an unknown substance turns blue litmus paper to red. which additional test can be used to confirm the substance identity

We can also use the pH paper test to confirm the substance identity.

Acidic substances can turn a blue litmus paper to red. Therefore, if we put the same substance in pH paper, the paper should turn yellow or red or any color between that, according to its acidity. Each color on the paper represents a pH degree.

On top of that, there are other methods that can also measure the pH or decide whether the substance is acidic or alkaline. Methods such as connecting a pH meter to a data logger can accurately measure a substance's pH. We can also use a universal indicator for measuring pH. The lower the pH value is, the more acidic a substance is.