Difference Between Density and Specific Gravity

Introduction

Whenever we do some work experiment with water or any liquid, we observe that some objects are immersed in water whereas some float over it. For example, ice cubes float on water. Because ice cubes are less dense than water. But if we put a stone in water it will sink. Because stone is denser than water. This is happen because every substance has a different density. Specific gravity is very closely related to Density. And it is also known as relative Density. At the industry level, various techniques, like hydrometer, hydrostatic balance, pycnometer, etc., are used to calculate the Density and specific gravity. In this tutorial, we briefly discuss Density and specific gravity.

What is the definition of Density?

Density is defined as the proportion of mass to the volume and it is as known as the specific mass. It is denoted by a greek letter ρρ abbreviated as rho. The formula used to estimate the Density of any material is given by

ρ=MVρ=MV

Here ρρ = Density

M is mass,and V is Volume.

Density depends on the composition of materials. And we know that matter exists in three forms i.e. solids, liquids, and gases. The composition of these states is different from each other. So every material has a different value of Density. The solid material is denser than liquid followed by gases. Because the atom is tightly bonded is solid and very loosely bonded in gases, whereas in liquid the distance between atoms is more than in solids.

The S.I. unit of Density is kgm−3kgm−3 and CGS unit is gcm−3gcm−3.

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Factor affecting the Density of the Material

Temperature: When any substance’s temperature increases, the molecules’ kinetic energy increases. As a result the Density of material s decreases.
Pressure: when pressure is increased, the atom of the material became closer and the Density of the material increases.

Measurement of Density

Density of a liquid is estimated by using an instrument called a hydrometer. It almost looks like a thermometer with a weighted bulb at the bottom. This weighted bulb helps the hydrometer to float in the liquid. Some marked lines are present on the hydrometer that indicate the Density of a particular liquid. If the hydrometer floats lower, it means the liquid is less dense. If the hydrometer floats higher, it means the liquid is denser. Some other applications of hydrometer.

Used to measure the salinity of water
Used in dairy farms to check the fat of the milk
Used to check the sugar level in beers and wines

What is meant by Specific Gravity?

Specific gravity is defined as the proportion of Density of sample material to the Density of reference material. It is also known as relative Density. It is estimated by the formula

SG=ρsamplematerialρreferencematerialSG=ρsamplematerialρreferencematerial

To calculate the specific gravity of liquids, water is taken as a reference liquid. Whereas in gases air is taken as a reference gas.

By estimating the specific gravity, we found the following conclusion

If the material has a specific gravity of less than one, it will float in water.
If the material has specific gravity more than water. It will immerse in water.
If the value of specific gravity is equivalent to one, it means two materials of equal volume carry the same mass.

The true specific gravity of liquids is estimated by the formula

SGtrue=ρsampleρwaterSGtrue=ρsampleρwater

The apparent specific gravity is estimated by the formula

SGapparent=Wair,sampleWair,waterSGapparent=Wair,sampleWair,water

It is a dimensionless quantity because it contains the ratio of same terms.

Applications of specific gravity

In physics, it is used to check whether the body will float or sink in the reference material
It is also used in the pharmaceutical industry to prepare TPN admixture.
The phase relationship of soils is also estimated using specific gravity.

Relation between Specific gravity and Density

We know that Density and specific gravity are different terms because the first term has dimension and the later one is dimensionless. But it becomes equivalent at three conditions given below

When Density is computed in kg per liter, grams per liter, or grams per cubic centimeter.
When both quantities are computed at the same temperature
When the Density of the referenced liquid is 4∘C4∘C.

The Density of the sample is estimated by the formula

ρsample=SG×ρreferenceρsample=SG×ρreference