47
$\begingroup$

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.

$\endgroup$
1
  • 2
    $\begingroup$ Comments removed. Be kind, people. $\endgroup$
    – rob
    May 27 at 12:56
58
$\begingroup$

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.

$\endgroup$
0
32
$\begingroup$

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.

$\endgroup$
1
  • 6
    $\begingroup$ I forgot an egg in a fridge for about a year once. It didn't rot, but it dried out completely. It was like a rattle. A dry cherry-sized mass of mummified yolk rolling around inside. Would have definitely floated. $\endgroup$ May 27 at 13:15
8
$\begingroup$

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!

$\endgroup$
5
  • 4
    $\begingroup$ I don't understand this answer. If egg is impermeable (as implied by the gasses being trapped) then this mechanism does not change the density of the egg. If it is permeable, why don't the gasses leave? $\endgroup$ May 26 at 17:12
  • 3
    $\begingroup$ @ComptonScattering The pores in eggs are microscopic. Permeation of water and gases goes with a given diffusion constant, so over many weeks a fresh egg will slowly lose moisture. When bacteria infect an egg, however, they can multiply rapidly and produce hydrogen sulfide at a rate faster than it can diffuse out of the egg, leading to a pressure increase. $\endgroup$
    – J...
    May 26 at 19:17
  • 1
    $\begingroup$ @J... ok thank you. Your comment though implies that gaseous build up from the bacterial growth does not contribute to the density change of the egg (as it is on a much shorter timescale than the time on which egg density responds). This seems to contradict the answer above. $\endgroup$ May 26 at 23:08
  • $\begingroup$ @ComptonScattering Usually an egg will have lost a lot of water well before it rots, so it would have already become a floater long before it actually spoiled. $\endgroup$
    – J...
    May 26 at 23:10
  • 3
    $\begingroup$ Had some eggs get buried in the mud once near the chicken coop. While mucking it out, we uncovered them. They exploded spontaneously once uncovered with a loud pop. $\endgroup$
    – Mike Vonn
    May 27 at 21:10
4
$\begingroup$

The density changes for the rotten egg.

Either

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

or

  1. 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.

$\endgroup$
1
  • 3
    $\begingroup$ There is also option 3: some of the water inside permeates out of the egg and is replaced by outside air, therefore decreasing its mass $\endgroup$
    – Kevin
    May 25 at 21:12
1
$\begingroup$

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.

$\endgroup$
1
$\begingroup$

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.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.