Whether osmosis occurred in a dialysis bag lab is determined by measuring the change in mass (weight) of the dialysis bag before and after it sits in the solution.

Because osmosis is the net movement of water into or out of the bag, water gain makes the bag’s mass increase, and water loss makes its mass decrease. Students typically record the initial mass of each bag, let the bags sit in their respective solutions for a set time, then dry and reweigh them to calculate the percent change in mass; this value shows both that osmosis occurred and its direction (into or out of the bag).