Animated Solution for Chemistry - Ionic Equilibrium: For the following Assertion and Reason, the correct option is Assertion (A) The pH of water increases with increase in temperature. Reason (R) The dissociation of water into ${\text{H}}^{+}$ and ${\text{OH}}^{-}$ is an exothermic reaction.

The relationship between the pH of water and temperature is one of the most fascinating and frequently tested concepts in physical chemistry. It challenges our basic intuition that "neutral always means pH 7." Let's dive deep into the thermodynamics of water to uncover the truth.

Imagine a beaker of pure water. Even without any added acid or base, water molecules are constantly colliding. Occasionally, a collision is energetic enough to break a water molecule apart into a hydrogen ion ($H^{+}$) and a hydroxide ion ($O{H}^{-}$).

Now, think about the nature of this reaction. We are breaking strong covalent O-H bonds. Bond breaking always requires an input of energy. Therefore, the autoionisation of water is an endothermic process ($\Delta H>0$).

This immediately tells us that Reason (R) is completely false. The dissociation of water is endothermic, not exothermic.

What happens when we heat this beaker of water? According to Le Chatelier's Principle, when we increase the temperature of a system at equilibrium, the system will shift in the direction that absorbs the added heat.

Since the forward reaction (dissociation) is endothermic, increasing the temperature shifts the equilibrium to the right. This means more water molecules break apart, leading to a higher concentration of both $H^{+}$ and $O{H}^{-}$ ions in the solution.

Here is where the trap lies. We know that the concentration of $H^{+}$ ions has increased. Let's look at the mathematical definition of pH:

Because of the negative sign in the logarithm, an increase in $[H^{+}]$ results in a decrease in pH. Therefore, as the temperature of water increases, its pH actually drops below 7.

This proves that Assertion (A) is also false. The pH of water decreases, not increases, with a rise in temperature.

Both the assertion and the reason are false. But there is a beautiful conceptual takeaway here: even though hot water might have a pH of 6.5 or 6.0, it is still perfectly neutral. Why? Because neutrality is not defined by pH 7; it is defined by the condition $[H^{+}]=[O{H}^{-}]$. Since every water molecule that breaks apart yields exactly one $H^{+}$ and one $O{H}^{-}$, the water remains neutral regardless of the temperature!