With increases in temperature pH of pure water

Options

A. increases
B. Decreases
C. Remains constant
D. May increase or decrease

Answer

To solve the question regarding the effect of temperature on the pH of pure water, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the pH Scale**:
- The pH scale ranges from 0 to 14, with 7 being neutral at 25°C (298 K). Below 7 is acidic, and above 7 is basic.

2. **Identify the pH of Pure Water at Room Temperature**:
- At room temperature (25°C or 298 K), the concentration of hydrogen ions \([H^+]\) and hydroxide ions \([OH^-]\) in pure water is \(1 \times 10^{-7} \, \text{mol/L}\). Therefore, the pH is calculated as:
\[ \text{pH} = -\log[H^+] = -\log(1 \times 10^{-7}) = 7 \]

3. **Effect of Temperature on Water Dissociation**:
- As the temperature increases, the dissociation of water increases. This means that more water molecules break apart into hydrogen ions and hydroxide ions.

4. **Calculate the pH at Higher Temperature**:
- For example, at 310 K, the concentration of \([H^+]\) and \([OH^-]\) increases to \(1 \times 10^{-6} \, \text{mol/L}\). The pH at this temperature can be calculated as:
\[ \text{pH} = -\log[H^+] = -\log(1 \times 10^{-6}) = 6 \]

5. **Conclusion**:
- Therefore, with an increase in temperature, the pH of pure water decreases. This is because the increase in temperature leads to greater dissociation of water, resulting in a higher concentration of hydrogen ions, which lowers the pH.

### Final Answer:
With an increase in temperature, the pH of pure water decreases due to the increased dissociation of water molecules.

---