Which of the following is the incorrect statement?

Options

A. NaCl has 6 : 6 coordination and CsCl has 8: 8 coordination
B. In `Na_2O` each oxide ion is coordinated by `8Na^+` ions and each `Na^+` ion by 4 oxide ions.
C. NaCl structure transform to CsCl structure on heating
D. NaCl is subjected to high pressure, it transforms into CsCl.

Explanation

To determine which statement is incorrect regarding the coordination numbers and structures of NaCl and CsCl, we will analyze each statement step by step.

### Step-by-Step Solution:

1. **Understanding NaCl Structure:**
- NaCl has a face-centered cubic (FCC) structure.
- In NaCl, Na⁺ ions are located at the edge centers, and Cl⁻ ions are at the face centers.
- The coordination number of Na⁺ is 6, as it is surrounded by 6 Cl⁻ ions.
- The coordination number of Cl⁻ is also 6, as it is surrounded by 6 Na⁺ ions.
- Therefore, the statement "NaCl has 6:6 coordination" is **correct**.

2. **Understanding CsCl Structure:**
- CsCl has a simple cubic structure.
- In CsCl, Cl⁻ ions are located at the corners of the cube, and the Cs⁺ ion is at the body center.
- The coordination number of Cs⁺ is 8, as it is surrounded by 8 Cl⁻ ions.
- The coordination number of Cl⁻ is also 8, as it is surrounded by 8 Cs⁺ ions.
- Therefore, the statement "CsCl has 8:8 coordination" is **correct**.

3. **Coordination Number in Tetrahedral Voids:**
- The coordination number of an atom in tetrahedral voids is 4.
- This statement is **correct**.

4. **Transformation of NaCl to CsCl on Heating:**
- The statement claims that the structure of NaCl transforms to CsCl when heated.
- This is **incorrect**; NaCl does not transform into CsCl upon heating. Instead, NaCl retains its structure unless subjected to significant pressure.
- Therefore, this statement is the **incorrect** statement.

5. **Transformation of NaCl to CaCl₂ under High Pressure:**
- The statement claims that NaCl transforms into CaCl₂ when subjected to high pressure.
- This statement is **correct**; under high pressure, NaCl can transform into a different structure, such as CaCl₂.

### Conclusion:
The incorrect statement among the options provided is the one that claims NaCl transforms to CsCl upon heating. Thus, the answer is:

**C: NaCl structure is transformed to CsCl structure on heating.**

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