The diagram shows an apparatus used to model the movement of substances between blood and body cells. A dialysis tube containing starch solution was tied at...
Question 1 Report
The diagram shows an apparatus used to model the movement of substances between blood and body cells. A dialysis tube containing starch solution was tied at both ends and placed in iodine solution. The wall of the tube acts as a partially permeable membrane.
After 20 minutes, the liquid inside the tube was blue-black. The iodine solution outside the tube remained orange-brown.
(a) Name the substance that moved into the tube. [1]
(b) Explain why this substance moved into the tube. [3]
(c) State the colour change that would show starch is present. [1]
(d) Describe why starch did not move out of the tube. [2]
(e) Complete the table by giving one variable that should be kept the same when this apparatus is repeated with different concentrations of iodine solution. [2]
| Variable | How it is controlled |
|---|---|
| ________________ | Use the same value in every tube |
Answer Details
(a) The substance that moved into the tube was iodine. [1]
(b) This tests diffusion. Iodine particles were at a higher concentration outside the dialysis tube than inside it. They therefore moved from high to low concentration, through the partially permeable tube wall. This net movement is diffusion. [3]
(c) Starch is identified by the colour change orange-brown to blue-black when iodine is present. [1]
(d) Starch did not leave because starch molecules are too large to pass through the pores in the partially permeable membrane. [2]
(e) One valid control is the volume of starch solution, controlled by using the same volume in every tube. Other valid paired answers include keeping temperature constant or leaving every tube for the same time. [2]
Exam reminder: Diffusion is movement of particles down a concentration gradient; a partially permeable membrane allows only sufficiently small particles through.
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