The pH of "pure" water at room temperature is 7.0
(Temperature will have an effect on pH of pure water. As the temperature increases so does the dissociation of hydrogen ions from H20)
The pH scale runs from roughly 0 to 14
(acids) 0 1 2 3 4 5 6 (neutral) 7 (bases) 8 9 10 11 12 13 14
pH is defined as the 'negative value of 10log(molar concentration of [H+]aq ),
so in pure water [H+]aq = 1,0 x 10-7 = 0,000.000.1 mol/liter,
pH = -10log(molar concentration of [H+]aq ) = 10log(1,0 x 10-7 ) = 7.0
Absolutely 100% pure water will have a pH of exactly 7. However, if exposed to normal air it will dissolve carbon dioxide and form carbonic acid (which lowers the pH to around 6, give or take half a pH).
This is a lot more complicated than it would appear.
The nominal answer is 7. If you have extremely pure water, however, in contact with air, it will absorb enough CO2 to have a pH of about 6.5, and a resistivity of about 1
However, pH is a measure of the hydronium ion in water, and in extremely pure water, there is no ionization, which is why it will have resistivity about 15 million times that of regular distilled or deionized water.
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Assuming the water is pure... there will be no change - since pure water has a pH of 7.
The water is nutral so it is at Ph 7
Adding 20 mL fresh pure water to fresh pure water: the pH remain unchanged.
Pure water is assumed to be pH 7, right in the middle. It is neither an acid nor a base.
Heating pure water the pH decrease.