The phenomenon of an egg floating in water is a common experiment that reveals the scientific principle of density. Density is the measure of an object’s mass relative to its volume. An object floats only if it is less dense than the fluid it displaces. This simple test serves as a reliable indicator of the egg’s age.
The Baseline: Why Fresh Eggs Sink
A freshly laid egg sinks immediately to the bottom of a container of tap water because its density is greater than the water surrounding it. The contents, primarily the yolk and albumen, are composed mostly of water, making the interior very dense. The average density of a fresh egg is approximately 1.03 grams per milliliter, which is higher than the 1.0 g/mL density of fresh water.
In a new egg, the internal air pocket, known as the air cell, is extremely small. This minimal air volume does little to offset the high density of the liquid contents. Therefore, the downward force of the egg’s mass easily overcomes the upward buoyant force exerted by the displaced water.
The Role of the Air Cell and Gas Exchange
The eggshell is not an impermeable barrier but is composed mainly of calcium carbonate. This shell contains thousands of microscopic pores, numbering between 7,000 and 17,000, which act as tiny channels for gas exchange. This porous structure allows the egg to “breathe,” a feature originally intended to support a developing embryo.
As an egg ages, the interior changes dramatically due to this constant gas exchange. Moisture within the egg slowly evaporates out through the shell’s pores over time. This continuous loss of mass is not matched by a reduction in the egg’s overall volume.
Air from the outside environment is drawn in to fill the vacant space, causing the internal air cell to steadily expand. Furthermore, as the internal contents break down, trace amounts of gases are produced, further contributing to the air pocket’s volume. The replacement of dense liquid mass with less dense gas mass decreases the egg’s overall density, eventually causing it to float.
Using the Float Test to Determine Freshness
The float test is a practical application of density principles, providing a simple visual cue for the egg’s relative age. A very fresh egg, with its small air cell, will sink completely to the bottom of the bowl and lie flat on its side. This position indicates maximum density.
An egg that is a few weeks old will still sink, but its slightly enlarged air cell causes it to stand upright. It rests on the pointed end with the wide, air-filled end tilted toward the surface. This upright posture signals that the egg is older but remains suitable for consumption.
If an egg rises and floats completely on the surface of the water, it signifies that the air cell has grown large enough. While a floating egg is old, the presence of such a large gas pocket is a strong indicator of advanced age and potential quality decline, making a follow-up sniff test advisable before cooking.