The Visking tubing is a selectively permeable membrane that allows small molecules like water to pass through, but restricts the movement of larger molecules such as iodine. When iodine solution was added to the water in the test tube, some of the iodine molecules diffused from the test tube into the Visking tubing bag, but the larger starch molecules in the bag could not diffuse out into the test tube due to their size.
Over time, the iodine molecules that diffused into the Visking tubing bag reacted with the starch molecules present in the bag, forming a blue-black complex. This complex is insoluble in water and therefore could not diffuse back out of the bag into the test tube.
As a result, the water in the test tube remained yellow in color, indicating that there was no iodine present in the water, while the contents of the Visking tubing bag were stained black due to the presence of the blue-black complex of iodine and starch. This demonstrates the process of diffusion and the selective permeability of the Visking tubing membrane.