If pH paper turns blue when testing a substance, that indicates the substance is a(n):

When pH paper turns blue during testing, it indicates that the substance has a higher pH level, which is characteristic of a basic or alkaline substance. The pH scale ranges from 0 to 14, with values below 7 indicating acidity, values around 7 indicating neutrality, and values above 7 indicating alkalinity. Thus, the change in color to blue strongly suggests that the substance being tested is a base.

In this context, choices pertaining to acidity or neutrality do not align with the observation of a blue color change. Since blue is associated specifically with higher pH levels, the notion of "concentration" does not relate directly to the reaction of the pH paper, but rather is a term regarding the amount of solute in a solution. The correct identification of the substance based on the pH paper's response is that it is indeed a base.