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 identifying the incorrect statement regarding coordination numbers and structural changes in solids, we will analyze each option step by step.

### Step-by-Step Solution:

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

2. **Analyzing Option A**:
- **Statement**: "At high pressure, the coordination number increases from 6:6 to 8:8."
- **Explanation**: When pressure is applied to a solid, the particles are forced closer together, leading to an increase in coordination number. Therefore, this statement is **correct**.

3. **Analyzing Option B**:
- **Statement**: "At high pressure, the coordination number decreases from 8:8 to 6:6."
- **Explanation**: This statement contradicts the principle that increasing pressure causes particles to come closer, which typically increases the coordination number. Hence, this statement is **incorrect**.

4. **Analyzing Option C**:
- **Statement**: "At high temperature, the coordination number decreases."
- **Explanation**: At high temperatures, the thermal energy causes particles to vibrate more and can lead to an increase in distance between them, resulting in a decrease in coordination number. Therefore, this statement is **correct**.

5. **Analyzing Option D**:
- **Statement**: "At 760 Kelvin temperature, CsCl structure changes into NaCl."
- **Explanation**: Cesium chloride (CsCl) can undergo a structural change to a sodium chloride (NaCl) type structure at elevated temperatures. This statement is **correct**.

### Conclusion:
- After analyzing all options, we conclude that **Option B** is the incorrect statement.

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

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