NAKED EGG

PURPOSE : To demonstrate the semi-permeability of a cell membrane.

MATERIALS : 1 raw egg in its shell
                           1 Jar with a lid ( the egg must fit inside the jar )
                           Clear vinegar
                           Flexible measuring tape

PROCEDURE :

• Measure and record the circumference around the center of the egg.
• Record the apperance of the egg.
• Place the egg inside the jar. Do not crack the shell.
• Cover the egg with vinegar.
• Close the lid.
• Observe immediately and then periodically for the next 72 hours.
• Remove the egg after 72 hours and measure its circumference.
• Compare the appearence of the egg before and after being in the vinegar.
• How has the egg changed in appearence and size?

RESULTS : The egg has a hard shell on the outside and the circumference will vary. Bubbles start forming on the surface of the egg's shell immediately and increase in number with time. After 72 hours the shell will be gone and portions of it may be seen floating on the surface of the vinegar. The egg remains intact because of the thin see-through membrane. The size of the egg has increased.

WHY ? The shell of the egg is made of calcium carbonate, commonly called limestone. When vinegar reacts with the limestone, one of the products is carbon dioxide gsa, those bubbles seen on the egg.The membrane around the egg does not dissolve in vinegar, but becomes more rubbery. The increased size is due to osmosis, the movement of water through a cell membrane. The water in the vinegar moves through the thin membrane onto the egg because the water inside the egg has more materials dissolved in it than does the vinegar. Water will always move through a membrane in the direction where there are more dissolved materials. The contents of the egg stayed inside the membrane because these molecules were too large to pass through the tiny holes. This selectiveness of materials moving through the membrane is called semi-permeability.