Metric Density and Specific Gravity
One of the fundamental properties of any particular material is its 'heaviness'. In solids we think of materials like cork or styrofoam as being very 'light' and lead and iron as being 'heavy'. Actually we have to consider the volumes in this discussion. We could have a ton of styrofoam, or a half-gram of lead. It depends on how large a sample we are using.
To calculate 'heaviness' or density we divide the mass of material in grams by its volume in cubic centimeters. (We might just as well divide the mass in kilograms by the volume in liters, or the mass in metric tons by the volume in cubic meters, the answer in each case would be numerically the same.
The densities of ordinary substances here on earth (as opposed to such things as gases at very low pressures or white dwarf stars) vary from the least dense, hydrogen gas, with a density of 0.000 089 grams per cubic centimeter to the element osmium which, at 22 grams per cubic centimeter, is only slightly more dense than gold or platinum. Density is such a characteristic property of each substance that it may even be used to identify the substance. A student of mine once brought in a heavy metal bar which he thought looked like gold. Measurement of its volume and mass yielded a density of about 8, the density of brass. We always laugh at the old movies in which a stagecoach is being robbed of its gold shipment and the bad guys are tossing the boxes of gold around. An ordinary suitcase full of gold bars would have a mass of a ton or more. A bank once had a cubic foot of gold in its vault and offered to give it to anyone who could carry it out of the building. A cubic foot of gold is 1200 pounds, about the same as three steel oil drums filled with water.
Since the gram was defined as the mass of a cubic centimeter of water, water must by definition have a density of 1 gram per cubic centimeter. Like all substances, its density varies with changes in temperature, but its density is exactly 1 somewhere around 3 or 4 Celsius degrees above freezing.
Specific gravity is the heaviness of a substance compared to that of water, and it is expressed without units. In the metric system specific gravity is the same as in the British system. If something is about 7.85 times as heavy as an equal volume of water (such as iron is) its specific gravity is 7.85. Its density is 7.85 grams per cubic centimeter, or 7.85 kilograms per liter, or 7.85 metric tons per cubic meter.