Which condition must be met for dipole-dipole forces to exist between two molecules?

• A. Both molecules must be polar.
• B. Both molecules must be nonpolar.
• C. Both molecules must be ionic.
• D. At least one molecule must contain hydrogen bonded to nitrogen, oxygen, or fluorine.

Answer: (A)

Verified step by step guidance

1. Understand that dipole-dipole forces occur due to the attraction between the positive end of one polar molecule and the negative end of another polar molecule.
2. Recall that polarity in a molecule arises when there is an uneven distribution of electron density, resulting in a permanent dipole moment.
3. Recognize that for dipole-dipole interactions to exist, both molecules involved must have permanent dipoles, meaning both must be polar.
4. Note that nonpolar molecules do not have permanent dipoles, so dipole-dipole forces cannot exist between two nonpolar molecules.
5. Distinguish dipole-dipole forces from hydrogen bonding (which requires hydrogen bonded to N, O, or F) and ionic interactions (which involve ions, not neutral molecules).