What is Density?
“Mass per unit volume of a material is called density.”Density is an important property of any material.A homogeneous material such as ice or iron has the same density throughout.We use (the Greek letter rho) for density.If a mass m of homogeneous material has volume V ,the density is:
Two objects made of the same material have the same density even though they may have different masses and different volumes.That’s because the ratio of mass to volume is the same for both objects.
Units for Density:
The S.I unit of density is the kilogram per cubic meter (1kg/m3).The cgs unit,the gram per cubic centimeter (1g/cm3),is also widely used:
The densities of several common substances at ordinary temperatures are given in table.
The densest material found on earth is the metal osmium (22,500 kg/m3),but its density pales by comparison to the densities of exotic astronomical objects such as white dwarf stars and neytron stars.
The specific gravity of a material is the ratio of its mass/volume to the mass/volume of water at 4.0c0,1000 kg/m3,it is a pure number without units.For example ,the specific gravity of aluminium is 2.7.”Specific gravity” is a poor term ,since it has nothing to do with gravity;’relative density’ would have would have been better.
The mass/volume of some materials varies from point to point within the material.One example is the material of the human body,which includes low density fat (about 940 kg/m3) and high density bone (from 1700 to 2500 kg/m3).Two others are the earth’s atmosphere (which are denser at greater depths).For these materials equation (1) describes the average density.In general ,the density of a material depends on environmental factors such as temperature and pressure.
Measuring mass/volume is an important analytical technique.For example,we can determine the charge condition of a storage battery by measuring the density of its electrolyte,a sulphuric acid solution.As the battery discharges ,the sulfuric acid combines with lead in the battery plates to form insoluble lead sulfate,decreasing the concentration of the solution.The density decreases from about 1.30 3 kg/m3 for a fully charged battery to 1.15 3 kg/m3 for a discharged battery.
Another automotive example is permanent type antifreeze,which is usually a solution of ethylene glycol (3 kg/m3 ) and water.The freezing point of the solution depends on the glycol concentration,which can be performed by using a device called a hydrometer.