Final answer:
Ionic substances generally have higher melting and boiling points than covalent substances due to their strong electrostatic interactions. Covalent compounds display lower melting and boiling points because they are held together by weaker intermolecular forces.
Step-by-step explanation:
The substances with the highest melting points tend to be ionic compounds because they are held together by very strong electrostatic interactions, translating to high enthalpies of fusion. Compared to covalent compounds, which have a wide range of physical properties, ionic compounds usually have significantly higher melting and boiling points. For example, sodium chloride has a melting point of 801°C and a boiling point of 1413°C, whereas water, a molecular compound, melts at 0°C and boils at 100°C. Ionic substances also tend to have substantial boiling points due to the strong intermolecular interactions that must be overcome to go from a liquid state to a gas, leading to high enthalpies of vaporization.
Moreover, when we compare ionic and covalent substances, we find that covalent compounds often have softer structures with lower melting and boiling points. They are also generally more flammable, show varying degrees of water solubility, and do not conduct electricity. These differences are due to the weaker intermolecular forces in covalent compounds compared to the strong ionic bonds in ionic compounds.