ubstance A is a nonpolar liquid and has only dispersion forces among its constituent particles. Substance B is also a nonpolar liquid and has about the same magnitude of dispersion forces among its constituent particles as substance A. When substance A and substance B are combined, they spontaneously mix. Why

Their particles exhibit the same type of intermolecular interaction

Explanation:

In chemistry, we commonly say that 'like dissolves like'. This implies that polar solvents dissolves polar solutes while nonpolar solvents dissolve nonpolar solutes.

This phenomenon of 'like dissolves like' is possible because, the dissolution of one substance in another involves intermolecular interaction between the solute and solvent molecules.

If the molecules of solute and solvent are both nonpolar and have about the same magnitude of intermolecular (dispersion) forces, interaction between the both molecules is significant hence the solute dissolves completely in the solvent.