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 how the pH of pure water changes with an increase in temperature, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding pH and Kw**:
- pH is a measure of the hydrogen ion concentration in a solution. It is calculated using the formula:
\[ \text{pH} = -\log[\text{H}^+] \]
- Kw is the ion product of water, defined as:
\[ K_w = [\text{H}^+][\text{OH}^-] \]
- At 25°C, Kw is \(1.0 \times 10^{-14}\).

2. **Effect of Temperature on Kw**:
- As temperature increases, Kw also increases. This is because the dissociation of water into H⁺ and OH⁻ ions is an endothermic process. Therefore, at higher temperatures, more water molecules dissociate.

3. **Increased Concentration of H⁺**:
- When Kw increases, the concentrations of both H⁺ and OH⁻ ions in pure water increase. Since water is neutral, at any temperature, the concentrations of H⁺ and OH⁻ are equal:
\[ [\text{H}^+] = [\text{OH}^-] = \sqrt{K_w} \]

4. **Calculating pH at Increased Temperature**:
- For example, if at a higher temperature, Kw increases to \(1.0 \times 10^{-13}\), then:
\[ [\text{H}^+] = [\text{OH}^-] = \sqrt{1.0 \times 10^{-13}} = 1.0 \times 10^{-6} \text{ M} \]
- The pH can then be calculated as:
\[ \text{pH} = -\log(1.0 \times 10^{-6}) = 6 \]

5. **Conclusion**:
- As the temperature increases, the pH of pure water decreases because the concentration of H⁺ ions increases. Thus, the pH of pure water at higher temperatures is lower than 7, indicating that it becomes more acidic.

### Final Answer:
The pH of pure water decreases with an increase in temperature.

---