The Floating Egg

Lying in the sea and reading a book at the same time – pictures you see often when people show you their photos from the Dead Sea. Only there can you lie on the water as if it were an air mattress. It works because the salt content of the water is extremely high.
In the Mediterranean, the salt content of water is about 3%. But in the Dead Sea, it is an incredible 30%! Only very few creatures can survive in such conditions. Which is where the name “Dead Sea” comes from. Without making swimming motions, people will sink in fresh water. And a fresh egg will sink to the bottom in fresh water as well.


The Floating Egg


The reason for this phenomenon is buoyancy: a body in water becomes lighter, precisely equivelant to the weight of the displaced water.

A 3 dm³ rock weighs about 7.5 kg in the air. In the water, it weighs 3 kg less, since it also displaces 3 dm³ of water (= 3 kg). So now it weighs only 4.5 kg.

If:

  • the weight is greater than the buoyancy (e.g. iron - 7.8 kg/dm³) , then the body will sink
  • the weight is less than the buoyancy (e.g. wood - 0.5 kg/dm³) , then the body will float
  • the weight is identical with the buoyancy (e.g. fish) , then the body will "hover" 

The egg has a density (=weight of 1 cm³) of about 1.1 g/cm³. Fresh water has a density of 1 g/cm³ --> the eggs sinks. Salt water from the Dead Sea has a density of ca. 1.3 g/cm³ --> the egg rises to the surface.