What is the pH of pure water at room temperature?

The pH scale measures how many hydrogen ions are floating free in a solution, and it runs logarithmically, so each step of one means a tenfold change in concentration. Pure water is not entirely inert: a tiny fraction of its molecules split apart into hydrogen ions and hydroxide ions. Because they are produced in equal numbers, neither dominates, and the result is neutral, which works out to pH 7 at around 25 degrees Celsius. Anything below 7 has an excess of hydrogen ions and is acidic; anything above has an excess of hydroxide and is alkaline. The temperature detail matters, because the splitting increases when water is heated, so hot pure water has a pH slightly below 7 while remaining perfectly neutral, since both ion counts rise together. Rainwater is usually mildly acidic even without pollution, because carbon dioxide from the air dissolves into it and forms carbonic acid.

Human blood is held in a narrow slightly alkaline band around 7.4 by chemical buffers.