Select the incorrect statement:

Options

A. At high pressure, the co-ordination number in- creases from `6:6` to `8:8`
B. At high pressure, the co-ordination number de- creases from `8:8` to `6:6`
C. At high temperature, co-ordination number de- creases
D. At 760 K temperature, CsCl structure changes into NaCl.

Answer

To solve the question of selecting the incorrect statement regarding coordination numbers under varying conditions, we will analyze each statement step by step.

### Step-by-Step Solution:

1. **Understanding Coordination Number**:
- The coordination number refers to the number of nearest neighboring atoms or ions surrounding a central atom in a crystal lattice.

2. **Analyzing Statement 1**:
- **Statement**: "At high pressure, the coordination number increases from 6 to 8."
- **Analysis**: When pressure increases, atoms are forced closer together, leading to a higher coordination number. This statement is correct.

3. **Analyzing Statement 2**:
- **Statement**: "At high pressure, the coordination number decreases from 8 to 6."
- **Analysis**: This contradicts the first statement. Under high pressure, the coordination number does not decrease; it actually increases. Therefore, this statement is incorrect.

4. **Analyzing Statement 3**:
- **Statement**: "At high temperature, the coordination number decreases from 8 to 6."
- **Analysis**: As temperature increases, particles gain energy and move more vigorously, which can lead to a decrease in the coordination number as they move further apart. This statement is correct.

5. **Analyzing Statement 4**:
- **Statement**: "At 760 Kelvin, the temperature of CsCl changes to NaCl structure."
- **Analysis**: CsCl has a coordination number of 8, while NaCl has a coordination number of 6. As temperature increases, the coordination number can decrease, which aligns with the behavior of the structures. This statement is also correct.

6. **Conclusion**:
- The incorrect statement among the options provided is **Statement 2**: "At high pressure, the coordination number decreases from 8 to 6."

### Final Answer:
The incorrect statement is: **"At high pressure, the coordination number decreases from 8 to 6."**