The question of whether an egg will sink or float in water is a common kitchen experiment that offers a direct lesson in physics and biology. Placing a raw egg into water provides an immediate method for assessing its condition. The result is variable, entirely dependent on the egg’s age and internal changes. The simple sink or float behavior reveals the relationship between density and buoyancy.
Freshness Determines Buoyancy
The placement of an egg in fresh tap water provides a quick three-tiered assessment of its freshness. A recently laid egg will sink swiftly to the bottom and settle flat against the container base. This indicates the egg’s internal contents are dense, confirming it is fresh and suitable for preparations like poaching or frying.
As the egg ages slightly, it will still sink, but the blunt end will tilt upward, causing the egg to stand upright or bob slightly. This moderate tilt suggests the egg is a few weeks old and is best used for baking or hard-boiling.
An egg that floats freely on the surface has aged considerably, and its buoyancy suggests it should be discarded.
The Role of the Air Cell and Density
The changes in the egg’s position are a direct consequence of a change in its overall density, which is controlled by the growth of an internal air cell. When an egg is first laid, a small air cell forms at the blunt end as the internal contents cool and contract.
The eggshell is highly porous, containing thousands of microscopic pores that allow for gas and moisture exchange. Over time, moisture from the egg’s interior evaporates through the shell’s pores, and this loss of water mass is replaced by air from the atmosphere.
This process causes the air cell to expand steadily, increasing the volume of air inside the shell. Simultaneously, carbon dioxide naturally present in the egg’s contents also escapes through the pores.
Because air is less dense than the liquid egg white and yolk it replaces, the overall mass of the egg decreases while its volume remains fixed by the shell. This reduction in mass relative to volume lowers the egg’s density until it becomes less dense than the surrounding water, causing it to float.
How Water Density Affects the Test
While the egg’s internal changes govern its buoyancy in a constant medium, the water itself can be manipulated to change the outcome of the test. Density is defined as mass per unit volume, and objects float only when their density is less than the density of the fluid they are in. Fresh tap water has a density of approximately 1 gram per milliliter, while a fresh egg has a slightly higher density, around 1.03 grams per milliliter, which is why it sinks.
Adding a soluble substance like salt to the water increases the fluid’s density by adding mass without significantly changing the total volume. As salt dissolves, it breaks down into ions, which occupy the spaces between the water molecules, making the solution heavier. This action increases the water’s buoyancy, which is the upward force exerted by the fluid. If enough salt is dissolved, the water’s density will eventually exceed the density of even a very fresh egg. A common experiment involves adding about three tablespoons of salt to a glass of water, which is typically sufficient to make a fresh, dense egg float, demonstrating the direct relationship between fluid density and buoyancy.