When placed in water, an egg sometimes stands upright instead of lying flat. This common household experiment sparks curiosity about what this behavior signifies. The way an egg positions itself offers a simple, visual clue about its internal condition.
Why Eggs Float or Sink
The behavior of an object in a liquid is governed by density and buoyancy. Density is how much mass is in a given volume, while buoyancy is the upward force exerted by a fluid. An object sinks if its density is greater than the fluid it displaces, and floats if its density is less. A fresh egg typically has a higher density than water, causing it to sink to the bottom and rest on its side.
The Air Cell’s Role in Egg Freshness
An egg naturally contains a small air cell, usually located at its larger end, nestled between the inner and outer shell membranes. Over time, an egg’s porous shell allows for a slow exchange of gases and moisture. As moisture evaporates and carbon dioxide escapes, the air cell expands. This enlargement increases the egg’s air volume, which reduces its average density and makes it more buoyant.
Interpreting the Egg Test Results
The “egg test” provides a practical way to assess an egg’s relative age based on its buoyancy. A very fresh egg will typically sink quickly to the bottom and lie flat on its side, indicating its high density and minimal air cell expansion. As an egg ages and its air cell grows, it may still sink but will stand upright on its larger end, with the smaller end pointing upwards. This position suggests the egg is older but still generally suitable for consumption. An egg that floats completely on the surface of the water has a significantly enlarged air cell, indicating it is quite old.
When an Egg is No Longer Safe
While an egg standing or floating in water indicates its age, this does not automatically mean it is unsafe to eat. The float test primarily assesses freshness, not spoilage. To determine if an egg is truly spoiled, one should rely on sensory indicators once the egg is cracked open. A spoiled egg will often emit a distinct, unpleasant sulfurous odor. Additionally, visual cues like discoloration of the yolk or white, or a slimy texture, also suggest that the egg should not be consumed.