Diffusion of a Substance across a Semi Permeable Membrane

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Name: Marion Paxton

Diffusion Lab

Introduction: In this lab you will observe the diffusion of a substance across a semi permeable membrane. Iodine is a known indicator for starch. An indicator is a substance that chances color in the presence of the substance it indicates. Watch as your teacher demonstrates how iodine changes in the presence of starch.

**Prelab Observations: Describe what happened when iodine came into contact with starch.

The cornstarch turned blue-black

1. Fill a plastic baggie with a teaspoon of cornstarch and a half a cup of water tie bag. (This may already have been done for you) 2. Fill a beaker halfway with water and add ten drops of iodine. 3. Place the baggie in the cup so that the cornstarch mixture is submerged in the iodine water mixture. 4. Wait fifteen minutes and record your observations in the data table 5. While you are waiting, answer the questions.


1. Define diffusion – the movement of particles from an area of low concentration to an area of high concentration

2. Define osmosis - the movement of particles from low solute concentration to and area of high solute concentration across a semi-permeable membrane.

3. Why is iodine called an “indicator”? Iodine is called an indicator because it can detect the presence of starch by changing it to a blue-black colour.

4. Molecules tend to move from areas of low concentration to areas of high concentration.

5. What does selectively permeable mean? Some substances are able to pass through but others are not.

What’s in the Bag?
We’re going to think about concentrations now, which substances are more or less concentrated depends on which one has the most “stuff” in it.

Which is more concentrated in starch? baggie
Which is more concentrated in iodine? beaker

If starch could cross the baggie, what color would the water in the beaker turn? Blue-black If...
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