After the students have completed this part of the exercise, discuss why the molecules moved faster in the warmer water. The second part of this exercise demonstrates the variation in the rate of diffusion of different molecules. For two gases at the same temperature, the average kinetic energy of the molecules is the same. Thus, from the formula given above, if the molecules of the two gases have different masses, they should be moving at different velocities, with the more massive molecules moving more slowly. If more than one trial is necessary, the tubes must be thoroughly cleaned and dried between trials.
Osmosis occurs when a membrane separates two solutions of different concentrations.
The membrane allows the solvent to pass through, but not the solute. On the side of the membrane with a lower concentration of solute, the water molecules strike the membrane and pass through it more frequently than on the side with the higher concentration of solute molecules, probably since the solute particles attract the solvent molecules and interfere with their direction of travel and because the solute molecules physically block the solvent molecules from striking and passing through the membrane. This difference in the rate of diffusion through the membrane causes the solvent to accumulate on the side of the membrane with the higher initial concentration until the solution concentrations on both sides of the membrane are equal and an equilibrium is established. Equilibrium can also occur when the pressure due to the added column of liquid in the high-concentration side is great enough to increase the flow of the solvent to the other side. This pressure is called the osmotic pressure, and it is capable of rupturing cells placed in solutions with low concentrations of solutes.