Assertion: The value of ionic product of water ` (K_w)` increases on increasing the temperature.
Reason: The ionisation of water increases with an increase in temperature.

Options

A. If both Assertion and Reason are CORRECT and a Reason is the CORRECT explanations of the Assertion
B. If both Assertion and Reason are CORRECT but Reaon is not CORRECT explanation of the Assertion.
C. If Assertion is CORRECT but Reasson is INCORRECT.
D. If Assertion is INCORRECT but Reason is CORRECT.

Answer

To solve the assertion-reason question regarding the ionic product of water (Kw) and its behavior with temperature, we can break it down into clear steps:

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that the value of the ionic product of water (Kw) increases with an increase in temperature.
- This is based on the principle that as temperature rises, the equilibrium shifts to favor the formation of ions.

2. **Analyzing the Reason**:
- The reason provided states that the ionization of water increases with an increase in temperature.
- This means that at higher temperatures, more water molecules dissociate into hydrogen ions (H⁺) and hydroxide ions (OH⁻).

3. **Relating Kw to Ionization**:
- The ionic product of water is defined as:
\[ K_w = [H^+][OH^-] \]
- If the ionization of water increases, the concentrations of both H⁺ and OH⁻ ions will increase.

4. **Effect of Temperature on Kw**:
- At 25°C, Kw is \(1.0 \times 10^{-14}\). As temperature increases, the value of Kw also increases.
- This is because the equilibrium constant for the ionization of water is temperature-dependent.

5. **Conclusion**:
- Since both the assertion and the reason are correct, we conclude that the assertion is true because the ionization of water does indeed increase with temperature, leading to an increase in Kw.

### Final Answer:
- Both the assertion and the reason are correct. The correct answer is A.