Question about flux and changes in net flux of molecules across a membrane
Diffusion is just the random movement of particles. Over time, the solute will randomly distribute throughout the entire available volume. Under initial conditions, there is no solute in side 2 and so it cannot move to side 1 (thus flux from side 2 to side 1 is 0). At equilibrium, the net flux is 0 but there is still flux from side 1 to 2 and from side 2 to 1 (which is equal and opposite) due to the random motion of the solute. So, flux from side 2 to 1 goes from 0 at initial conditions to some non-zero value at equilibrium (ie it increases).