The pH of pure water at 25 degrees Celsius is conventionally 7. At this temperature, water is considered neutral because its hydrogen-ion and hydroxide-ion activities are equal.
The pH scale describes acidity on a logarithmic scale. A pH of 7 represents neutrality, values below 7 indicate increasing acidity, and values above 7 indicate increasing alkalinity. Pure water is not chemically inactive: a tiny fraction of its molecules continuously split into hydrogen and hydroxide ions in a process called self-ionization.
The familiar value of 7 is a temperature-specific benchmark, not a universal definition of neutrality. The ionization constant of water changes with temperature, so neutral water has a pH slightly above 7 at cooler temperatures and below 7 at warmer temperatures. Dissolved salts and gases can also change a water sample’s measured pH.
A common mistake is to treat 0 and 14 as the only possible endpoints. They describe approximate values for certain concentrated solutions, while pH itself can extend beyond that range in unusual conditions. The number 7 is the standard answer for pure water at 25 °C.