Diffusion of Substances Across Semi-Permeable Membranes

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Background: You need the answers to these questions. You may need to refer to Chapters 2, 3 and 4 for your answers.

1. What is an ion? What is a molecule? Which would be bigger?

What is an ion? An atom or molecule bearing a positive or negative charge due to the loss or gain of an electron.

What is a molecule? A chemical structure that contains two or more atoms that are held together by chemical bonds.

Which would be bigger? Molecule

2. What is the meaning of semi-permeable? A membrane whether selective, partially or differentially is a membrane that will allow certain molecules or ions to pass through it by diffusion.

3. What is osmosis? How is osmosis different from diffusion?

What is osmosis? The movement of water across a semi permeable membrane toward a solution containing a relatively high solute concentration.

How is osmosis different from diffusion? Osmosis covers the movement of water across a membrane toward a solution containing a high solute, whereas diffusion covers passive movement from an area of higher concentration to an area of lower concentration. This is where the two differ.

4. What is filtration? Movement of a fluid across a membrane whose pores restrict the passage of solutes on the basis of size.

Hypothesis: A filter will have openings that permit small objects to go through.

Warning: Iodine solutions may be stored behind the counter at the pharmacy alongside the Sudafed (and for the same reason). It’s okay. No one will track you down for a science experiment!

Procedure: You will need to gather the following items. You will need to leave your experiment set up for about 24 hours.

• 4-6 plastic food storage bags (cheap ones are best!)

• One potato

• Tincture of iodine (available in the first-aid section of the store)

• 4-6 bowls, cups or whatever will hold the bags in their baths

1. Cook the potato until it is quite done. Save the water for your experiment, but you may add butter, salt and pepper and eat the potato while you finish setting up your lab.

2. Mix a drop of iodine into a spoonful of potato water. Observe the color change. (If it doesn’t turn blue-black, you may not have iodine.) Rinse the spoon – do not eat iodine.

3. Fill two of your baggies about halfway with potato water. Tie the bags tightly closed.

4. Mix a tablespoon of iodine in two cups of water. Fill two of your baggies with the iodine water. Tie the bags tightly closed.

5. Fill two of your bowls with iodine water.

6. Fill two of your bowls with potato water.

7. Now, put one potato bag in a bowl of iodine water. Put the other potato bag in potato water.

8. Put one iodine bag in a potato water bowl. Put the other iodine bag in iodine water.

9. Complete the observation table.

|Bag |Bowl |Before colors |After colors | |Potato |Iodine |Cloudy/blue |Light blue liquid in the bag | |Potato |potato |Cloudy mixture |No changes in colors, but still cloudy mixture | |Iodine |potato |Cloudy dark blue almost black color |Lighter blue, a dark film on the side of the bowl | |Iodine |iodine |Black film |Didn’t really notice a difference, if there was one it was a | | | | |slight one | |Both |Potato |Dark blue color with a cloudy look |Top of the bowl is a blackish color. The bottom is a much darker| | | |...
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