Absolute density

Absolute density is the magnitude that expresses the relationship between the mass and volume of a substance or solid object. Its unit in the International System is kilogram per cubic meter (kg/m³), although it is also frequently expressed in g/cm³. Density is an intensive quantity.

Relative density

The relative density of a substance is the relationship "Division (mathematical)") between its density and that of another reference substance; Consequently, it is a dimensionless magnitude (without units).

For liquids and solids, the usual reference density is that of liquid water at the pressure of 1 atm "Atmosphere (unit)") and the temperature of 4 °C. Under these conditions, the absolute density of distilled water is 1000 kg/m³, that is, 1 kg/dm³.

For gases, the usual reference density is that of air at a pressure of 1 atm and a temperature of 0 °C, which is 1.29 g/L, that is, 0.00129 g/ml.

Average density and point density

For a homogeneous system, the mass/volume expression can be applied to any region of the system, always obtaining the same result.

However, a heterogeneous system does not have the same density in different parts. In this case, the "average density" must be measured, dividing the mass of the object by its volume or the "point density" which will be different at each point, position or "infinitesimal" portion of the system, and which will be defined by:

However, it should be taken into account that the hypotheses of continuous media mechanics are only valid up to scales of , since at atomic scales the density is not well defined. For example, the size of the atomic nucleus is approximately That is, at the atomic scale the density is far from uniform, since atoms are essentially empty, with practically all the mass concentrated in the atomic nucleus.

Apparent density

Apparent density is a magnitude applied to materials of heterogeneous constitution, and among them, porous ones such as soil, which form heterogeneous bodies with interstices of air or another substance, so that the total density of a volume of the material is less than the density of the porous material if it were compacted. In the case of a material mixed with air we have:.

The apparent density of a material is not an intrinsic property of the material and depends on its compaction. The apparent density of soil () is obtained by drying a soil sample of a known volume at 105 °C to constant weight.

It should be considered that for soil samples that vary in volume at the time of drying, such as soils with a high 2:1 clay concentration, the water content that the sample had at the time of taking the volume should be expressed.

In construction, the apparent density of work elements is considered, such as a brick wall, which contains bricks, cement or plaster mortar and air voids (when the brick is hollow or perforated).

In general, the density of a substance varies when pressure or temperature changes, and with changes of state. In particular, it has been established empirically:

The effect of temperature and pressure on solids and liquids is very small, so typically the compressibility of a liquid or solid is 10 bar "Bar (unit of pressure)") (1 bar = 0.1 MPa) and the coefficient of thermal expansion is of the order of 10 K. The above considerations lead to the fact that an equation of state for an ordinary substance must satisfy the following restrictions:

On the other hand, the density of gases is notably affected by pressure and temperature. The ideal gas law mathematically describes the relationship between these three quantities:

where is the universal constant of ideal gases, is the pressure of the gas, its molar mass and the absolute temperature. That means that an ideal gas at 300 K (27 °C) and 1 atm "Atmosphere (unit)") will double its density if the pressure is increased to 2 atm while keeping the temperature constant or, alternatively, its temperature is reduced to 150 K while keeping the pressure constant. As can be seen, relations () are also satisfied in ().

Contenido

La densidad puede obtenerse de forma indirecta y de forma directa. Para la obtención indirecta de la densidad, se miden la masa y el volumen por separado y posteriormente se calcula la densidad. La masa se mide habitualmente con una balanza, mientras que el volumen puede medirse determinando la forma del objeto y midiendo las dimensiones apropiadas o mediante el desplazamiento de un líquido, entre otros métodos.

Los instrumentos más comunes para medir la densidad son:.

Otra posibilidad para determinar las densidades de líquidos y gases es utilizar un instrumento digital basado en el principio del tubo en U oscilante. Cuyo frecuencia de resonancia está determinada por los materiales contenidos, como la masa del diapasón es determinante para la altura del sonido[8]​.

Units

The most used units of measurement are:

In the International System of Units (SI):.

Other units to express density are:.

In the Anglo-Saxon system of units:.

Density of chemical elements

Density of chemical elements under laboratory conditions, expressed in g·cm (elements with a density greater than that of osmium or iridium have only a theoretical densityː super-heavy radioactive elements are produced in quantities too low or decay too quickly to allow measurement):.

Density of the elements at their melting point[9]​ in g·cmː.

