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Im completely n00b in biology, just a question arised during the saturday night warmup and there is no better place to get answers :)

So, as probably most of you know, if you want to decide an egg is rotten/gone bad/addled you just put it in a bowl filled up with cold water and some salt. If the egg stays on the bottom of the bowl then it is good. If it os floating on the top of the water then it is rotten/addled.

I cannt imagine other then there is an air bubble in the rotten egg, thats why it is floating.

But how would that air bubble get in the egg?

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As an egg ages, it becomes progressively less dense and this is a natural consequence of the development of the embryo. If an egg becomes contaminated by bacteria, then it will also become less dense. The reason for this is that as the contents of the egg develop (or decompose, as the case may be) gases are produced. These gases dissolve into fluids of the egg and then migrate out of the egg into the surrounding environment.

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I would not use the terms "rotten/addled". The test you describe is a simple kitchen test to determine if eggs are "old" vs "fresh" (before the days of refrigeration). Fresh eggs will sink while old eggs will tend to float. This is because all eggs have a "bubble of air" at the blunt end (presumably for the chick to breath while hatching). As the egg gets older, this bubble will increase in size due to simple desiccation (moisture loss) over time.

You can try the test using fresh vs. expired eggs from the market. You will find that many more of them will float when they are weeks past their expiration date, but they are definitely not rotten/addled and some people will still use them. It is more a test for the cautious person to avoid possible health risks from older eggs. A rotten/addled egg will give off the characteristic "rotten egg" smell due to bacterial growth.

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