Silver nitrate crystals don't appear to react when mixed with sodium chloride crystals, but when mixed together in water at room temperature, a reaction is observed to quickly take place. This happens because

Substance 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. when substance a and b are combined, they spontaneously mix.