Question

Assertion: pH of pure water increases with increase in temperature.
Reason: degree of dissociation of water increases with increases in 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 the assertion.
C) Assertion is correct but reason is incorrect.
D) Assertion is incorrect but reason is correct.

Answer 1

pH means power of hydrogen. The solution is acidic when the pH is less and the solution will be alkaline if the pH of solution is high. pH scale is from 1 to 14. Below 7 is the acidic region and above 7 is the basic region. The pH of pure water is 7.

Complete step by step answer:
-A neutral solution is that is neither acidic nor basic that means concentration of hydrogen ion is equal to hydroxide ion. And pure water is a neutral solution. But the pH depends on the temperature.Here it is explained that how pH of pure water depends on the temperature:The formation of hydrogen ions and hydroxide ions from water is an endothermic process that means it require heat or absorbs heat when dissociate. The chemical equilibrium reaction for dissociation of water is shown as;
$2{H_2}O \rightleftharpoons {H_3}{O^ + } + O{H^ - }$
${K_{eq}} = {K_w} = {[{H_3}O]^ + }{[OH]^ - } = 1.0 \times {10^{ - 14}}$ at ${25^o}{\text{C}}$
-The forward reaction absorbs heat. And according to Le Chatelier’s Principle, if there is the change in the condition of a reaction in the equilibrium, the position of equilibrium shift according to the change that has made. Therefore when we increase the temperature of this system, the equilibrium shifts to lower temperature side that means it will favor the forward direction. And therefore the concentration of hydrogen ion and hydroxide ion increases and value of Kw increases. So the temperature increases -degree of dissociation increases hence the concentration of hydrogen ion increases and therefore the pH value decreases.So, the assertion is incorrect, the pH of water decreases with increase in temperature, but the reason is correct that degree of dissociation increases with increase in temperature.
Correct option is D.
Note: when acid and base reacts they form water and salt. If strong acid and base reacts than neutral salt will form and solution will be nearly neutral, but if one of them is stronger than other than the pH of that solution depends on whether acid was strong or the base.