What is the pH of pure water?

Answer 1

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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• At 25° C the pH of pure water is very close to 7.
• Acids have a pH less than 7 while bases have a pH greater than 7.
• Because it has a pH of 7, water is considered to be neutral. It is neither an acid nor a base, but is the reference point for acids and bases

Answer 2

This pH is 7.