Passage VII

The net movement of H₂O molecules across a semipermeable membrane from a region of higher H₂O concentration to a region of lower H₂O concentration is called osmosis. A semipermeable membrane, such as dialysis tubing, is a barrier with tiny pores that allow only molecules under a certain size, such as H₂O molecules, to pass through. Other small molecules can also migrate from higher concentration regions to lower concentration regions.

A dilute starch solution and a dilute iodińe (I₂) solution were each prepared in H₂O. The I₂, solution had a higher concentration of dissolved particles than did the starch solution. Therefore, the starch solution had a higher concentration of H₂O than did the I₂ solution. Molecules of I₂ are larger than H₂O molecules, but are much smaller than starch molecules. A starch solution is milky-white. An I₂,solution is reddish-brown. Mixing starch and I₂ solutions will result in a blue solution.

Experiment 1

A scaled bag made of dialysis tubing (a dialysis bag) containing the I₂ solution was submerged in a beaker containing the starch solution. After 1 hr, the bag had swollen from its original size. The solution around the outside of the bag was blue, but no blue coloration was evident within the bag.

Experiment 2

A dialysis bag containing the starch solution was submerged in a beaker containing the I₂ solution. After 1 hr, the bag had shrunk. The solution inside the bag was blue, while the appearance of the solution outside the bag did not change.

Experiment 3

A dialysis bag containing the I₂ solution was submerged in a beaker containing distilled H₂O. After 1 hr, the bag was swollen, and the solutions both inside the bag and outside the bag were reddish brown.