In this experiment we will be observing the the movement of molecules through a semi permeable membrane. Students will be able to observe how some molecules (starch) are too large to pass through a membrane, while smaller molecules (iodine) can freely move.
Although this is a simple experiment it is effective in illustrating movement across membranes.
Materials
Dialyses tubing
Scissors
Beakers
Iodine
Starch
Paper clip
Method
When doing this experiment, you can let the kids decide how to approach it. They may choose to place the iodine in the dialyses tubing and starch in the beaker or Vise Versa.
In this experiment I will be doing starch in the Dialyses tubing and Iodine in the beaker.
Cut a piece of Dialyses tubing off
Place the Dialyses tube in water and open it.
Tie off one end of the tubing and poor starch solution into the tube.
Tie off the top of the dialyses tube. The tube should not be leaking
Place the tube in a Iodine and water solution. You can hold the tube up using the paper clips.
Leave the beaker for a few minutes.
The Iodine should being to diffuse into the starch, turning the solution from white to dark purple.
Science behind the experiment
The terms you need to understand are:
Diffusion
Semi-permeable membrane
Concentration gradient.
So what happened?
The Dialysis tubing provides a semi-permeable membrane. Only allowing smaller molecules to pass through it. Iodine molecules are small enough to pass freely through the membrane, however starch molecules are complex and too large to pass through the membrane.
Initially there was a higher concentration of iodine outside than inside the tube. Thus iodine diffused into the tube with the starch. This turned the starch a darker colour.
