What Does Mass Density Mean?
The mass density of an object is defined as its mass per unit volume. This parameter can be expressed using several different units, including kilograms per meter cubed (kg/m^{3}) and pounds per square foot (lb/ft^{2}). Mass density is represented by the lowercase Greek letter rho, ρ, although the Latin letter 'D' has also historically been used.
Mathematically, mass density is expressed using the following formula:
ρ = M/V
Where:
ρ = mass density
M = mass of the fluid or solid
V = the volume of the fluid or solid
Mass density is a representation of the amount of mass (or the number of particles) of a substance, material or object in relation to the space it occupies. However, this measurement is not necessarily constant; mass density depends on several factors, including temperature and pressure.
For example, gases expand when they are heated. In other words, their volume increases with increasing temperature. Based on the formula above, if the volume of the gas expands, its density decreases.
Mass density can also be related to an object's level of corrosion. As iron corrodes, the metal oxidizes to form rust, which occupies more space than the original metal. This expansion and reduced density can lead to several defects, such as cracking and spalling in concrete.
Mass density is known by several different terms, including density, volumetric mass density and specific mass.
Corrosionpedia Explains Mass Density
To simplify the comparison of densities of different materials across different units of measurement, other terms such as relative density or specific gravity are used. These dimensionless parameters represent the ratio of the mass density of a material to that of a standard material, typically water. Therefore, if the relative density of an object is less than one, that means that it is less dense than water and will likely float.
As mentioned previously, density varies depending on environmental temperature and pressure. As such, densities for various materials and elements are usually measured and recorded at standard conditions for temperature and pressure (STP), i.e., 273.15°K (0.0°C) and 100 kPa (0.987 atm).
Increasing the pressure on an object or substance almost always increases its density. For example, a sponge ball starts with a particular density. If you were to squeeze the ball in your hand, its weight remains the same; however, its volume changes as it is being compressed. As the same mass is contained in a smaller volume, based on ρ = M/V, its density will increase.
The inverse is true for the density of materials subjected to changing temperature. Increasing the temperature of a substance almost always decreases its density due to expansion and increasing volumes. There are, however, a few notable exceptions. For example, water increases in density between its melting point at 0°C and 4°C.
Densities of Some Common Materials
Listed below are the densities for common elements and materials. Note that the following values are given at standard temperature and pressure (STP) conditions.
Material  Density, ρ (kg/m^{3})

Water
 1000 
Concrete  2400 
Steel  7,750  8,050 (depends on alloying elements)

Hydrogen
 0.0898 
Air (sea level)  1.2 