This is an experiment to observe diffusion across a membrane. The baggie is permeable to substances that can pass through its tiny pores and impermeable to molecules larger than the pores. Diffusion occurs when there is a concentration gradient and molecules will move from high to low concentrations. The reason iodine and starch are being used in this experiment is that iodine is an indicator and when it combines with starch, a blue-black color change occurs. In this case, if a baggie is filled with iodine solution and sealed and placed in a beaker filled with starch solution, after several minutes, a color change will occur. The colorless starch solution will turn a blue-black color inside the beaker. The iodine solution inside the baggie will still be the same original amber color it was when the iodine was placed inside. This indicates that iodine was able to diffuse across the membrane(in this case, the baggie) causing a color change to occur in the beaker with the starch solution. Iodine is small enough to diffuse through the pores in the baggie thus, using a baggie to store iodine would only make a mess.