What is the pH value of pure water at 25°C?

• pH is a measure of the hydrogen ion concentration ([H⁺]) in a solution. It is defined as:

  $$H2O⇌H++OH−\text{H}_2\text{O} \rightleftharpoons \text{H}^+ + \text{OH}^-H2O⇌H++OH$$^-
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  At 25°C, the concentration of both H⁺ and OH⁻ ions in pure water is 1×10−71 \times 10^{-7}1×10−7 mol/L.

• $$pH=−log⁡[H+]\text{pH} = -\log[\text{H}^+]pH=−log[H+]$$​$$pH=−log⁡(1×10−7)=7\text{pH} = -\log(1 \times 10^{-7}) = 7pH=−log(1×10−7)=7$$​​

• Since the concentrations of H⁺ and OH⁻ are equal, the pH of pure water is:

Thus, the pH of pure water at 25°C is 7, which is considered neutral on the pH scale (ranging from 0 to 14).