We did an osmosis experiment where we put a dialysis bag containing a solution o

Question

We did an osmosis experiment where we put a dialysis bag containing a solution of starch + sugar in a beaker containing water + iodine.

After 90 minutes, the color inside the tube went from white to grayish blue (iodine moved in).

After 90 minutes the color of the water didn't change - remained yellow.

There was a trace of glucose in the water at the end of the experiment, and the dialysis tube seemed to "puff out" - like water had entered it..

a. Why did the color inside the dialysis tube change?
b. Why did some of the glucose leave the dialysis bag?
c. Why didn't the water change color?
d. Why did the volume of wtter in the dialysis bag seem to increase?

Please help!

Explanation / Answer

Remember that in simple diffusion, things move from a region of high concentration to low concentration. a) The color inside the tube changed because iodine, which is small enough to diffuse through the partially permeable membrane (the dialysis bag), entered the bag through simple diffusion (high to low concentration) and reacted with the starch to give a blue black color. This is the classic iodine test for starch and the color change is the result of the formation of polyiodide chains from the reaction of starch and iodine. The amylose, or straight chain portion of starch, forms helices where iodine molecules assemble, forming a dark purple/black color. b) The glucose is the simplest sugar and is small enough to diffuse through the bag to a region of lower concentration, i.e the water. c) The water color did not change since the starch, which is necessary for the color to change is too large to diffuse through the pores of the partially permeable membrane. d) The water diffused from a region of high concentration (i.e the beaker) to a region of low concentration (the bag since the bag has lots of solute molecules so water concentration is relatively lower) via osmosis. Hope that this helps.

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