Why is the pH of pure water 6.5? Pure water is suppose to have no acid or base added, and theoretically the pH should be 7. Pure water breaks down by itself into a small amounts of hydrogen cations and hydroxide anions (see figure below glass), one ion of each for every ten million (10,000,000) molecules of water, and the number of hydrogen and hydroxyl ions are equal and balanced, in other words, pH balanced to pH 7. So where does the pH 6.5 come from? Pure water upon standing absorbs carbon dioxide creating carbonic acid. Pure water has a pH of 6.5, because it contains some carbonic acid. https://en.wikipedia.org/wiki/Self-ionization_of_water