arrange these seven substances in order from highest melting point to lowest melting point.

H2O, MgO, CCl4, CH2Cl2, CaCl2, N2, PF3

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.

To arrange these substances in order from highest melting point to lowest melting point, you need to consider the nature of the substances and their intermolecular forces. The strength of intermolecular forces determines the level of difficulty in breaking the bonds between particles, which ultimately affects the melting point.

Here's a step-by-step guide to arranging the substances in order of their melting points:

1. Categorize the substances:
- H2O and MgO are ionic compounds.
- CCl4, CH2Cl2, CaCl2, and PF3 are covalent compounds.
- N2 is a diatomic covalent molecule.

2. Consider the type of bonding:
- Ionic compounds (H2O and MgO) have strong electrostatic forces between oppositely charged ions.
- Covalent compounds have weaker intermolecular forces. Depending on the type, these forces can include London dispersion forces, dipole-dipole interactions, and hydrogen bonding.

3. Review the factors affecting melting points:
- Ionic compounds tend to have high melting points due to the strong attraction between ions.
- Covalent compounds' melting points are generally lower than those of ionic compounds since intermolecular forces are weaker. However, compounds with hydrogen bonding tend to have higher melting points.

4. Compare substances and order them accordingly:
Based on the information above, we can determine the order from highest melting point to lowest as follows:

MgO > CaCl2 > H2O > CH2Cl2 > PF3 > CCl4 > N2

Explanation:
- MgO is an ionic compound and has the highest melting point due to the strong attraction between magnesium and oxygen ions.
- CaCl2 is also an ionic compound, but it has a slightly lower melting point than MgO.
- H2O has hydrogen bonding, making it higher in melting point than covalent compounds without hydrogen bonding.
- CH2Cl2, PF3, and CCl4 are covalent compounds. They have weaker intermolecular forces, so they have lower melting points than H2O and the ionic compounds.
- N2, being a diatomic covalent molecule without any significant intermolecular forces, has the lowest melting point of all the substances.

By following these steps and considering the different factors, we can successfully arrange the substances in order of their melting points.