Which one of the following statements is incorrect ?

Options

A. Crystalline solids have sharp melting and boiling points
B. FCC possesses closest packing of atoms in it
C. Increase in pressure in ionic solids may decrease coordination number
D. NaCl has 6:6 coordination number

Answer

To determine which of the statements is incorrect, let's analyze each statement one by one:

### Step 1: Analyze the First Statement
**Statement:** Crystalline solids have sharp melting and boiling points.

**Analysis:** This statement is correct. Crystalline solids have a well-defined structure, which leads to sharp melting and boiling points. For example, ice (a crystalline solid) melts at exactly 0°C, while amorphous solids like wax melt over a range of temperatures.

### Step 2: Analyze the Second Statement
**Statement:** FCC possesses the closest packing of atoms in it.

**Analysis:** This statement is also correct. The Face-Centered Cubic (FCC) structure has a packing efficiency of 74%, which is higher than that of other structures like Body-Centered Cubic (BCC) with 68%. Therefore, FCC is indeed one of the closest packing arrangements.

### Step 3: Analyze the Third Statement
**Statement:** An increase in pressure on ionic solids may decrease the coordination number.

**Analysis:** This statement is incorrect. When pressure is increased, the ions in an ionic solid are forced closer together, which typically leads to an increase in the coordination number. Therefore, this statement is the one that is incorrect.

### Step 4: Analyze the Fourth Statement
**Statement:** NaCl has a 6:6 coordination number.

**Analysis:** This statement is correct. In the NaCl crystal structure, each Na⁺ ion is surrounded by six Cl⁻ ions, and each Cl⁻ ion is surrounded by six Na⁺ ions, resulting in a 6:6 coordination ratio.

### Conclusion
After analyzing all the statements, the incorrect statement is the third one: "An increase in pressure on ionic solids may decrease the coordination number."

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