The density, or more precisely, the volumetric mass density, of a substance is its mass per unit volume. The symbol most often used for density is ρ (the lower case Greek letter rho). Mathematically, density is defined as mass divided by volume:
Different materials usually have different densities. Osmium and iridium are the densest known elements at standard conditions for temperature and pressure but certain chemical compounds may be denser.
Density may be relevant to buoyancy, purity and packaging. (In science, buoyancy is an upward force exerted by a fluid that opposes the weight of an immersed object. Packaging is the technology of enclosing or protecting products for distribution, storage, sale, and use.)
A graduated cylinder containing various coloured liquids with different densities.
To simplify comparisons of density across different systems of units, it is sometimes replaced by the dimensionless quantity "specific gravity" or "relative density", i.e. the ratio of the density of the material to that of a standard material, usually water. Thus a specific gravity less than one means that the substance floats in water.
The density of a material varies with temperature and pressure. This variation is typically small for solids and liquids but much greater for gases. Increasing the pressure on an object decreases the volume of the object and thus increases its density. Increasing the temperature of a substance (with a few exceptions) decreases its density by increasing its volume. In most materials, heating the bottom of a fluid results in convection of the heat from the bottom to the top, due to the decrease in the density of the heated fluid. This causes it to rise relative to more dense unheated material.