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# Assertion:$pH$ Of pure water is always $7.$ Reason:Pure water contains equal concentration of hydronium and hydroxyl ions at room temperature.$A.$ Both Assertion and reason are correct and Reason is the correct explanation for Assertion.$B.$ Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion.$C.$ Assertion is correct but reason is incorrect.$D.$ Assertion is incorrect Reason is correct.

Last updated date: 29th Feb 2024
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Hint: $pH$ is a measure of how acidic and basic solution is. The range of $pH$ is $0 - 14$ . The neutral solution have $pH$ equal to $7$, $pH$ of acidic solution is always less than $7$ and $pH$ of basic solution is always greater than $7.$

Complete answer:$pH$ of a solution is depend on temperature and concentration of $O{H^ - }$ and ${H^ + }$ ions i.e. if we increase the temperature of solution the solution become acidic it lower down its $pH$ less than $7$ because rise in temperature of a solution is associated with increased molecular vibrations, on increasing the temperature, the concentration of ${H^ + }$ ion also increases due to a decreased tendency to form hydrogen bonds, as a result $pH$ of a solution decrease. For example $pH$ of pure water is $7$ at ${25^ \circ }C$ and $pH$ of pure water is less than $7$ at ${60^ \circ }C.$
We also know that $pH$ of solution also depend on concentration of $O{H^ - }$ and ${H^ + }$ ions i.e. in acidic solution concentration of ${H^ + }$ ions is more than $O{H^ - }$ while in basic solution concentration of ${H^ + }$ ions is less than concentration of $O{H^ - }$ ions and in neutral solution concentration of ${H^ + }$ and $O{H^ - }$ ions are equal.
Hence, on behalf of the above discussion we can say that $pH$ of pure water is always not equal to $7$ but pure water contains equal concentration of hydronium and hydroxyl ions at room temperature.
Therefore the given Assertion is incorrect but Reason is correct.

So the correct option is $D.$

Note: If pH of the solution is less than 7,it is an acidic solution.If pH of solution is more than 7, it is basic in nature.if it is 7,then the solution is neutral ; water is an example of neutral solution.