Answer:
Molecules don’t have to collide with each other in order to experience intermolecular forces.
Molecules with permanent dipoles, held in fixed orientation, such as water molecules in the solid state (ice), interact with a potential proportional to the inverse cube of distance. This intermolecular force always exists, no matter how far apart the molecules are; every water molecule interacts with every other water molecule. However, because it diminishes rapidly with distance, it tends to only be important at short distances. (In contrast, the potential between two ions goes as 1/r, and that between an ion and a dipole goes as 1/r^2.)
The van der Waals forces, of which London dispersion forces are the most important, are described by a potential proportional to the inverse sixth power of distance. Theoretically, those forces are always operating between any two atoms or molecules, but again, only become important at short distances.
Explanation:
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