Osmosis and Diffusion with Dialysis Tubing

Dialysis tubing is filled with a starch-glucose solution and placed in iodine solution. The starch remains inside, glucose diffuses out, and iodine diffuses in, showing that small molecules can cross the semi-permeable membrane while larger molecules cannot.

1. Soak 15 cm lengths of dialysis tubing in water for at least 15 minutes.
2. Prepare 2% starch solution (2 g in 100 mL boiling water) and 30% glucose solution (30 g in 100 mL water).
3. Combine equal volumes of starch and glucose solutions to make a 1% starch and 15% glucose mixture.
4. Measure and record initial glucose concentration of the mixture using glucose test strips (after diluting 1 mL sample into 9 mL water).
5. Prepare iodine solution by adding 1 mL iodine/potassium iodide (I₂/KI) solution to 100 mL water in a beaker. Test for glucose; record results.
6. Tie one end of dialysis tubing securely. Half-fill with the starch–glucose mixture using a pipette, then tie the other end to make a sealed pouch.
7. Place the tubing pouch in the iodine solution and leave for ~15 minutes.
8. Observe color changes: note whether the tubing solution or surrounding solution changes color.
9. Measure glucose concentration in the iodine solution after 15 minutes with a test strip.
10. Carefully empty the tubing into a beaker, dilute 1 mL into 9 mL water, and test for glucose again. Record results.
11. Compare initial and final glucose levels inside and outside the “cell” pouch.

Dialysis Tubing Experiment - Bright Biology:

Osmotic Pressure Dialysis Tubing - North Carolina School of Science and Mathematics:

📄 Osmosis and Diffusion - Southern Biological: https://www.southernbiological.com/osmosis-and-diffusion/?srsltid=AfmBOorvzo8tSvppswkB0ZCZqcCD31m4W6TPRhMDXG5X5SjZsbn1OROl

• Use a pouch made of parchment paper instead of the dialysis tubing, and fill with water and food colouring.
• Leave a pouch overnight to show longer-term diffusion of glucose toward equilibrium.
• Repeat with different solutes to test permeability (for example, sucrose vs glucose).

• Wear gloves, goggles, and a lab coat; iodine stains skin and clothing.
• Handle glassware carefully to prevent spills and breakage.
• Avoid direct contact with iodine solution and wash hands thoroughly after the experiment.
• Dispose of chemical solutions as instructed by your teacher.

• Why does the solution inside the tubing turn blue? (Iodine passes in and reacts with starch to form a starch–iodine complex.)
• Why doesn’t starch diffuse out of the tubing? (Starch molecules are too large to pass through the membrane pores.)
• Why is glucose detected outside the tubing after the experiment? (Glucose molecules are small enough to diffuse through the membrane.)
• What does this experiment demonstrate about cell membranes? (They are selectively permeable—some molecules pass freely, others cannot.)
• How does molecular size affect diffusion across membranes? (Smaller molecules diffuse more easily, while larger ones are restricted.)