Gather these materials:Several fresh, uncooked eggs; a glass tube or drinking straw; a small glass (just the right size to hold an egg, but not large enough to let the egg fall in); a penkmfe ; and a candle.
Follow this procedure: Notice that the two ends of an egg are not exactly alike. One end is more rounded than the other. Choose one egg and decide which end is rounder. Using the point of the sharp edge of your penknife, scrape the eggshell on the rounded end until you can see a thin skin on the inside. There is one thin skin directly attached to the shell and another about
t of an inch beneath it. You cannot help breaking the first skin, but don't break the second one. Now carefully chip off the eggshell until it has a hole slightly smaller than a dime. The contents of the egg will not drop out if you haven't broken the inside skin. If you have, put the egg aside, return it to your mother (she can still use it for cooking), and try again with another egg. Don't be discouraged if you break the inside skin on several eggs. This requires patience and a steady hand. Practice helps, too.
When you have succeeded in this part of the experiment, fill the small glass with water, and set the egg on the rim so that the open hole is beneath the surface of the water. Take your penknife again, and in the pointed end of the egg, make a very small hole, deeper than the other one. Make this hole go right into the egg white. This hole will be easier to make because you don't have to worry about breaking the skin. Put one end of the glass tube or drinking straw into the egg white through this second hole. Make the opening around the tube airtight by sealing the space between the eggshell and the tube with candle wax.
You can do this by lighting a candle and, when the wax begins to melt, tipping
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the candle over so that the wax falls' on the area you want sealed. Now let the egg, glass, and tube stand undisturbed overnight. In the morning, break the egg and examine the egg white.
Results: By morning the level of the fluid in the tube was above the surface of the eggshell. The water in the glass was at a lower level than it was originally.
When you examined the egg white it was much thinner than ordinary uncooked egg white. What actually happened?
Any living skin tissue which is very, very thin is called a membrane. Like all living things, membranes are made of cells, the basic structural unit of plants and animals. Membranes seem to be strong and solid, but if you looked at oner under a microscope, you would see that the walls of the cells are porous.
Because of these pores, osmosis can take place. Osmosis is the process by which molecules pass through the pores in the cell walls of a membrane. Osmosis, in other words, is diffusion through a membrane.
In the experiment with the egg, water molecules from the glass of water passed through the thin skin of the egg and pushed the egg white up into the tube. Actually, while the water molecules passed up into the egg, some of the egg white molecules also passed down into the water. But because molecules move faster from a thin liquid, like water, to a thicker one, like egg white, there was not yet enough egg white in the water to be clearly visible.
The process of osmosis is very important in our lives because all the air we breathe, the food we eat, and the wastes our bodies must remove pass through the cell membranes in this manner.
ANOTHER WAY OF OBSERVING OSMOSIS
Gather these materials: Two thistle tubes or small glass funnels; 2 pint jars;
2 upright stands and clamps; 3 test tubes; an alcohol burner; some transparent cellulose wrapping paper; 2 Tubber bands; sucrose (C12H 2 20 11 ) ; crystals of copper sulfate (CuS04) ; and some Benedict's solution.
Follow this procedure: 1. First you will have to learn how to test for the presence of sugar, because you will need to know this later in the experiment.
Label the jars I and 2. Fill jar No. I halfway with warm water. Dissolve 3 teaspoonfuls of sugar in the water. Now pour a little of the sugar solution into a test tube and add 3 teaspoonfuls of Benedict's solution. Heat the test tube over the alcohol burner until the liquid boils. Now place 2 teaspoonfuls of plain water in a fresh test tube and add an equal amount of Benedict's solution.
Heat the test tube over the alcohol burner until the liquid boils.
Results: When you boiled the sugar solution with Benedict's solution added, it turned brick red. The color of the plain water with Benedict's solution added
stayed the same when you boiled it. Benedict's solution provides a chemical test for sugar. When you add it to a solution containing sugar and boil the two together, the liquid turns brick red. This only happens if the solution contains sugar.
2. Fill one of the thistle tubes with plain water. Cover the mouth with the cellulose wrapping paper and hold it firm with a rubber band. Invert the thistle tube and place it in the jar of sugar solution (jar No.1). With the clamp, attach the thistle tube to the upright stand at such a height that the paper is well below the surface of the sugar solution. (The diagram shows you how to fix your
apparatus.) Allow this to stand at least for several hours or preferably overnight.
Fill jar No.2 with plain water. Put! cup of warm water in the other thistle tube and add 3 teaspoonfuls of copper sulfate to it. Then cover the mouth of the thistle tube with wrapping paper, and invert the thistle tube in the jar of plain water, as you did before. Hold it in the right place with the clamp. Allow this to stand overnight also.
Results: Since plain water is colorless and copper sulfate is blue, you had no difficulty observing what happened in jar No.2. The blue solution in the thistle tube was less blue, whereas the formerly colorless water in the jar turned blueish.
This means that molecules of copper sulfate got into the water in the jar and vice versa.
Since the sugar solution in jar No. 1 and the plain water in the thistle tube are both colorless, you can't see any changes with your eyes. But you can tell if there were any by testing the plain water for sugar. Take 2 teaspoonfuls of the liquid from the thistle tube and pour it into a fresh test tube. Add 2 tea- spoonfuls of Benedict's solution. Heat the test tube over the alcohol burner until the liquid boils. Notice that the color turns brick red, proving that the
"plain water" now contains sugar. If you test the sugar solution in the jar, too, you will find that it does not turn quite such a deep brick red, showing that it lost some sugar. Sugar molecules from the jar got into the thistle tube and water molecules from the thistle tube got into the jar.
You have just proved again that molecules pass back and forth through membranes. In this case, the wrapping paper acted as a membrane.
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