The fact that pure acetic acid does not turn blue litmus to red is a common chemistry experiment.
One of the most common chemistry experiments students perform in high school involves mixing a solution of blue litmus with a solution of pure acetic acid. When this happens, the mixture turns red, but if there is any water present in the mixture, it will turn blue instead. This is because acetic acid reacts with water to form acetic acid and hydrogen ions. The reaction that takes place between litmus and acetic acid is the same one that takes place between an acidic solution and a base solution, which causes both solutions to change color.
However, when pure acetic acid is mixed with water, no reaction occurs. This means that there are no hydrogen ions present for the two solutions to react with each other. The lack of hydrogen ions also means that there are no protons present for the two solutions to react with each other either. As such, no reaction occurs between pure acetic acid and water.
