Diffusion in a Bag

Okay, science lovers! Here is a must-do-lab to try in your biology classroom. This one will go with your diffusion and osmosis lessons. It is perfect for the cell unit when discussing the transport of material in and out of the cell. In this experiment, a plastic bag will demonstrate the role of the cell membrane has in the cell. It is one thing for students to read about diffusion in the notes, but quite another thing for them to see it with their own eyes.

After students have learned about the cell structure and its organelles, you will want to teach them about the different types of transport systems that occur across the cell membrane (diffusion, osmosis, facilitated transport, and active transport). This iodine and cornstarch lab will help students better understand the term, semipermeable after the lab is completed.

I have a worksheet on this lab, if you want to look at it click here.

Why do I like this lab? First, because it builds critical thinking skills. Second, the term diffusion and semipermeable will make more SENSE to your kiddos after they see it in action in this lab. And third, because it’s really EASY to set up. Easy breezy.

OKAY, so once students have been introduced to the terms semipermeable, diffusion and osmosis, they are ready to try this lab. What will they need?

Materials:

1. A 500mL beaker (or another clear container about that size)

2. Sandwich baggies (Use the cheap, thin bags with the fold over top)

3. 1 tablespoon or more cornstarch

4. 50 mL water

5. 10 drops of iodine solution (For science)

6. Plastic pipette

Procedures:

1. Fill your beaker with 500ml water.

2. Next, place the plastic baggie over the top of the beaker and pour half a cup of water

to it. Then add 1-2 tbsp of cornstarch to the water in the baggie. Folding it over the

glass should prevent the starch from entering the beaker, as it needs to be kept in the

bag. *See the image below to see how the baggie is folded over the top of the beaker.

You can place the baggie over the beaker as shown below, then take it out and tie it

tightly. The main thing is you DON"T want cornstarch to enter the beaker-the

cornstarch must remain in the baggie ONLY for the entire experiment.

4. Once the cornstarch water is in the bag, remove it and tie or seal it tightly.

5. Using a pipette, add 10 large drops of iodine to the water in the beaker.

6. Place the cornstarch baggie in the iodine water solution as shown below, and let it

remain submerged for 20-30 minutes.

7. Students will record the changes as they occur.

This is NOT the finished color. Let the experiment sit overnight to show the full effect of this lab. Students will be amazed at the drastic color change.

Extensions:

1. Let the baggie sit in the iodine water overnight to show students a GREATER effect.

2.You can also ask students what they think might happen if the experiment was

reversed where you place the iodine water solution in the baggie and the cornstarch

and water in the beaker. Have students give you their predictions beforehand, then

have them try it out for themselves, or you can do a demonstration for them. Then later

they will see if their prediction was correct or not. Have them EXPLAIN the results to

you, using what they learned about diffusion.