The intensive physical property that relates the mass of an object to its volume is called density. Density, which is simply the mass of an object divided by its volume, is expressed in the SI derived unit g/ml for a liquid or for a solid. The densities of some common materials are given in the fallowing table.
Because most substances change in volume when heated or cooled, densities are temperature dependent. At 3.98 °C, for example, a 1.0000 mL container holds exactly 1.0000 g of water (density = 1.0000 g/mL). As the temperature is raised, however, the volume occupied by the water expands so that only 0.9584 g fits in the 1.0000 mL container at 100 °C (density = 0.9584 g/mL) When reporting a density, the temperaturemust also be specified.
Although most substances expand when heated and contract when cooled, water behaves differently. Water contracts when cooled from 100 °C to 3.98 °C, but below this temperature it begins to expand again. Thus, the density of liquid water is at its maximum of 1.0000 g/mL at 3.98 °C but decreases to 0.999 87 g/mL at 0 °C. When freezing occurs, the density drops still further to a value of 0.917 g/cm3 for ice at 0 °C. Ice and any other substance with a density less than that of water will float, but any substance with a density greater than that of water will sink.
