Rubber Egg

Angel Lee Gilbert             Douglass Middle Academy 
                              543 N. Waller
                              Chicago IL 60644
                              (312) 534-6176

Objectives:

     To demonstrate the semi-permeability of a cell membrane
     To lay ground-work for discussion of osteoporosis

Materials Needed:

     1 raw egg in its shell              clear vinegar
     1 jar with a lid                    flexible tape
       (egg must fit inside the jar)

Procedure:

     1.  Measure and record the circumference around the center of the egg.
     2.  Record the appearance of the egg.
     3.  Place the egg inside the jar.  DO NOT crack the shell.
     4.  Cover the egg with vinegar.
     5.  Close the lid.
     6.  Observe immediately, and then periodically for the next 72 hours.
     7.  Remove the egg after 72 hours and measure its circumference.
     8.  Compare the appearance of the egg before and after being in the 
         vinegar.

Strategy:

     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. 
     The shell of the egg is made of calcium carbonate, commonly called 
limestone.  When vinegar chemically reacts with the limestone, one of the 
products is carbon dioxide gas, 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 into 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 the molecules were too large to pass through the tiny homes.  
This selectiveness of materials moving through the membrane is called semi-
permeability. 

Performance Assessment:

This is a pass/fail activity.