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.
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 $\mathrm{\rho}$ abbreviated as rho. The formula used to estimate the Density of any material is given by
$$\mathrm{\rho =\frac{M}{V}}$$
Here $\mathrm{\rho}$ = 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 $\mathrm{kg\:m^{-3} }$ and CGS unit is $\mathrm{g\:cm^{-3} }$.
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.
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
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
$$\mathrm{SG=\frac{\rho _{sample\:material}}{\rho _{reference\:material}}}$$
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
$$\mathrm{SG_{true}=\frac{\rho _{sample}}{\rho _{water}}}$$
The apparent specific gravity is estimated by the formula
$$\mathrm{SG_{apparent}=\frac{W _{air,sample}}{W _{air,water}}}$$
It is a dimensionless quantity because it contains the ratio of same terms.
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.
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 $\mathrm{4^{\circ}C}$.
The Density of the sample is estimated by the formula
$$\mathrm{\rho _{sample}=SG\times \rho _{reference}}$$
| Parameter | Density | Specific gravity |
|---|---|---|
| Definition | It is the proportion of mass to volume. | It is the relative proportion of the Density of the sample to the Density of the reference. |
| Calculation | To calculate the Density of any material weight and volume of the substance must be known. | To calculate the specific gravity, the Density of the sample material is divided by the Density of the reference material. |
| Representation | It represents an absolute term. | It represents a relative term. |
| Units | Its unit is kg per meter cube | It is a dimensionless quantity. |
The terms specific gravity and Density are two confusing properties. But both play a vital role in the cases of liquids. This tutorial covers all the important information about Density and specific gravity.
Density shows how tightly the atoms are bonded with each other. And each substance has a different value of Density. We calculate the Density by dividing the mass by volume. Whereas specific gravity is the ratio of two densities, one a sample and the other reference material. We also discuss their applications and the relationship between them.
Q1. The water shows maximum Density at $\mathrm{4^{\circ}C}$. why?
Ans. At $\mathrm{4^{\circ}C}$ the molecules are becomes closer hence volume gets gets decreases and density increase. And below this temperature, the water will freeze and volume will increase hence the density decreases.
Q2. What happens to the least dense immiscible liquids?
Ans. If the liquids are immiscible and not stirred they will separate into two layers, with the less dense floating on the more dense liquid.
Q3.
Ans. Given,
$$\mathrm{\mathit{M}\:=\:1160\:\mathit{kg}}$$
$$\mathrm{\mathit{V}\:=\:1m^{3}}$$
We know that
$$\mathrm{\rho =\frac{M}{V}}$$
$$\mathrm{\rho =\frac{1160}{1}}$$
$$\mathrm{\rho =1160\:\mathit{kgm^{-3}}}$$
Q4. Which metal has the highest and lowest specific gravity?
Ans. Lead has the highest specific gravity and lithium has the lowest specific gravity.
Q5. Give one application for Density.
Ans. Ships and Submarines. One well-known application of Density is determining whether or not an object will float on water.
Q6. What is the application for specific gravity?
Ans. Specific Gravity is used to check on the progress of reactions and concentration of the solution. It is also used in auto mechanics to test battery fluid.
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