Why do rotten eggs float in water? My mom told me that one can check whether an egg is rotten by sinking it in a glass of water. If it floats, then it is rotten.
I didn't find any explanation for this phenomenon. If anyone knows one, please answer.
 A: As the yolk decays, they emit hydrogen sulphide, which builds up inside the egg. Really rotten eggs can pop quite easily, which is something you need to be careful of when clearing out old nests.
If left for long enough, a good few months, the yolk and the white will evaporate altogether, leaving just a shell and an awful lot of gas. They make a surprisingly loud bang when you break them!
A: The domestic chicken's egg shell has about 7000 pores that allow the embryo to breathe. When an egg rots the yolk and surrounding materials decompose and they give off gasses which can pass through the shell. This allows mass to leave the interior of the egg resulting in less density for the volume of the egg making it more buoyant.
A: The density changes for the rotten egg.
Either

*

*the volume changes, i.e. gases expand the egg slightly

or


*the mass changes - maybe the shell is slightly permeable to gas and the rotten egg expels some gas thus becoming less massive.

Sorry not sure which.
A: It doesn't need to be rotten. When the egg is getting old, it evaporate water and looses mass while drying. Even if not rotten.
A: As time passes the egg yolk decomposes and gases escape though the pores in the shell.
So its mass decreases.
As the volume of the egg remains constant:
the weight decreases.
Therefore the egg floats, as upthrust on the egg remains constant.
Additionally an egg floating means that the egg is not fresh and does not neccesarily mean that it is rotten.
A: Rotten egg sinks because eggs are porous and Eggs have an air cell that becomes larger as the egg ages and acts as a buoyancy aid. An egg can float in water when its air cell has enlarged sufficiently to keep it buoyant. This means the egg is older, but it may be perfectly safe to use.
