Fresh eggs typically sink when placed in water, but older eggs often rise to the surface. This change in behavior is a straightforward demonstration of physical laws. The scientific explanation for why an egg moves from sinking to floating provides a reliable method for assessing its age.
Understanding Density and Buoyancy
The physical principle that determines whether an object sinks or floats is the relationship between its density and the density of the fluid it is submerged in. Density is defined as mass per unit volume, and an object will float if its average density is less than the density of the surrounding water. Conversely, an object sinks if it is denser than the water it displaces, a concept known as Archimedes’ Principle.
Tap water has a density of approximately 1.0 gram per milliliter (g/mL). A fresh egg typically has an average density of about 1.03 g/mL, making it slightly heavier than the same volume of water. This small difference in density causes a newly laid egg to overcome the water’s buoyant force and sink. As the egg ages, however, this density balance shifts, altering its interaction with the fluid.
The Role of the Air Cell and Shell Porosity
The egg’s changing buoyancy lies in the structure of its shell. The shell is composed primarily of calcium carbonate and is riddled with thousands of microscopic pores. These pores allow the exchange of gases, such as oxygen and carbon dioxide, which is necessary for a developing embryo.
This gas exchange continues even after the egg is collected and stored. Over time, moisture from the egg’s interior evaporates through the shell’s porous structure. Since the eggshell prevents the overall volume from shrinking, the space once occupied by water is gradually replaced by air.
The air replaces the evaporated water and collects in a small space known as the air cell. Since air is less dense than water, the continuous expansion of this air cell directly lowers the egg’s average density. The egg loses mass (water) but maintains its volume, meaning its density progressively decreases. Once the air cell has expanded enough to make the egg’s average density less than that of the water, the egg becomes buoyant and floats.
Interpreting Egg Float Positions
Testing an egg’s position in water provides a simple measure of its age based on the size of its air cell. An egg that sinks completely and lays flat on its side is very fresh, typically less than one week old, indicating a minimal air cell size. If the egg sinks but stands upright on one end, it is moderately older, suggesting the air cell is beginning to expand.
An egg that floats on its side or fully rises to the top is considered old, often three weeks or more past its lay date. This floating behavior confirms that the internal air cell has grown large enough to reduce the egg’s density. While floating indicates age and potential loss of quality, it does not automatically confirm spoilage, which is caused by bacterial growth. If a floating egg exhibits an off-putting sulfurous smell when cracked, it should be discarded; otherwise, it may still be safe if cooked thoroughly.