Specific gravity is a measure of density. Determining the specific gravity of an object can tell you a lot about it, like whether it will float or sink in water. Usually specific gravity refers to an object's density when compared with the density of water, so this value is a ratio.
The specific gravity of a substance is calculated by dividing the specific gravity of that substance by the specific gravity of water. Thus, the specific gravity of pure water would be a number divided by itself, which will always equal one.
The specific gravity of water depends on the type of water being measured. Pure water at 4 degrees Celsius has a specific gravity of one. If the water has salts in it, the water is denser and the specific gravity will be greater than one. Sea water, for instance, is denser than fresh or pure water. Water with a specific gravity of one has a weight of one gram per one milliliter.
Anything with a specific gravity less than 1 (indicating it is less dense than water) will float on water. Because specific gravity is temperature dependent (the lower the temperature the less the density), the specific gravity of ice is less than that of liquid water. This makes ice float. This is important because, for example, if ice did not float, lakes and other bodies of water in northern climates would be frozen over much of the year.
Specific gravity has many uses. In the process of beer brewing, for instance, alcohol decreases the specific gravity of the wort (the beer before it is carbonated or alcoholic). The amount of alcohol in the beer is a measure of the change in specific gravity over the course of fermentation. Medically, the specific gravity of urine is measured as a part of a routine health work-up. A high specific gravity may be due to something as simple as dehydration (which would make the urine more concentrated) to something as serious as heart failure.
A hydrometer is a tool that determines specific gravity. This tool consists of a small tube, closed on each end and shaped like a fishing bobber. The hydrometer floats in the liquid being measured, and the level of the fluid is read on a scale. In water (at 4 degrees Celsius) the hydrometer should read 1.0.