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Activity 5 - Journal
Diffusion Across a Semi-permeable Membrane
In this experiment you will see how substances can diffuse across the cell
membrane and what determines if a substance gets through or not. Iodine is
an indicator of starch. In other words, when iodine comes into contact with
starch, it will turn the starch a new color.
1. Make predictions/hypotheses. Iodine (which by itself is brown in color) reacts with starch to
form colors that range from pink to black.
- Will any color changes occur either in the iodine solution or in the starch inside the
plastic bag? What do you predict will happen and why?
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Will there be a difference in the results between the solution with just 10 drops of
iodine and the one with 25 or 50 drops? If so, what will the difference be?
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2. Fill your beaker or plastic cup with about 1 cup (240mL) of water.
3. Place a spoonful of starch into your plastic bag. Make sure not to spill any of the starch on the
outside of the bag. Put 50mL of water in the plastic bag. Try to get as much air out of the bag as
possible without spilling any of the solution and then tie the bag so it will not leak. Be careful
not to put too much pressure on the bag or it might tear.
4. Using the eye dropper, place the number of drops of iodine into the water as directed by your
teacher (10, 25, or 50).
5. Place the bag with the starch solution into the beaker, making sure that the starch solution is
covered by the iodine mixture.
Diffusion Across a Semi-permeable Membrane
Activity 5 Worksheet
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Name___________________________________ Date_____________________
6. Watch the solution for 10 minutes and record what happens.
Beaker 1 (10 drops I-)
Questions
Beaker 2 (25 drops I-)
Beaker 3 (50 drops I-)
At what time did
the starch
solution start to
change color?
(Put “never” if it
does not change
color)
What was the
final color of the
starch solution
after 10
minutes? (Give
the number)
What percent of
the starch
solution turned
color?
What was the
final color of the
iodine solution
after 10 minutes
compared to its
initial color?
1
white
2
pink
3
4
5
purple
6
7
black
7. Discussion Questions
-
Did the starch solution change color in any of the beakers? Why?
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Did the iodine solution change color in any of the beakers? Why?
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Diffusion Across a Semi-permeable Membrane
Activity 5 Worksheet
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Circle which substance(s) the plastic bag is permeable to?
IODINE
-
STARCH
WATER
How is the plastic bag like the cell membrane?
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How is the plastic bag different than the cell membrane in relation to movement of
substances?
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Did changing the concentration of the iodine solution affect the color of the starch
solution and affect the time in which the starch solution changed color? If so, why?
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Diffusion Across a Semi-permeable Membrane
Activity 5 Worksheet
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