Laboratory 1 Diffusion and Osmosis


EXERCISE 1A: Diffusion

Materials:

dialysis tubing, 15% glucose/ 1% starch solution, I2KI solution, Beaker

Procedure:

  1. Obtain a 15 cm piece of 2.5-cm dialysis tubing that has been soaking in water. Tie off one end of the tubing to form a bag. To open the other end of the bag, rub the end between your fingers until the edges separate.
  2. Place 10 mL of the 15% glucose/1% starch solution in the bag. Tie off the other end of the bag leaving sufficient space for the expansion of the contents in the bag. Rinse the bag and record the color of the solution in Table 1.1.
  3. Test the 15% glucose/l% starch solution with glucose Testape. Record the results in Table 1.1.
  4. Fill a 100-mL beaker two-thirds full with I2KI (potassium iodide + iodine) solution and record the color of the solution in Table 1.1. Test this solution with glucose Testape and record the results in Table 1.1.
  5. Immerse the bag in the iodine solution in the beaker and record the time.
  6. Allow your setup to stand approximately 30 minutes or until you see a distinct color change in the bag or in the beaker. Record the final color of the solution in the bag and of the solution in the beaker in Table 1.1.
  7. Test the liquid in the beaker and in the bag with glucose Testape (or with Benedict's solution. Record the results in Table 1.1.

Table 1.1

solution color
Testape Results (Presence of Glucose)

Initial Contents
Initial
Final
Initial
Final

Bag

15% glucose & 1% starch

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.

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.

Beaker

H2O + IKI

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.

.

.

Analysis of Results:

  1. Which substance(s) are entering the bag and which are leaving the bag. What experimental evidence supports your answer?

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  2. Explain the results you obtained. Include the concentration differences and membrane pore size in your discussion.

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  3. Quantitative data uses numbers to measure observed changes. How could this experiment be modified so that quantitative data could be collected to show that water diffused into the dialysis bag?

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  4. Based on your observations, rank the following by relative size, beginning with the smallest: glucose molecules, water molecules, IKI molecules, membrane pores, starch molecules.

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  5. What results would you expect if the experiment started with a glucose and IKI solution inside the bag and only starch and water outside? Why?

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