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Eggs contain something called "calcium carbonate". This is what makes them hard.
Vinegar is an acid known as acetic acid.
When calcium carbonate (the egg) and acetic acid (the vinegar) combine, a chemical reaction takes place and carbon dioxide (a gas) is released. This is what the bubbles are made of.
The chemical reaction keeps happening until all of the carbon in the egg is used up -- it takes about a day.
When you take the egg out of the vinegar it's soft because all of the carbon floated out of the egg in those little bubbles.
Leave the same egg sitting out on the table for another day.
Now feel it again.
The calcium left in the egg shell stole the carbon back from the carbon dioxide that's in the air we breath.
- OR -
If you were using a raw egg, once the shell has softened, you can place the egg in water and it'll absorb and expand via osmosis until the shell finally bursts. (Thanks to James for sharing this tip!)
What makes our bones hard? That's right! Calcium carbonate -- the same thing that made the egg shells hard.Optional: Knotted Bones Printable Activity Sheet
Take some thin chicken bones and drop them in vinegar for a day. Take them out and they'll be soft just like the egg shells were.
Now you can tie them in a knot, just like a piece of string.
Leave them sitting out on the table and they'll get hard again!
Take them to school for sharing time and see if your classmates can figure out how you did it (or do this at school and take them home to stump mom and dad!)