Question

Will the pH of water be same at $4^{o}C$ and ${25}^{o}C$? Explain.

Answer 1

We know that
$K_w=\left[H^{+}\right]\left[{OH}^{-}\right]$
At ${25}^{o}C$
$\left[H^{+}\right]=\left[{OH}^{-}\right]={10}^{-7}M$
$pH=-{\log }_{10}\left[H^{+}\right]=-{\log }_{10}\left[{10}^{-7}\right]=7$
At $4^{o}C$, ionisation of water will be less than that at ${25}^o$
$\therefore$ at ${25}^{o}C$ $K_w=\left[H^{+}\right]\left[{OH}^{-}\right]$ will be less than ${10}^{-14}$. Thus pH of water will be more than $7$ at $4^{o}C$