Select the correct statement(s)

Options

A. At high pressure, the co-ordination no, increases from 6 : 6 to 8 : 8
B. At high pressure, the co-ordination no decreases from 8 : 8 to 6 : 6
C. At high temperature, co-ordination number decreases
D. At 760 K temperature, CsCl structure changes into NaCl.

Answer

To solve the question regarding the correct statements about coordination numbers at high pressure and high temperature, we can break down the information provided in the video transcript step by step.

### Step-by-Step Solution:

1. **Understanding Coordination Number**:
- The coordination number refers to the number of atoms, ions, or molecules that a central atom or ion can bond with in a complex or crystal structure.

2. **Effect of High Pressure**:
- At high pressure, the coordination number tends to **increase**. This is because the atoms or ions are forced closer together, allowing for more neighboring atoms to be coordinated with the central atom.
- For example, in some structures, the coordination number can change from 6 to 8 when pressure is increased.

3. **Effect of High Temperature**:
- At high temperature, the kinetic energy of the molecules increases, causing them to move more vigorously. This can lead to an **increase in volume** and a **decrease in coordination number**.
- For instance, in certain cases, the coordination number may decrease from 8 to 6 as the temperature rises.

4. **Specific Example of CaCl2 and NaCl**:
- At a temperature of 760°C, the structure of CaCl2 changes to NaCl.
- In CaCl2, the coordination number is 8, while in NaCl, it is 6. This exemplifies the decrease in coordination number at high temperatures.

5. **Conclusion**:
- Based on the analysis, we can summarize the correct statements:
- At high pressure, the coordination number increases.
- At high temperature, the coordination number decreases.
- The transition from CaCl2 to NaCl at high temperature also supports this observation.

### Final Answer:
The correct statements are options A, C, and D.

---