The ${\mathrm{K}}_{\mathrm{w}}$ is a constant for the extent that pure water molecules dissociates into its component ions, protons $\left({\mathrm{H}}^{+}\left(\mathrm{aq}\right)\right)$ and hydroxide ions $\left({\mathrm{OH}}^{-}\left(\mathrm{aq}\right)\right)$. The ${\mathrm{K}}_{\mathrm{w}}$ value is the product of the hydrogen ion and hydroxide ion concentrations (or alternatively the square of the hydrogen ion concentration).

Table compares the extent that pure water molecules dissociates at different temperatures, and very precise measurements of pH values.

Explain why the pH changes in response to the dissociation of water molecules. What does pH measure?