It can be surprising to see an egg float when placed in a glass of water. The phenomenon of an egg either sinking or floating is directly related to its internal composition and how it interacts with the surrounding water.
The Principle of Density
Density describes how much mass is contained within a given volume. For example, if two objects are the same size, the heavier one is denser because more mass is packed into the same space. If an object is denser than the fluid it is placed in, it will sink.
Conversely, if an object is less dense than the surrounding fluid, it will float. Water has a density of approximately one gram per cubic centimeter. Objects with a density greater than one gram per cubic centimeter will sink in water, while those with a density less than one gram per cubic centimeter will float.
How Eggs Change Over Time
An egg’s shell is porous, containing tiny pores across its surface. These openings allow for the exchange of gases and moisture between the egg’s interior and its external environment. Over time, moisture from inside the egg evaporates through these pores into the surrounding air. As water escapes, it is replaced by air, causing the air cell, located at the wider end of the egg, to gradually expand.
This continuous loss of water and the corresponding increase in air within the egg contribute to a reduction in its overall mass. While the egg’s volume remains relatively constant, its mass decreases due to water evaporation. This reduction in mass, without a significant change in volume, directly lowers the egg’s average density. Eventually, an older egg becomes less dense than the water it is submerged in, leading it to float instead of sink.
What a Floating Egg Reveals
When an egg floats in water, it indicates the egg has aged and lost a significant amount of moisture. The expanded air cell is a reliable sign of an older egg, as this process occurs gradually over days or weeks. This does not necessarily mean the egg is spoiled or unsafe for consumption. Many older eggs are still safe for cooking and eating.
To determine if a floating egg is spoiled, rely on other sensory indicators, primarily smell. A spoiled egg produces a distinct, unpleasant sulfurous odor when cracked, regardless of whether it floats or sinks. The “float test” is best used as an indicator of an egg’s age rather than its safety.