Why does pure water have a neutral pH at 25 degrees Celsius?
Answer:
By definition, pH = -log[H+]; [H+] is the hydrogen ion concentration with units of moles/liter. When water is neutral (pure; neither acidic nor basic), the concentrations of H+ and OH- (hydroxide ion) are equal. At 25 degrees Celcius, neutral water has only a small dissociation and forms both H+ and OH- in equal amounts. The concentration of H+ at 25oC is 1.0x10-7M.
As the tempertaure increases, the amount of dissociation also increases since there is greater energy available to break the water molecules into their ions. As the [H+] increases, the pH goes down at higher temperatures. This effect is very small however, and only measurable using sensitive equipment. Also, 25oC is simple the standard temperature at which pure water is defined as having a pH = 7.
Pure water has a pH of 7 regardless of the temperature. If something is dissolved in it, then its ph changes; however, it is then no longer pure water, but a solution. This is why it is so commonly used as a base in chemistry esperiments, and also one of the reasons why life cannot exist without it.
Nmoses1977 is correct. In addition to that, though, you have to keep in mind that the pH scale, a human invention to measure acidity/alkalinity, was created that way. Water is the neutral baseline, neither acid nor alkaline, and the rest of the scale is based on acidity/alkalinity as compared to water, so it was assigned the value of zero, which is seven on the scale, in the middle. 25 degrees celsius is also the temperature baseline of the scale; the ideal temp at which you determine the pH of any substance.
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