How is pH scale affected by increasing temperature?

Answer

To understand how the pH scale is affected by increasing temperature, we can break down the explanation into several steps:

### Step-by-Step Solution:

1. **Understanding Water Dissociation**:
- Water (H₂O) dissociates into hydrogen ions (H⁺) and hydroxide ions (OH⁻).
- The equilibrium for this dissociation can be represented as:
\[ H_2O \rightleftharpoons H^+ + OH^- \]

2. **Effect of Temperature on Equilibrium**:
- According to Le Chatelier's Principle, if we increase the temperature of a system at equilibrium, the system will adjust to counteract this change.
- In the case of water, increasing the temperature shifts the equilibrium to favor the formation of more H⁺ and OH⁻ ions.

3. **Increase in Ion Concentration**:
- As the temperature rises, the dissociation of water increases, leading to a higher concentration of H⁺ and OH⁻ ions in the solution.
- This means that the values of [H⁺] and [OH⁻] will increase.

4. **Understanding Kw**:
- The ion product of water (Kw) is defined as:
\[ K_w = [H^+][OH^-] \]
- With the increase in temperature, Kw also increases because both [H⁺] and [OH⁻] are increasing.

5. **Impact on pH**:
- The relationship between pH and Kw is given by:
\[ pK_w = -\log(K_w) \]
- As Kw increases, pKw decreases, which in turn affects the pH.
- At 25°C, pH + pOH = 14. However, as temperature increases, this relationship changes.

6. **Conclusion on pH Scale**:
- As the temperature increases, the pH value of pure water decreases.
- For example, at approximately 50°C, the pH can drop to around 12, indicating that the solution is more acidic compared to its state at 25°C.

### Final Answer:
As the temperature increases, the pH of water decreases due to the increased dissociation of water into H⁺ and OH⁻ ions, resulting in a higher Kw value.

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