Why Rotten Eggs Float

One of the ways to tell whether an egg is rotten or still good is to use the flotation test. To perform the test, you place the egg in a glass of water. Fresh eggs typically rest at the bottom of the glass. An egg that sinks but rests with the large end facing up may be a bit older but is still fine for cooking and eating. If the egg floats, it's old and may be rotten. You can test this for yourself, although to be scientific about it, you'll need to crack open the egg to observe its appearance and smell it to be certain eggs are good or bad (trust me, you'll know the bad ones). You'll find the test is fairly accurate. So, you may be wondering why the bad eggs float.

Why Bad Eggs Float

Fresh eggs sink because the egg yolk, egg white, and gases have enough mass that the density of the egg is greater than the density of water. Density is the mass per unit of volume. Basically, a fresh egg is heavier than water.

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When an egg starts to go "off" decomposition occurs. Decomposition gives off gases. As more of the egg decomposes, more of its mass is converted to gases. A gas bubble forms inside the egg so an older egg floats on its end. However, eggs are porous, so some of the gas escapes through the eggshell and is lost to the atmosphere. Although gases are light, they do have mass and affect the density of the egg. When enough gas is lost, the density of the egg is less than that of water and the egg floats.

It's a common misconception that rotten eggs float because they contain more gas. If the inside of an egg rotted and the gas couldn't escape, the mass of the egg would be unchanged. Its density would also be unchanged because the volume of an egg is constant (i.e., eggs don't expand like balloons). Changing matter from the liquid state to the gas state doesn't change the amount of mass! The gas has to leave the egg for it to float.

Gas With a Rotten Egg Smell

If you crack open a rotten egg, the yolk may be discolored and the white may be cloudy rather than clear. More likely, you won't notice the color because the overwhelming stink of the egg will repulse you immediately. The smell is from the gas hydrogen sulfide (H₂S). The gas is heavier than air, flammable, and toxic. 

It's worth noting the rotten egg smell isn't just from bacterial decomposition of an egg. Over time, the egg yolk and egg white become more alkaline. This occurs because eggs contain carbon dioxide in the form of carbonic acid. Carbonic acid slowly escapes the egg as carbon dioxide gas that passes through the pores in the shell. As the egg becomes more alkaline, the sulfur in the egg becomes better able to react with hydrogen to form hydrogen sulfide gas. This chemical process occurs more rapidly at room temperature than at cooler temperatures.

Brown Eggs vs. White Eggs

You may be wondering whether it matters if you try the flotation test on brown eggs versus white eggs. The results will be the same. There is no difference between brown eggs and white eggs except for their color, assuming the chickens were fed the same grain. Chickens with white feathers and white earlobes lay white eggs. Brown or red chickens that have red earlobes lay brown eggs. The egg color is controlled by a gene for eggshell color that does not affect the thickness of the shell.

There are also chicken eggs with blue shells and some with speckled shells. Again, these are simple color differences that do not affect the structure of the eggshell or the result of the flotation test.