Question

2. When diffusion has reached equilibrium, we say that

Answer 1

Answer:

See Below.

Explanation:

The key word here is net. The net movement has reached zero when a system is in equilibrium but there are still motion's going back and forth due to statistics and just random brownian motion.

Think of it this way, if there are 100 people walking forwards in a crowd but 2 are moving against the crowd, the net movement is still forwards because the bulk of people are going in that direction. However, there are still 2 people moving against.

Same here, if we are talking about a diffusion, let's say in the case of osmosis, if most of the solute is moving across a membrane then we'd say its net direction is that way but that doesn't mean that there aren't processes happening in the other direction. Water molecules in osmosis mostly diffuse, chemically speaking (because you can say this biologically in a different way), from the probability of water molecules colliding with each other and passing the membrane so even if there is a net movement in a certain way their random motion can make them go to the other side just as well. If the fact that motion stops at equilibrium were the case a lot of systems, both chemical and biological, would not exist as we know it.

Think net = bulk NOT total or entire.