Actually most water is a dilute solution of carbolic acid with a pH around 5.5 because of the dissolved CO2 in the water.

This sometimes catches people out when they use pure water to store something like DNA which will breakdown over time at this pH. It is in theory why you should use a buffer like Tris/ETDA to store DNA. I have found that storing DNA for a short time in water is normally OK, but if you want to keep the DNA for more than a month it is a good idea to use a buffer.