Introduction:
The pH of pure water decreases with an increase in temperature. This phenomenon can be explained by understanding the concept of autoionization of water and the effect of temperature on this process.

Autoionization of Water:
Water has the ability to self-ionize, meaning it can undergo a chemical reaction where some of its molecules dissociate into ions. This process is known as autoionization of water. In this reaction, water molecules can donate or accept protons (H+) to or from other water molecules, resulting in the formation of hydronium ions (H3O+) and hydroxide ions (OH-).

The pH Scale:
The pH scale is a measure of the acidity or alkalinity of a solution. It ranges from 0 to 14, where 7 is considered neutral, values below 7 are acidic, and values above 7 are alkaline. The pH of a solution is determined by the concentration of hydrogen ions (H+) present in the solution.

Effect of Temperature on Autoionization:
The autoionization of water is an endothermic process, meaning it requires energy to occur. As temperature increases, the kinetic energy of water molecules also increases. This leads to an increase in the rate of molecular collisions and consequently, an increase in the rate of autoionization.

Le Chatelier's Principle:
According to Le Chatelier's principle, when a system in equilibrium is subjected to a stress, it will respond by shifting its equilibrium position to counteract the effect of the stress. In the case of the autoionization of water, increasing the temperature can be considered as a stress.

Effect of Temperature on pH:
When temperature is increased, the equilibrium of the autoionization reaction is shifted towards the right, meaning there is an increase in the concentration of hydronium ions (H3O+). As a result, the concentration of hydroxide ions (OH-) decreases, leading to a decrease in the pH of the water.

Conclusion:
The decrease in pH of pure water with an increase in temperature is a result of the increased rate of autoionization of water at higher temperatures. Understanding this phenomenon is important in various fields such as chemistry, biology, and environmental science, as it helps explain the behavior of water and its impact on chemical reactions and natural processes.