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发表于 2008-9-29 12:36:39
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来这里半年了,边学习边打工，忙忙碌碌,学会了使用面包机做面包,生活也渐渐适应了.收获最大的是孩子,在高中学习的轻松,他说愿意学习,是这样的,看见他的老师给发来的电子邮件,在网上老师的回复.下面是他的生物的实验报告:A determination of the permeability of sandwich bags to starch and iodine
Purpose: The purpose of this experiment was to study the permeability of different kinds of sandwich bags to starch and iodine.
Background Information: First, the meaning of permeability must be explained. The dictionary definition of permeability is a measure of a material’s ability to transmit liquids. In this experiment, we are going to find out about whether the molecules of starch and iodine can pass through materials like sandwich bags.
The basis of this movement of molecules is called diffusion. Diffusion is the movement of particles from a higher concentration to low concentration by random molecular movement. It’s a spontaneous process that does not require extra energy. Diffusion only happens when there’s a difference of concentration, or what is called, concentration gradient occurs. If there is a concentration gradient, the solute molecules in the highly concentrated region would move to the region of lowly concentrated, so that the whole region is equally concentrated, or what is called, equilibrium.
To understand diffusion better, the meaning of entropy must be explained. For any state of any system, there is a number that describes basically how ‘messy’ it is. That number is called entropy. For example, when you have a box with a board in the middle of it, each side of which is chicks of two different color, let’s say, white and black. In this system, the entropy is relatively low, because it’s not messy. You can pick white chicks from one side of the board and black chicks from the other. You can not pick black chicks from the white-chick-side, and you can not pick white chicks from the black-chick-side.
But when you withdraw the board, within a few hours, the chicks would mix together. Now, the entropy increased. Why? Because the system had just turned relatively ‘messy’. Now when you randomly pick a chick from the box, you can not be sure whether you are going to pick a white one or a black one.
Now, according to the Second Law of Thermodynamics, any spontaneous process (that is, process that does not require extra energy) will increase the system’s entropy. We now know that diffusion is a spontaneous process, is the entropy of the system increased? Let’s say a drop of methylene blue was dropped into a glass of water. Soon, the whole glass would turn into a blue color. Now, the process of methylene blue dissolve is actually the diffusion of methylene blue molecule. Before the methylene blue is fully dissolved, there’s a huge ‘clump’ of methylene blue molecules. The entropy is relatively low, because the methylene blue molecules separated with the water quite orderly. But after the diffusion, the entropy increased. Get a spoon of water, and it’s impossible to get only just water or only just methylene blue. So, we can see that the basis of diffusion is the Second Law of Thermodynamics.
In this experiment, when a deliberately produced concentration gradient occurs between two regions separated by sandwich bags, diffusion would happen if the bag is permeable to the solute molecules, that is, the molecules can pass through the sandwich bag. And if there is a concentration gradient but no diffusion occurred, one can determine that the sandwich bag is not permeable to these solutes.
What is the method of testing whether solute molecules appear on the other side of the testing material? In this case, the special property of starch and iodine are used. When iodine and starch are mixed, the mixture would turn into a purplish color, which is very obvious and easy to recognize. The chemical reaction can be shown in a way like this:
nI2 + (C6H10O5)6n = (C18H30O15I)2n
So in this experiment, we can observe the color change of the mixture to determine whether iodine and starch passed through the sandwich bags.
Variables: Manipulated Variables: Type of the sandwich bags, types of solute.
Controlled Variables: Concentration of the solute, air tightness of the device, temperature.
Responding Variables: The color of the mixture, both inside and outside the sandwich bag.
Hypothesis: The sandwich bag is permeable to both iodine and starch.
Materials: 50.0 mL graduated cylinder, test tube, 400 mL beaker, 10.0 mL graduated cylinder, plastic sandwich bag, aqueous starch solution, aqueous iodine solution, droppers, metal twist tie, glass stir rod, distilled water.
Procedure: Step 1: Measure 8 mL of iodine solution and 100 mL of water in the cylinder, mix them in a clean and dry 400 mL beaker. Stir to make the iodine fully dissolve.
Step 2: Take a zip-lock sandwich bag, fill it with made starch solution. Zip the sandwich bag so it becomes completely air tight.
Step 3: Put the zip-lock sandwich bag into the beaker containing the iodine solution. Make sure it’s totally submerged.
Step 4: Leave it undisturbed. Wait for a period of time. Observe the color of the solutions, both inside and outside the bag.
Step 5: Repeat Step 1 to 4 with different kinds of sandwich bags.
Observations:
Type of bags
The color
of solutions Type A Sandwich bag
(Co-op) Type B Sandwich bag
(Safeway) Type C Sandwich bag
Color of the solution inside the bag before the experiment Colorless and transparent Colorless and transparent Colorless and transparent
Color of the solution outside the bag before the experiment Yellow Yellow Yellow
Color of the solution inside the bag after the experiment Blue Blue Blue
Color of the solution outside the bag after the experiment Almost colorless Almost colorless Almost colorless
Analysis: From the data shown in the table above, we can see the change of color both inside and outside the bags. We can see that solutions in all the bags turned blue while the solutions outside these bags not only did not turn blue but also turned into an almost transparent color, it means after the experiment, there’s iodine inside the bag, and low concentration of iodine outside the bag after the experiment. First, because the solution inside the bags turned blue, we can determine that the iodine diffused passed the sandwich bags and reacted with the starch inside to produce a blue color. It also resulted in the form of concentration of iodine outside the bag decreased (the color outside the bags turned almost colorless). On the other hand, starch did not diffuse out of the bag, otherwise the solution outside would turn blue. So basically the starch didn’t diffuse through the bag while the iodine diffused through.
Conclusion: All sandwich bags tested are permeable to iodine, but not permeable to starch.
Evaluations: This experiment can be improved by observing the time of the color change. When there’s a first sign of color change, record the time from submerging the bags inside the beakers. Thus, the bag that is most permeable to the solute can be determined. Also, there might be a possibility that the starch did diffuse out, but with iodine diffused inside the bag, causing low concentration outside, the color change may not be noticed. We should check the outside solution with iodine after the experiment is done.
2008-9-23 |
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