Hydroxonium(H3O+)=hydroxide(OH-) one acts as a strong acid and the other a strong base- the instant they come to be, we have water, which results in a neutral PH in isolation; the PH is altered, in solution, by changing relative concentrations of these ions caused by exposure to molecules such as atmospheric carbon dioxide(distilled water can absorb more CO2 and hence tends to be slightly acidic) which creates carbonic acid in solution with water. This equilibrium is also part of the fundamental nature of pure water. H2O (the molecule) is an oxide of hydrogen mathematically. Water is bound H2O plus a relatively tiny amount ofhydrogen; hydroxide ions(oxides are binary compounds of oxygen with another element). Additionally "water", under various circumstances can have oxygen dissolved at varying levels(from very low dissolved oxygen (fish breathe) in the open underwater areas of the sea to relatively high oxygen in babbling brooks and rivers) and carbon dioxide (added by contact with air)- so there are many forms of "water" but "pure water in isolation" is without these common additions. Water is in a category of it's own. There is much yet to be learned about this wonderful substance, with all of it's unique properties, including long held beliefs regarding bonds and bond strength in my opinion, but that is another discussion. The bound molecule, H2O, has been categorized as a neutral, non-metal oxide albeit with significant unique aspects as water which have not been fully explored. The molecule of H2O is existant, in our reality, in the form of water. Water is not H2O; H2O is not water. H2O, should it be isolated, is not water and does not have the properties of water. Additionally, all H2O molecules are not the same, as a small percentage of H2O molecules, in water, are deuterium, as opposed to the aneutronic hydrogen(protium), which is far more common(with the definition that "pure water" is not "heavy water"(water with a high amount of deuterium)). Both protium (most H2O) and deuterium are stable non-radioactive isotopes of hydrogen.
Water Ph, in solution, changes up or down depending upon the relative concentrations of the two ions, hydroxonium (acid) and hydroxide(base). When the acid ions outnumber the base there is an acid and when the base ions outnumber the acid there is base. That is, when there are more hydroxonium ions than hydroxide ions, we have an acid and when there is more hydroxide we have a base- pure water has an equal number of these 2 ions and hence is neutral, by default. The confusing part of the process is the value of "neutral PH" which changes as the temperature of water changes, matching the value of the PH of water at a given temperature. Water, on it's own, always maintains a perfectly neutral PH value, even upon change in temperature, albeit it water can react amphoterically- as an acid or a base when in solution with other substances. The equilibrium point of water shifts when the temperature is raised (an effort, at reattaining balance, by effectively moving against change). Thus, water responds with an opposite reaction to the change in temperature. The counterbalance, to heating, is an effort to move in the direction of cooling. Heating pure water also causes both of water's ions to increase in numbers(equally); this causes a seeming decrease in the numerical value of the PH of water. However, this doesn't cause the water to become any more acidic as temperature increases. Water, in solution, is only acidic with an excess of hydrogen ions, as i have already said above. Pure H2O, is PH neutral, even if it's PH value changes due to the shifting of it's equilibrium point by increasing temperature because the new PH value instantly becomes the neutral PH value for water at that particular temperature. As another way of looking at it, the neutral PH value of water at room temperature, PH 7, would actually be alkaline for water tested at higher temperatures. Effectively, the neutral PH value, of water, lowers, as water raises in temperature. The "real PH" effectively remains unchanged within the instantly stable equalibria of water at a given temperature. The same result is seen, but in the other direction, as the temperature of water is lowered from room temperature. The PH value of water rises as the temperature of water falls from room temperature but so to does the neutral PH value of water at the new temperature. The unaffected PH state of pure water, at any temperature, is a neutral PH- meaning that water is not acidic and not base in addition to meaning an equal number of hydroxonium and hydroxide ions.