What is the pH value of pure water?

The pH value of pure water is 7, which represents the neutral point on the pH scale. This value is derived from the autoionization of water, a process where water molecules spontaneously dissociate into hydronium (H3O+) and hydroxide (OH-) ions [1]. At a standard temperature of 25°C, the ion-product constant of water (Kw) is 1.0 × 10^-14 [1]. In pure water, the concentrations of hydrogen ions [H+] and hydroxide ions [OH-] are equal, each measuring 1.0 × 10^-7 M. Since pH is defined as the negative logarithm of the hydrogen ion concentration (pH = -log[H+]), the calculation -log(1.0 × 10^-7) results in a pH of 7.00 [1]. While temperature changes can slightly shift the Kw and resulting pH, 7 remains the standard reference for neutrality in pure water.

Sources

1. [1] https://www.khanacademy.org/science/biology/water-acids-and-bases/acids-bases-and-ph/a/acids-bases-ph-and-bufffers