At 90 °C, pure water has $\left[{\mathrm{H}}_3{\mathrm{O}}^{+}\right]={10}^{-6}\mathrm{mol} {\mathrm{dm}}^{-3}$ The value of $\mathrm{p}{K}_{\mathrm{w}}$ at 90°C is

For pure water, $\left[{\mathrm{H}}_3{\mathrm{O}}^{+}\right]=\left[{\mathrm{OH}}^{-}\right]$

$K_{\mathrm{w}}=\left[{\mathrm{H}}_3{\mathrm{O}}^{+}\right]\left[{\mathrm{OH}}^{-}\right]={10}^{-12}{\left(\mathrm{mol} {\mathrm{dm}}^{-3}\right)}^2$;  $\mathrm{p}{K}_{\mathrm{w}}^{\circ }=-\log K_{\mathrm{w}}^{\circ }=12$

Deionized or distilled water are terms used to describe pure water. All of the dissolved components that were combined with the water have been evaporated out of distilled water. Water evaporates, distils, or evaporates while leaving salt behind.