If a solution turns blue litmus paper red, it is an acidic solution.
Litmus paper is a simple way to test whether a solution is acidic or basic. Blue litmus paper turns red in the presence of acids due to the change in pH level. In an acidic solution, the concentration of hydrogen ions (H+) is higher than that of hydroxide ions (OH–), which is what causes the blue litmus to change color. Hence, if you encounter a solution that has this effect on blue litmus paper, you can confidently classify it as acidic.