Only bismuth and water share this characteristic. Density (the mass per unit volume or mass/volume) refers to how compact or crowded a substance is. For instance, the density of water is 1 g/cm³ (gram per cubic centimeter) or 1 kg/l (kilogram per liter); the density of a rock is 3.3 g/cm³; pure iron is 7.9 g/cm³; and Earth (as a whole) is 5.5 g/cm³ (average). Water as a solid (i.e., ice) floats.