SPECIFIC GRAVITY OF SOLIDS
 

Another important characteristic of a solid is its density relative to that of pure liquid water at 4°C (about 39°F). Water is at its most dense at this temperature and is assigned a relative density of 1. Substances with relative density greater than 1 will sink in pure water at 4°C, and substances with relative density less than 1 will float in pure water at 4°C. The relative density of a solid, defined in this way, is called the specific gravity.You often will see this abbreviated as sp gr. It is also known as relative density. You certainly can think of substances whose specific gravity numbers are greater than 1. Examples include most rocks and virtually all metals. However, pumice, a volcanic rock that is filled with air pockets, floats on water. Most of the planets, their moons, and the asteroids and meteorites in our solar system have specific gravities greater than 1, with the exception of Saturn, which would float if a lake big enough could be found in which to test it!

Interestingly, water ice has specific gravity of less than 1, so it floats on liquid water. This property of ice is more significant than you might at first suppose. It allows fish to live underneath the frozen surfaces of lakes in the winter in the temperate and polar regions of the Earth because the layer of ice acts as an insulator against the cold atmosphere. If ice had specific gravity of greater than 1, it would sink to the bottoms of lakes during the winter months. This would leave the surfaces constantly exposed to temperatures below freezing, causing more and more of the water to freeze, until shallow lakes would become ice from the surface all the way to the bottom. In such an environment, all the fish would die during the winter because they wouldn’t be able to extract the oxygen they need from the solid ice, nor would they be able to swim around in order to feed themselves. It is difficult to say how life on Earth would have evolved if water ice had a specific gravity of greater than 1.