select the correct statement (s)

Options

A. CsCl change to NaCl structure on heating
B. NaCl changes to CsCl structure on applying pressure
C. Co-ordination number decreses on applyping pressure
D. Co-ordination number increses on heating

Answer

To solve the question of selecting the correct statements regarding the changes in coordination numbers and structures of certain compounds, we will analyze each statement step by step.

### Step-by-Step Solution:

1. **Analyze Statement A: "CHCl changes to NaCl structure on heating."**
- CHCl (Cesium Chloride) has a coordination number of 8:8 (8 chloride ions surrounding 1 cesium ion).
- Upon heating, the kinetic energy of the ions increases, which can lead to a decrease in coordination number.
- The structure of NaCl has a coordination number of 6:6 (6 chloride ions surrounding 1 sodium ion).
- Therefore, it is correct that CHCl can change to a NaCl structure upon heating due to the decrease in coordination number.
- **Conclusion:** Statement A is correct.

2. **Analyze Statement B: "NaCl changes to CaCl structure on applying pressure."**
- NaCl has a coordination number of 6:6.
- When pressure is applied, the volume decreases, leading to an increase in the number of ions per unit volume.
- This increase in density can lead to an increase in coordination number.
- CaCl has a coordination number of 8:8.
- Thus, it is true that NaCl can change to CaCl structure under high pressure due to the increase in coordination number.
- **Conclusion:** Statement B is correct.

3. **Analyze Statement C: "Coordination number decreases on applying pressure."**
- This statement contradicts the previous analysis.
- Applying pressure generally increases the coordination number as ions are forced closer together.
- **Conclusion:** Statement C is incorrect.

4. **Analyze Statement D: "Coordination number increases on heating."**
- As established in the analysis of Statement A, heating typically leads to a decrease in coordination number.
- Therefore, this statement is also incorrect.
- **Conclusion:** Statement D is incorrect.

### Final Conclusion:
The correct statements are:
- Statement A: "CHCl changes to NaCl structure on heating." (Correct)
- Statement B: "NaCl changes to CaCl structure on applying pressure." (Correct)