An egg that floats in boiling water can be an indication that it's not fresh. However, there are a few other possible explanations for what it means when eggs are floating while boiling in hot water. Density is the key reason and is a simple, scientific explanation for why eggs sink in fresh water but float in salted water.
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Eggs in Water Have Density and Buoyancy
An object will sink if it is denser than a buoyant force — in this case the water — and an object will float if its weight is less than the buoyant force. Because fresh water is less dense than salt water, the egg drops. Because salty water is more dense than the egg, it floats.
Why Else Do Eggs Float?
Salty water aside, there is a second reason that may cause an egg to float in boiling water. An egg can float in water when its air cell is sufficiently large enough to keep the egg buoyant. Soon after an egg is laid, it naturally cools, creating a small air space as its contents contract. The air cell is found at the larger end of the egg between the inner and outer membranes.
Why Do Rotten Eggs Float?
As an egg ages, this air cell grows larger as the yolk and white further contract. Therefore, an older egg is less dense and more likely to float in boiling water. Store-bought eggs can last several weeks beyond their sell-by date or expiration date. To reduce the chances of eggs going bad, keep them refrigerated.
Why Do Cracked Eggs Float?
If the egg's shell is cracked, egg white will leak out into the boiling water. This reduces the mass of the egg, making it easier for the less-dense egg to float. Some bits of boiled egg white will float around in the saucepan of boiling water, which is unattractive but harmless.
How to Boil an Egg
Take care not to crack the eggshell when lowering an egg into boiling water. It's best to use a spoon to gently lower the egg into the saucepan. Alternatively, start with the eggs covered by an inch of cold water and bring the water to a boil. There's no need to add salt to the water or to cover the saucepan. Hard-boiled eggs require 10 to 12 minutes in boiling water.
Floating Eggs Experiment
Young school children can safely observe a simple chicken eggs float test as a little experiment away from boiling water. The only items needed are an egg, a tall glass, water, table salt and a tablespoon.
- Fill a glass halfway to the rim with water.
- Place an egg in the glass of water. The egg will sink.
- Add table salt and stir.
- Keep adding and stirring 1 tablespoon of salt at a time until the egg floats. Three tablespoons of salt should be enough to make the average-size chicken egg float.
The experiment can also be conducted with two eggs and two separate glasses, one for the unsalted water and the other for the salted water.
The Sniff Test
If there's any uncertainty, test the quality of an egg before consuming it. Remember, rotten eggs have a very distinct odor. Just give a quick sniff when cracking an egg open to ensure its freshness, as rotten eggs can cause food poisoning. Any question about whether an egg is safe for eating will be readily resolved.