverse is true. This specific property of litmus paper allows users to quickly identify the nature of a solution without having to measure the pH directly.