Density Facts

So in the end, in relation to the floatability of a body the most important thing is not the absolute density, but the relative density of the object and the fluid in which the object is immersed. For example, the wood is a dense object and heavy in the air, but it is not if its in the water, because the density of the water is bigger than the wood’s.

This concept is used in many aspects of our life. The most common gases used are fuels, butane, and natural gas, and the most common residue from combustion, the carbon dioxide, all are denser than air, therefore they do not float in it, but occupy the lowest part of their location. The vents on kitchens and other places where these gases are used or produced are located in the bottom part of doors and walls that lead outside to allow this dense gases to flow out. The vents located in the upper part allow the entrance of new fresh air.

Salty water

The dissolution of Salt in the water increases its density. This is the reason for the sea water has more density than river water. The difference is not that big, but enough to allow some objects with density almost the same as river water to float in sea water.

Couple of tourists in the Dead Sea. They are no sitting down on the bottom: they float because of the high salinity

The composition of sea water is not the same in every part of the world. In closed seas, or almost closed, and medium depth, as the Mediterranean, the Black Sea or the Dead Sea, the higher evaporation of the water produces bigger concentrations of salt and therefore increased density. The open and deep oceans like the Atlantic or Pacific have waters with less density.

Swimming in the Dead Sea

 An extreme case of high density caused by high salt concentrations is the Dead Sea. The dead Sea is, in reality, a salty lake of about 80 Km long and 17 Km wide, located in Israel and Jordan. Its depth is low, about 40 m average, and it changes a lot depending on the seasons. The low depth and the fact that its surface is located about 390 m under any other ocean and sea has favored the evaporation of the water for thousand of years, producing a high concentration of salt like any other sea. The density of these waters is high, so floating in them is quite easy. In fact the hard part in this sea is to drown, but at the same time these conditions have banished any kind of vegetal or animal life form in it.


Source: Ciencia Recreativa (1992) - Planeta Agostini