• # question_answer Will $pH$ of water be same or different at $4{}^\circ C$and $25{}^\circ C$? Explain.

$pH$ of water will be different at these temperatures. Rather $pH$ of $4{}^\circ C$will be more than$pH$ at $25{}^\circ C$. Actually, for water (neutral): $[{{H}^{+}}]\times [O{{H}^{-}}]={{K}_{w}}={{10}^{-14}}$ at $25{}^\circ C$ Similarly, $pH\times pOH=p{{K}_{w}}=14$ at $25{}^\circ C$ or $pH=pOH\,=7$ ($\because$ Water is neutral and$pH=pOH$) On increasing the temperature, dissociation of water increases and so is the value of ${{K}_{w}}$. This means that at lower temperature (say $4{}^\circ C$), the value of ${{K}_{w}}$ will decrease or$p{{K}_{w}}$  will be more than 14. This also means that at lower temperature, the value of $pH$ (or$pOH$ ) will be more than 7.