Under the hard outer shell of a chicken egg is a semipermeable membrane that allows air and moisture to pass through. Because water molecules can move into and out of the egg but larger molecules cannot, the semipermeable egg membrane allows for an exploration of the concepts of diffusion and osmosis. Osmosis is the movement of a solvent, such as water, through a semipermeable membrane into a solution of higher solute concentration that tends to equalize the concentrations of solute on the two sides of the membrane.
The process of removing the outer shell while leaving the underlying membrane intact presents an additional opportunity for observations, as an acid-base reaction occurs when you make the "naked" eggs for the osmosis experiment.
Make Naked Eggs
Before the egg osmosis experiment can begin, the egg's hard outer shell must be removed. This is done by placing the eggs in a container of white vinegar, covering the container, and placing it in the refrigerator. Replace the vinegar after about 24 hours and let it continue react with the eggshells for another 24 hours, if needed. Carefully rinse any remaining eggshell off the egg under water, rubbing the egg gently, if needed. The removal of the eggshell occurs through an acid-base reaction; the eggshell, comprised mostly of calcium carbonate, is basic, while the vinegar is an acid.
Make Initial Observations
Once the eggs' shells are removed and the eggs are rinsed clean and dry, measure the eggs. If a scientific or kitchen scale is available, weigh each egg. If there is no scale available, use a soft measuring tape or a piece of string and a ruler to measure the horizontal and vertical circumferences of each egg. Record the weight or dimensions of each egg in a table, as you will measure each egg again later.
A variety of materials may be used for this experiment. Try any or several of the following: distilled water, water with food coloring, a solution containing 20 percent salt by weight, soda pop, or corn syrup. Put 100 milliliters or about 1/2 cup of each solution in a separate cup, beaker or other suitable container. Label the container to note which solution it contains. Place an egg in each container and record which egg was placed where so you can compare subsequent measurements to your initial measurements.
Observe the Results
After about a day in the solutions, carefully remove each egg, one at a time, using a spoon. Rinse egg with water and let it dry, then measure it and record its weight or dimensions. Observe the solution in each container. If it is in a beaker or container with measurements, observe any change in the solution volume.
If you wish, after weighing and measuring the eggs, place any eggs that decreased in size in a container with distilled water, then remove and weigh or measure the eggs at 10 minute or other intervals. Observe any changes.
Understanding What Happened
The semipermeable egg membrane allows some materials, like water, to pass into or out of the egg. Molecules tend to move from an area of high concentration to an area of low concentration. This process is called diffusion. Osmosis is the diffusion of water through a selectively permeable membrane. Osmosis is the reason why, when the "naked" egg is placed in distilled water, the egg gets bigger as water passes from an area of high concentration -- the distilled water -- to an area of lower concentration -- the egg. Water moves in the opposite way when the egg is placed in corn syrup, for example, because the concentration of water in corn syrup is lower than the concentration of water in the egg.
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