Density - Definition, Applications and SI Units of Density

Density

The density of a substance is defined as the amount (mass) of the substance per unit volume. It is often denoted using the Latin letter D or ρ; the lower case Greek letter pronounced rho. The formula of density is given by ρ = m/v, where m is for the mass of the substance and v is for its volume present. From the equation, it is clear that density is directly proportional to mass and indirectly proportional to volume.

Factors that affect the density values of a substance are pressure and temperature. Increasing the pressure and decreasing the temperature will bring up the density value of a substance as the volume is reduced by these changes. Similarly, by increasing the temperature and decreasing the pressure, the density value can be brought down as the volume value goes up. The effect of temperature and pressure on the density of a substance can be significantly noticeable in gasses when compared with solids or liquids. Generally, the densest materials are solids, followed by liquids and then gasses, in that specific order. This is because solids, by nature, have much firmly packed, tight molecules, and on the other end, gases have much more loose, freely packed molecules, and liquids lie in the middle in terms of how closely the molecules in it are packed.

In the case of a pure substance that does not contain any impurities, the value of density is equal to the mass concentration of that substance. Different materials typically have their very own value of density, depending on that material’s buoyancy, purity and packaging. In some cases, the reciprocal of the density of a substance is called the specific volume of that substance. The specific volume of a substance is defined as the total volume occupied by a certain mass of that substance. The specific volume of a substance is an intrinsic property, while the density of the substance is an intensive property.

Units of Density

Since the formula for the density of a substance is the mass of that substance divided by the volume of that substance, density can technically be expressed in terms of any unit of mass divided by any unit of volume. However, for standardization, the SI unit of density, kilogram per cubic meter (kg/ m 3 ), is the most widely used unit. In addition, the cgs (centimeter gram second system of units) unit of gram per cubic centimeter (g/cm 3 ) is also used prominently. But since the measured density of a sample of any substance can range from very small values to significantly high values, it is more practical to use an appropriate unit of mass and volume to arrive at the best unit of density specific to that industry. Some other units of density are listed below.

• gram per milliliter (g/mL)
• metric ton per cubic meter (t/m 3 )
• kilogram per liter (kg/L)
• kilogram per cubic decimeter (kg/dm 3 )

Applications of Density

The knowledge of the density of different substances is very handy and serves a great deal of purpose in various fields. Some are discussed below.

• Whether an object will float on water or just sink is influenced by one major factor, density. If the overall density of the object is less than that of the surrounding water, it will float on the surface. On the other hand, if the overall density of the object is more than the water, it will not be able to float and sink. This principle is very important in the construction of ships and submarines. Submarines have ballast tanks, and ships have huge, hollow cavities called the hull. The hull and the ballast tanks have one purpose, to store large volumes of air. This enables the ships and submarines to have an overall relatively low density, compared with the surrounding water, despite the use of heavy materials like iron, steel etc., in their construction. Ballast tanks in submarines have an added feature in that they can release the air and fill up with water on command so that the submarine can go below the surface when needed.
• The density values of different materials come into play during separation techniques. In the case of oil spills in the ocean, the fact that oil is less dense than water is very useful in the cleanup process because oil tends to float on the surface of the water, making the cleanup process much easier.
• When designing the plumbing systems for any type of building, be it a household or a commercial establishment, considering the liquid's density is useful in designing an effective plumbing system.)