Question

pH of water is 7 at 25°C. If water is heated to 80°C, its pH will:

• A. Decrease
• B. Remain the same
• C. \( \text{H}^+ \) concentration increases, \( \text{OH}^- \) concentration decreases
• D. Increase

Hint:

As temperature increases, the ionization of water increases, which leads to a decrease in pH.

Answer 1

The Correct Option is A

The question addresses the effect of temperature on the pH of water. To understand the answer, we need to discuss the relationship between temperature and the dissociation of water. 

1. Understanding pH and Water Ionization:
   Water dissociates into hydrogen ions (\( \text{H}^+ \)) and hydroxide ions (\( \text{OH}^- \)) as follows: 

\[\text{H}_2\text{O} \leftrightarrow \text{H}^+ + \text{OH}^-\]

1.  
   At 25°C, the concentration of both ions is \(10^{-7}\) mol/L, giving water a neutral pH of 7.
2. Effect of Temperature on Dissociation:
   Increasing temperature causes the dissociation of water to increase. As temperature rises, more \( \text{H}^+ \) and \( \text{OH}^- \) ions are produced. This is due to the fact that the process is endothermic.
3. Change in pH with Temperature:
   Because higher temperature leads to more \( \text{H}^+ \) ions, the pH of water will decrease. This does not indicate the water becomes acidic; neutral pH simply shifts to lower numerical values at higher temperatures.
4. Conclusion:
   Hence, when water is heated to 80°C from 25°C, the pH will decrease due to increased dissociation of water molecules.

Answer: Decrease

Answer 2

The pH of pure water depends on the ionization of water, which increases with temperature. At higher temperatures, the ionization of water increases, leading to a higher concentration of \( \text{H}^+ \) and \( \text{OH}^- \) ions, but since both increase equally, the pH will decrease.
Thus, the correct answer is (1) Decrease.