Absolute density

Absolute density is the magnitude that expresses the relationship between the mass and volume of a substance or solid object. Its unit in the International System is kilogram per cubic meter (kg/m³), although it is also frequently expressed in g/cm³. Density is an intensive quantity.

Relative density

The relative density of a substance is the relationship "Division (mathematical)") between its density and that of another reference substance; Consequently, it is a dimensionless magnitude (without units).

For liquids and solids, the usual reference density is that of liquid water at the pressure of 1 atm "Atmosphere (unit)") and the temperature of 4 °C. Under these conditions, the absolute density of distilled water is 1000 kg/m³, that is, 1 kg/dm³.

For gases, the usual reference density is that of air at a pressure of 1 atm and a temperature of 0 °C, which is 1.29 g/L, that is, 0.00129 g/ml.

Average density and point density

For a homogeneous system, the mass/volume expression can be applied to any region of the system, always obtaining the same result.

However, a heterogeneous system does not have the same density in different parts. In this case, the "average density" must be measured, dividing the mass of the object by its volume or the "point density" which will be different at each point, position or "infinitesimal" portion of the system, and which will be defined by:

However, it should be taken into account that the hypotheses of continuous media mechanics are only valid up to scales of , since at atomic scales the density is not well defined. For example, the size of the atomic nucleus is approximately That is, at the atomic scale the density is far from uniform, since atoms are essentially empty, with practically all the mass concentrated in the atomic nucleus.

Apparent density

Apparent density is a magnitude applied to materials of heterogeneous constitution, and among them, porous ones such as soil, which form heterogeneous bodies with interstices of air or another substance, so that the total density of a volume of the material is less than the density of the porous material if it were compacted. In the case of a material mixed with air we have:.

The apparent density of a material is not an intrinsic property of the material and depends on its compaction. The apparent density of soil () is obtained by drying a soil sample of a known volume at 105 °C to constant weight.

It should be considered that for soil samples that vary in volume at the time of drying, such as soils with a high 2:1 clay concentration, the water content that the sample had at the time of taking the volume should be expressed.

In construction, the apparent density of work elements is considered, such as a brick wall, which contains bricks, cement or plaster mortar and air voids (when the brick is hollow or perforated).

In general, the density of a substance varies when pressure or temperature changes, and with changes of state. In particular, it has been established empirically:

The effect of temperature and pressure on solids and liquids is very small, so typically the compressibility of a liquid or solid is 10 bar "Bar (unit of pressure)") (1 bar = 0.1 MPa) and the coefficient of thermal expansion is of the order of 10 K. The above considerations lead to the fact that an equation of state for an ordinary substance must satisfy the following restrictions:

On the other hand, the density of gases is notably affected by pressure and temperature. The ideal gas law mathematically describes the relationship between these three quantities:

where is the universal constant of ideal gases, is the pressure of the gas, its molar mass and the absolute temperature. That means that an ideal gas at 300 K (27 °C) and 1 atm "Atmosphere (unit)") will double its density if the pressure is increased to 2 atm while keeping the temperature constant or, alternatively, its temperature is reduced to 150 K while keeping the pressure constant. As can be seen, relations () are also satisfied in ().

Contenido

La densidad puede obtenerse de forma indirecta y de forma directa. Para la obtención indirecta de la densidad, se miden la masa y el volumen por separado y posteriormente se calcula la densidad. La masa se mide habitualmente con una balanza, mientras que el volumen puede medirse determinando la forma del objeto y midiendo las dimensiones apropiadas o mediante el desplazamiento de un líquido, entre otros métodos.

Los instrumentos más comunes para medir la densidad son:.

Otra posibilidad para determinar las densidades de líquidos y gases es utilizar un instrumento digital basado en el principio del tubo en U oscilante. Cuyo frecuencia de resonancia está determinada por los materiales contenidos, como la masa del diapasón es determinante para la altura del sonido[8]​.

Units

The most used units of measurement are:

In the International System of Units (SI):.

Other units to express density are:.

In the Anglo-Saxon system of units:.

Density of chemical elements

Density of chemical elements under laboratory conditions, expressed in g·cm (elements with a density greater than that of osmium or iridium have only a theoretical densityː super-heavy radioactive elements are produced in quantities too low or decay too quickly to allow measurement):.

Density of the elements at their melting point[9]​ in g·cmː.