London dispersion forces as the sole intermolecular interaction

Steps:

• Identify intermolecular forces: temporary dipole-induced (London dispersion) for nonpolar molecules; permanent dipole-dipole for polar; hydrogen bonding for molecules with H bonded to N, O, or F.
• Examine bromine (Br₂): symmetric diatomic, nonpolar, no permanent dipole.
• Check others: hydrogen chloride (HCl) polar covalent; ethanol (C₂H₅OH) has O-H; water (H₂O) has O-H.
• Conclude only bromine relies solely on temporary dipoles.

Why A is correct:

• Bromine (Br₂) is nonpolar, so intermolecular forces are exclusively London dispersion forces from temporary dipoles, as defined for nonpolar substances.

Why the others are wrong:

• B: HCl is polar, with permanent dipole-dipole forces in addition to dispersion.
• C: Ethanol has O-H bonds, enabling hydrogen bonding beyond temporary dipoles.
• D: Water has O-H bonds, leading to strong hydrogen bonding, not just temporary dipoles.

Final answer: A