What characterizes equimolar counter diffusion?

Equimolar counter diffusion refers to a process where, in a given system, the number of moles of one species diffusing in is equal to the number of moles of another species diffusing out, maintaining a constant overall balance of species. This characteristic ensures that as moles of substance A leave a region, an equivalent number of moles of substance B enter that same region.

In this context, the answer correctly identifies that moles of A diffusing out are replaced by moles of B diffusing in. This replacement is crucial for maintaining equimolarity in the diffusion process. The underlying principle is driven by concentration gradients as A and B seek to achieve equilibrium; thus, the fluxes of both species adjust to ensure that the system remains in equilibrium while allowing continuous transfer of materials.

This process can be contrasted with options suggesting unidirectional fluxes or unequal diffusion rates, which would indicate a lack of equilibrium, countering the fundamental idea of equimolar counter diffusion.