On increasing the temperature of pure water

Options

A. both pH and pOH increase
B. both pH and pOH decrease
C. pH increases and pOH decreases
D. pH decreases and pOH increases

Explanation

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

### Step-by-Step Solution:

1. **Understanding Water as a Weak Electrolyte:**
- Pure water (H₂O) is a weak electrolyte, which means it does not completely dissociate into ions. It undergoes a self-ionization process:
\[ H_2O \rightleftharpoons H^+ + OH^- \]

2. **Equilibrium Constant (K_w):**
- The equilibrium constant for the self-ionization of water is denoted as \( K_w \):
\[ K_w = [H^+][OH^-] \]
- At a given temperature, \( K_w \) has a specific value. For pure water at 25°C, \( K_w = 1.0 \times 10^{-14} \).

3. **Effect of Temperature on \( K_w \):**
- When the temperature of pure water is increased, the value of \( K_w \) also increases. This is because the self-ionization of water is an endothermic process, meaning it absorbs heat.

4. **Concentration of Ions:**
- As \( K_w \) increases with temperature, the concentrations of both \( H^+ \) and \( OH^- \) ions will also increase. This can be expressed as:
\[ [H^+] = [OH^-] = \sqrt{K_w} \]

5. **Calculating pH and pOH:**
- The pH is calculated using the formula:
\[ pH = -\log[H^+] \]
- Since the concentration of \( H^+ \) increases, the pH value will decrease.
- Similarly, pOH is given by:
\[ pOH = -\log[OH^-] \]
- Since the concentration of \( OH^- \) also increases, the pOH value will decrease as well.

6. **Conclusion:**
- Therefore, when the temperature of pure water is increased, both the pH and pOH will decrease.

### Final Answer:
Both pH and pOH decrease when the temperature of pure water is increased.

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