Question

The pH of an aqueous solution of ${\text{NaCl}}$ at $85^\circ {\text{C}}$ should be:
A.7
B.>7
C.<7
D.0

Answer 1

Hint: pH can be measured on the pH scale from 0(highly acidic) to 14(highly basic). Higher the hydrogen ion concentration of a substance, lower is the pH value.
For a neutral solution, pH value is 7. If the pH value is less than 7, it represents an acidic solution and if it is higher than 7, it represents a basic solution.

Complete step by step answer:
Sodium chloride is a salt of a strong acid which is hydrochloric acid and a strong base which is sodium hydroxide and is neutral in nature. The hydrolysis of a salt of a strong acid and a strong base is not possible.
Since the salt is neutral in nature, its pH value should be 7. But, the temperature is more than $25^\circ {\text{C}}$ .
Now, at $25^\circ {\text{C}}$ , the value of the ionic product of water is ${10^{ - 14}}$ . The ionic product of water is $\begin{gathered}
  {{\text{K}}_{\text{w}}}{\text{ = }}\left[ {{{\text{H}}^{\text{ + }}}} \right]\left[ {{\text{O}}{{\text{H}}^{\text{ - }}}} \right] \\
   \Rightarrow {10^{ - 14}}{\text{ = }}\left[ {{{\text{H}}^{\text{ + }}}} \right]\left[ {{\text{O}}{{\text{H}}^{\text{ - }}}} \right] \\
\end{gathered} $
At $25^\circ {\text{C}}$ , $\left[ {{{\text{H}}^{\text{ + }}}} \right] = \left[ {{\text{O}}{{\text{H}}^{\text{ - }}}} \right]$ .
So, $\left[ {{{\text{H}}^{\text{ + }}}} \right] = {10^{ - 7}}$ . Thus, pH is 7 at $25^\circ {\text{C}}$ .
Since the degree of ionization is proportional to temperature, the value of the ionic product of water increases with increase in temperature. So, the ionic product will increase at a temperature more than $25^\circ {\text{C}}$ .
Therefore, at $85^\circ {\text{C}}$ , the ionic product will increase and so the hydrogen ion concentration will increase and thus, the pH will become less than 7.

So, the correct option is C.

Note:
pH has a great importance in biochemical reactions such as digestion of food. Human blood has a pH of $7.4$ . If it changes by $0.2$ pH units, then death results.
pH also has a great importance in agriculture. Soil is tested to determine whether acidic or basic fertilizer is required for a particular crop to be grown in that soil.