The observation of an egg standing vertically in a glass of water has long been a source of fascination. This phenomenon, where an egg settles upright instead of lying on its side, is purely a matter of physics and internal composition. The explanation is rooted in predictable scientific principles that govern how objects behave in liquid. This simple test offers a direct look into the concepts of buoyancy, stability, and the natural aging process of an egg.
Understanding Buoyancy and Stability
The primary force determining whether an object sinks or floats is buoyancy, the upward force exerted by a fluid that opposes the weight of an immersed object. An object sinks if its average density is greater than the surrounding fluid. A fresh chicken egg has an average density of about 1.03 grams per milliliter, making it denser than fresh water (approximately 1.00 gram per milliliter). Consequently, a new egg sinks completely to the bottom of a container of tap water.
When settling, a fresh egg usually rests on its side due to the location of its center of gravity. For an object to remain stable in a fluid, its center of gravity must be positioned directly below its center of buoyancy. The egg’s contents are initially uniform, and its elliptical shape causes the center of gravity to be low, resulting in a stable horizontal resting position.
Why Older Eggs Stand Naturally
The reason an egg eventually stands upright in water is due to a natural, internal change that occurs over time. An eggshell is porous, allowing moisture and gases to be exchanged with the outside air. As the egg ages, water evaporates and carbon dioxide escapes, causing the internal contents to shrink slightly.
This shrinkage creates and enlarges an air pocket, known as the air cell, located at the blunt end of the egg. Since air is less dense than the egg’s liquid contents, the increasing size of this air cell changes the egg’s overall average density and internal balance. The enlarged air pocket acts like a small flotation device concentrated at one end.
This shift repositions the egg’s center of gravity higher up inside the shell and closer to the air cell. When the older egg is placed in water, the heavier, pointed end sinks first. The lighter, blunt end, buoyed by the larger air cell, rises and points upward. This vertical orientation is the egg’s new position of stability, causing it to stand on its narrow end. This “float test” is a reliable indicator that the egg is older.
The Salt Water Experiment
A different way to make an egg stand or float involves manipulating the surrounding liquid rather than waiting for the egg to age. This demonstration relies on increasing the density of the water by dissolving salt into it. Salt water is denser than fresh water because the dissolved salt adds mass to the same volume of liquid.
By adding enough salt, the water’s density can be raised to match or exceed the average density of the egg. If the water becomes denser than the egg, the egg will float fully on the surface, even if it is fresh. When the density of the salt solution is adjusted to be exactly equal to the density of the egg, the egg becomes neutrally buoyant.
In this state, the egg remains suspended wherever it is placed in the water. To make the egg stand upright in the middle of the glass, layers of water with different densities are created, with the densest water at the bottom. The egg settles at the precise point where the fluid’s buoyant force perfectly counteracts its weight.