The question of whether an egg floats or sinks in water is a classic kitchen query. An egg’s behavior in water acts as a direct, physical indicator of its age. The difference between sinking and floating is governed entirely by the principle of density, which changes as the egg ages after being laid. This simple home experiment, often called the float test, reveals the physical transformations occurring inside the shell.
The Sinking Truth About Fresh Eggs
A newly laid egg will sink immediately to the bottom of a container filled with fresh water and lie completely on its side. This occurs because the egg’s internal contents—the yolk and albumen—are significantly denser than the water. Density is a measure of mass per unit volume; a fresh egg has a density of approximately 1.03 grams per milliliter, compared to water’s 1.0 g/mL.
The shell of the egg is porous, covered in thousands of microscopic pores. While a very small air cell is present at the broad end of even the freshest egg, it is negligible in size. The mass of the liquid interior overcomes the buoyancy provided by this tiny pocket of air, ensuring the egg sinks flat and confirming its peak freshness.
The Science of Why Eggs Start to Float
The shell’s porosity allows the egg’s density to decrease over time. As an egg ages, moisture from the interior contents slowly evaporates through the pores into the surrounding atmosphere. This loss of liquid mass is replaced by air drawn into the egg, causing the internal air cell to enlarge steadily.
Replacing dense liquid with less dense air reduces the egg’s overall mass without changing its volume, effectively lowering its density. The chemical breakdown of the egg’s proteins also contributes to the air cell’s growth by releasing gases, such as carbon dioxide. As the air cell expands, the egg’s buoyancy increases until its overall density becomes less than that of the water. When this happens, the egg begins to lift off the bottom of the container.
Using the Float Test to Determine Freshness
The float test provides a clear, three-stage indicator of an egg’s age. An egg that sinks completely and lies flat on its side is very fresh and ideal for any use, such as poaching or frying.
If the egg sinks but stands upright on its pointed end, with the blunt end raised toward the surface, it is considered older, likely one to three weeks old. This upright position is due to the expanded air cell concentrating the air in the blunt end. Such an egg is still safe to eat but is better suited for hard-boiling, as the larger air cell aids in easier peeling.
When an egg floats entirely on the surface of the water, it indicates a significantly enlarged air cell and a serious reduction in density. This egg is old and should be discarded, as the extensive breakdown of internal material suggests it has passed its prime.