Question

The $pH$ value of pure water at $300K$ is :
A. exactly $7$
B. slightly $ > 7$
C. zero
D. slightly $ < 7$

Answer 1

Hint: We know that the concept of $pH$ is used to determine whether a solution is acidic or basic in nature , An acidic solution has more concentration of $[{H^ + }]$ and a basic solution has more concentration of $[O{H^ - }]$. The range of $pH$ of an acidic solution is less than $7$ and the $pH$ of a basic solution is greater than $7$ and less than $14$. The fact is very important to note that the $pH$ of pure water at $298K$ is $7$.

Complete step by step answer:
We know that water can be dissociated into hydrogen ions and hydroxide ions . That is ${H_2}O \rightleftarrows {H^ + } + O{H^ - }$. So when we increase the temperature of the pure water from $298K$ to $300K$, the degree of dissociation of the pure water will also increase. With the increase in degree of dissociation, the concentration of hydrogen ions also increases. That means there will be a slight increase in $[{H^ + }]$ at $300K$ as compared to the $298K$.Now we know that the $pH$ of a solution is given by $pH = - \log [{H^ + }]$. At $298K$ the $pH$ of water was $7$. So the $pH$ at $300K$ will be slightly less than $7$, as there is a slight increase in $[{H^ + }]$ , due to the slight increase in temperature.

Hence the correct option is D. slightly $ < 7$ .

Additional information:
  The $pH$ of rain water is between $5$ and $5.5$ . The $pH$of the drinking water should be in the range of $6.5 - 8.5$. A slight change in the $pH$of the water we consume can be very harmful for our human body.

Note:
Always remember that the $pH$ of pure water at $298K$ is $7$. When we increase the temperature , the degree of dissociation also increases. The $pH$ of a solution is given by $pH = - \log [{H^ + }]$. The $pH$is just a number and it does not have any unit.