1. Pure Water:
* At room temperature (25°C): Pure water is neutral with a pH of 7. This is because the concentration of hydrogen ions (H+) and hydroxide ions (OH-) are equal.
* As temperature increases: The auto-ionization of water (H2O <=> H+ + OH-) increases. This means more H+ and OH- ions are formed, but the increase in both ions is equal, so the pH remains near 7. The pH of pure water slightly decreases with increasing temperature. This is because the equilibrium constant for the auto-ionization of water increases with temperature.
2. Impure Water (Solutions):
* For acidic solutions: As temperature increases, the ionization of weak acids increases. This leads to an increase in the concentration of H+ ions and a decrease in pH.
* For basic solutions: As temperature increases, the ionization of weak bases increases. This leads to an increase in the concentration of OH- ions and an increase in pH.
In summary:
* For pure water, the pH decreases slightly with increasing temperature.
* For acidic solutions, the pH decreases with increasing temperature.
* For basic solutions, the pH increases with increasing temperature.
Important Note: The effect of temperature on pH is generally small, especially for pure water. However, it can be significant for solutions containing weak acids or bases.
