What is the ph of pure water?

Pure water at 25°C has a pH of 7, which is neutral on the pH scale, meaning it is neither acidic nor basic. This value arises because the concentration of hydrogen ions (H⁺) in pure water is 10⁻⁷ moles per liter, and pH is defined as the negative logarithm of this concentration. However, this neutral pH value is temperature-dependent: at 0°C, the pH of pure water is slightly above 7 (around 7.47), while at 100°C, it drops to about 6.14, but the water remains neutral at each temperature because the concentrations of H⁺ and OH⁻ ions stay equal.

In practice, pure water exposed to air quickly absorbs carbon dioxide, forming carbonic acid and lowering the pH to around 5.2–5.8, which is why laboratory measurements of ultrapure water must be done under controlled, air-free conditions to obtain the theoretical pH of 7.

The pH of pure water is not a fixed number but a function of temperature and exposure to the atmosphere.