Thursday, March 26, 2009

Biology ~ Experiment of Visking tubing

Appendix ~ Experiment 3.1

The movement of substances across a semi- permeable membrane

Title : The movement of substances across a semi permeable membrane

Objective : To study the movement of substances across a semi permeable membrane

Problem statement : What factor influences the diffusion of substances through a semi permeable membrane?

Hypothesis : The diffusion of molecules across a semi permeable membrane is based on the size of the molecule


a) Manipulated : Size of solute molecules

b) Responding : Colour of solution

c) Fixed : Temperature and time

Materials : Iodine solution, Benedict’s solution, 1% starch suspension, 30% glucose solution and distilled water

Apparatus : A Visking tube, cotton thread, test tubes, beaker and water bath


- Soak the Visking tube in water for 5 minutes to soften it. Open the Visking tube and tie one end of the tube with cotton thread to prevent leakage.

- Fill the Visking tube with 15ml of glucose solution and 15ml of starch suspension. Tie the other end of tube tightly with cotton thread. Record the colour of the solution

- Rinse the outer surface of Visking tube with distilled water.

- Mix 400ml of distilled water with 15ml iodine solution in a beaker. Record the coilour of the solution

- Place the Visking tube in the beaker and leave it for 40 minutes.

- After 40 minutes, take the Visking tube out and put aside

- Observe and record the colour of solution in the Visking tube and in the beaker.

- Test both solutions for the presence of glucose using Benedict’s solution. Pour 2ml of each solution into separate test tube and add 1ml of Benedict’s solution. Heat the mixture in the water bath for 5 minutes and record the colour change.

Result :

Solution in




Benedict’s test

Initial colour

Final colour

Visking tube

15 ml sucrose solution + 15 ml starch solution

White precipitate

Blue black
( positive for starch)

Brick red colour

(positive for sucrose)


Distilled water +

20 drops iodine solution


Yellow (negative for starch)

Brick red precipitate (positive for sucrose)


  1. Based on the results, make an inference regarding the size of the iodine molecules as compared to the size of the starch molecules.
    - comment on size of iodine compared to starch molecules
    - then relate the size of iodine to the ability of iodine to diffuse through Visking tubing

  2. What are inference can you make regarding the size of the starch molecules when compared to the size of the pores on the Visking tube?
    comment on size of starch compared to iodine molecules
    - then relate the size of starch to the ability of starch to diffuse through Visking tubing

  3. What do the results of Benedict’s test show? What inference can you make from the result?
    - Benedict test is use to test the presence of glucose (sucrose) in a solution.
    - 2ml of solution which we want to test is pour in a test tube, then add several drops of benedict's solution before heating the test tube for about 5 minutes.
    - If the color of solution change into brick red precipitate at the end, that show that positive for glucose test ( means glucose is present)

  4. Predict the outcome of the result if the experiment is repeated by filling the Visking tube with glucose solution and the beaker with starch suspension?
    - What will happen if we change the solution for this experiment?
    - Glucose solution will diffuse from region to what region? What will be happen in iodine test and Benedict's test?

  5. Based on the experiment, what analogy can you make between the Visking tube and the digestive system?

- we know that digestive system function in absorbing nutrients from foor digestion.

- So, both only allow certain molecules to pass through depend upon the size


  1. Comment on your hypothesis. Is your hypothesis accepted or rejected?


  1. sucrose is a non-reducing sugar. without doing the test on non-reducing sugar benedict's test, how did you obtain brick red precipitate?

  2. Iodine is an indicator. Why would you need up to 15ml?? A few drops would be enough.