To arrange the substances in order from highest melting point to lowest melting point, we need to consider factors such as intermolecular forces and molecular structure. Here's the list in the correct order:
1. CaCl2 (calcium chloride): This compound has strong ionic bonds between the calcium and chloride ions, resulting in a high melting point.
2. MgO (magnesium oxide): Similar to calcium chloride, magnesium oxide also has strong ionic bonds due to the attraction between magnesium and oxygen ions.
3. H2O (water): Water molecules form hydrogen bonds with each other, resulting in stronger intermolecular forces compared to the substances below. This leads to a relatively high melting point for water.
4. PF3 (phosphorus trifluoride): Although PF3 is a polar molecule, it lacks hydrogen bonding. The dipole-dipole interactions in PF3 are weaker than those in water, leading to a lower melting point.
5. CH2Cl2 (dichloromethane): CH2Cl2 has polar covalent bonds, resulting in dipole-dipole interactions. However, the strength of these interactions is weaker compared to those in PF3, resulting in a lower melting point.
6. CCl4 (carbon tetrachloride): Similar to CH2Cl2, CCl4 is a nonpolar molecule that lacks significant dipole-dipole interactions. Consequently, CCl4 has a lower melting point than CH2Cl2.
7. N2 (nitrogen gas): Nitrogen exists as diatomic molecules held together by relatively weak London dispersion forces. Consequently, N2 has the lowest melting point among the listed substances.
So, the substances arranged in order from highest melting point to lowest melting point are: CaCl2, MgO, H2O, PF3, CH2Cl2, CCl4, N2.