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ON
DAY 1 BUBBLES OF CARBON DIOXIDE BEGIN TO FORM ON THE EGG SHELL. |
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DAY 7 A TRANSLUCENT EGG IS LEFT AFTER THE SHELL HAS DISSOLVED. |  | | | | |
| Here
is a classic experiment with eggs. If you soak a raw egg in
vinegar, over the course of time, the vinegar will dissolve
the eggshell. What you’re left with is the egg’s translucent
membrane to protect the egg. Since the shell is made up mostly
of calcium carbonate -- it contains calcium carbonate (94%),
magnesium carbonate (1%), calcium phosphate (1%), and 4% organic
matter-- vinegar which contains acetic acid will dissolve
the shell. |
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Procedure:
Place the
egg in a tall glass or jar and cover the egg with vinegar. Wait
a few minutes and look at the jar. You should see bubbles forming
on the egg. Leave
the egg in the vinegar for a full 24 hours in the refrigerator.
After the 24 hours, carefully pour the old vinegar down the drain
and cover the egg with fresh vinegar. Place the glass with the
vinegar and egg back in the refrigerator for a full week. One
week later pour off the vinegar and very carefully rinse the egg
with water. The egg looks translucent because the outside shell
is gone. Notice that some
of the vinegar has permeated through the egg's membrane and caused
the egg to get a little bigger.
What
happened? The
egg shell is composed of calcium carbonate (CaCO3). When calcium carbonate comes
in contact with vinegar which contains acetic acid (CH3CO2H)
a chemical reaction occurs. The
first thing that happens is that the carbonate (CO3--) part
of calcium carbonate is protonated by acetic acid to make carbonic acid (H2CO3).
Along with
this, the calcium and acetate from calcium acetate.
2
CH3COOH + CaCO3 = H2CO3 + Ca(CH3COO)2.
In the next step, the carbonic acid breaks down to form carbon dioxide and water,
H2CO3 = H2O + CO2.
The overall
reaction is just the sum of the two reactions:
2
CH3COOH + CaCO3 = H2O + CO2 + Ca(CH3COO)2.
CaCO3(s) +
2CH3COOH(l) Ca(CH3COO)2(s) +
H2O(l) + CO2(g)
Water and
excess vinegar evaporate upon standing. Carbon dioxide (a gas)
is released into the air during the reaction.
After
you dissolve the eggshell, the egg is surrounded by a membrane. (Actually, it’s
two membranes, but they are held tightly together). To get a better idea of see
Anatomy of an Egg. What
does a naked egg look and feel like? See this mpg
from University of Iowa. Where
to go from here? Here
are some excellent classroom experiments using naked eggs that demonstrate diffusion
and osmosis. | 