Which of the following statements is not true about crystalline solids?

Options

A. Polar molecular solids have higher enthalpies of vaporisation than those of not-polar molecular solids.
B. Graphite , though covalent solid is a good conductor of electricity.
C. Ionic solids are condcutors in molten state.
D. Non-polar molecular solids have London forces between the constituents and have higher melting points than polar moelcular solids.

Answer

To determine which statement is not true about crystalline solids, we need to analyze each statement provided in the question.

### Step-by-Step Solution:

1. **Statement 1: Polar molecular solids have higher enthalpy of vaporization than those of non-polar molecular solids.**
- **Analysis:** This statement is true. Polar molecular solids have stronger intermolecular forces (like dipole-dipole interactions) compared to non-polar molecular solids (which primarily exhibit London dispersion forces). Hence, they require more energy to vaporize.

2. **Statement 2: Graphite, though a covalent solid, is a good conductor of electricity.**
- **Analysis:** This statement is also true. Graphite has a layered structure where each layer consists of carbon atoms bonded in a hexagonal lattice. The layers can slide over each other, and the presence of delocalized electrons allows graphite to conduct electricity.

3. **Statement 3: Ionic solids are conductors in molten state.**
- **Analysis:** This statement is true. Ionic solids do not conduct electricity in solid form because their ions are fixed in place. However, when melted, the ions are free to move, allowing the molten ionic solid to conduct electricity.

4. **Statement 4: Non-polar molecular solids have London forces between constituents and have higher melting points than polar molecular solids.**
- **Analysis:** This statement is incorrect. Non-polar molecular solids do indeed have London dispersion forces, but they generally have lower melting points compared to polar molecular solids due to weaker intermolecular forces. Therefore, this statement is false.

### Conclusion:
The statement that is not true about crystalline solids is **Statement 4**.