Question:
For the following Assertion and Reason, the correct option is:
Assertion: The $\mathrm{pH}$ of water increases with increase in temperature.
Reason: The dissociation of water into $\mathrm{H}^{+}$and $\mathrm{OH}^{-}$is an exothermic reaction.
Correct Option: , 2
Solution:
Temperature plays a significant role on $\mathrm{pH}$ measurements. As the temperature rises, molecular vibrations increase which results in greater ability of water to ionise and form more hydrogen ions.
As a result, the $\mathrm{pH}$ will drop. So assertion is incorrect. The dissociation of water molecules into ions is bond breaking and is therefore an endothermic process (energy must be absorbed to break the bonds). So reason is also incorrect.