Now to begin, let's suppose we have a system of one liter of pure water that has a PH of seven. Let's examine what happens to our PH when we add a small amount of acid or base to our system. Well, let's begin with the acid. Suppose we add zero one mo of HCL to our one liter system. Let's see the new PH. Well, the PH is equal to negative log zero one, and that gives us a PH of two.