The pH scale in water contexts

pH is logarithmic, so each unit is a tenfold change. That is why small-looking differences in a water report can matter a great deal.

Why low pH matters at the tap: Acidic water is more aggressive toward copper and lead plumbing, so pH is adjusted upward before distribution.
Why high pH matters: Chlorine disinfection is markedly less effective above about pH 7.5, because the active hypochlorous acid form gives way to the weaker hypochlorite ion.
Averaging pH: Because the scale is logarithmic, taking the arithmetic mean of pH values is strictly incorrect. It is nonetheless done routinely.

Text description

A scale from pH 0 to 14. Below 7 is acidic, 7 is neutral, above 7 is alkaline. Marked positions include battery acid near 0, lemon juice near 2, acid rain near 4, natural rainfall near 5.6, soft upland water near 6.5, pure water at 7, typical treated tap water between 7 and 8, chalk groundwater near 7.5 to 8, seawater near 8.1, and bleach near 12.