Alien Egg – part 1
- Acetic acid (vinegar)
- Steel wool
This is the first of our experiments called “The Alien Egg”. In these experiments we dissolve the shell of an egg and then submerge the egg in solutions of different solutes, with different concentrations. The membrane of the egg remains intact, resulting in the egg expanding or contracting as water enters or escapes. While not really an “alien egg”, it looks like one and kids enjoy this experiment a lot.
Phase one of “The Alien Egg” relies on the solubility of calcium carbonate in acetic acid.
Here we pre-treat eggs by buffing the shells lightly with superfine steel wool to remove some of the food wax that is sometimes sprayed onto eggs when they are processed for sale. We then rinse away any remaining wax residue with a product called vegetable wash, which is intended to remove food grade waxes from produce. You can quantify the time saved by preparing your Alien Egg specimens this way by running a control setup that consists of an untreated egg soaked in acetic acid under the same conditions (temperature, light) as our treated specimens. The reaction time for the treated egg is typically one third that of the untreated version.
In a warm room, the eggshell is typically ready for removal after 45 minutes. Once the calcium carbonate shell has softened and corroded in the acetic acid, you may rinse off the remaining shell under warm water by rubbing (very gently) with your finger tips. Attempting to scratch away the calcium carbonate reside normally results in tearing the delicate inner membrane we are trying to expose. If you are teaching a large class, you can “batch process” a few dozen eggs in a long, flat clear plastic storage container but setting the eggs into a bath of vinegar before leaving at the end of the day, then rinsing off the membranes on the following morning. To remove the smell of vinegar from the storage bin, fill it with warm soapy water, seal it, and shake it vigorously.
2CH3COOH + CaCO3 -> Ca(CH3COO)2 + H2O + CO2
You may notice the carbon dioxide bubbles forming quite quickly if your classroom is warm and if you have pre-treated the eggshells with the steel wool and vegetable wash.
Besides water and carbon dioxide, the reaction produces a compound called calcium acetate, Ca(CH3COO)2, a common food additive used as a stabilizer in the manufacturing of candy products like taffy and also in preserving the texture of canned vegetables.