Density is mass per unit volume — how much matter is packed into a given space. It is an intensive property, meaning it does not depend on how much of the substance you have. A gram of aluminium and a tonne of it have the same density.
Units and the convenient accident
The SI unit is kg/m³, but g/cm³ is used constantly because water is almost exactly 1.000 g/cm³ at 4 °C. That makes specific gravity — density relative to water — the same number, which is why the two get used interchangeably.
Floating and Archimedes
An object immersed in a fluid experiences an upward force equal to the weight of the fluid it displaces. Whether it floats therefore comes down to a comparison of densities. Ice at 917 kg/m³ floats in water at 1000 kg/m³, with 91.7% of its volume submerged — which is the origin of the iceberg cliché, and it is very nearly correct.
A steel ship floats because the relevant density is that of the whole hull including the air inside it, not of the steel. Change that average and the ship sinks.
Common densities
| Material | kg/m³ |
|---|---|
| Air (sea level) | 1.2 |
| Balsa | 160 |
| Ice | 917 |
| Water | 1000 |
| Aluminium | 2700 |
| Steel | 7850 |
| Lead | 11340 |
| Gold | 19300 |