I am a bit confused by the definition of H+ activity. I have previously learned that activity and concentration follow each other in diluted solutions but when they get more and more concentrated the activity becomes closer and closer to 1. As an example, the activity of pure water is 1 although the molarity/molality is around 55, and another example is that precipitated salt is also counted to have an activity of 1. What confuses me is that pH values can be negative, which means that the activity can be above 1. How is that possible if the value of 1 normally means that it is a pure substance? Are there two different definitions of activity?