Which type of force determines the physical properties of molecules?

• A. Intramolecular
• B. Covalent
• C. Intermolecular
• D. Ionic

Answer: (C)

Intermolecular forces are the attractions between separate molecules, and they largely determine a substance’s physical properties—melting and boiling points, vapor pressure, solubility, and phase behavior—because they govern how strongly molecules stick to each other in a bulk sample. Inside a molecule, covalent (intramolecular) bonds hold the atoms together and set chemical structure and reactivity, not the bulk physical properties. Ionic interactions are strong forces seen in salts between ions, which also aren’t the general basis for the physical properties of typical molecular substances. Thus, the forces between molecules best explain why the physical properties vary from one substance to another.