JANET: The density of a substance is defined
as the mass of the substance
per unit volume of the substance.
And this is given in units
such as grams per centimeter cubed,
kilograms per meter cubed
or pound mass per feet cube.
And we use the Greek symbol Rho to designate density.
So density is often used as a conversion factor
relating mass to volume.
For example, if you're given the volume of a substance
we can use the density of it to come up with the mass.
So if Rho is equal to the mass times the volume
then the mass is gonna be equal to the density
or Rho times the volume.
And you can do at the same way
if you know the mass and the density
you can find the volume.
So another property of a substance is it specific volume.
And it usually is indicated by this
since specific refers to per unit mass,
sometimes per unit mole
but here we're gonna look at it per unit mass.
The specific volume is gonna be equal
to the total volume divided by its mass.
And here the units are going to be things like
this volume centimeter cubed divided by g or grams.
And as you can see this specific volume
is equal to the inverse of the density.
Where do we see this specific volume
is often used in thermodynamics
where for example, steam tables use
specific volume as a property.
So where do you find density values?
Well, you can use a standard reference book
or you can go online and type in
what substance you're looking for the density of.
However many tables provide what's known
as the specific gravity of a substance.
So that's abbreviated capital S, capital G
and the specific gravity of a substance
is equal to the density of whatever the substance is
divided by the reference density.
So the most common reference densities we have.
So for solids and liquids
what we use is water at four degrees C.
And so therefore our reference density
is gonna be equal to 1.00 grams per centimeter cubed
or 1000 kilograms per meter cubed or in English
or American units 62.43 pound mass per feet cubed.
And as you can see by this hopefully
that the specific gravity is unitless.
Note in particular this reference density
because many students think the reference density
should be one kilogram per meter cube
rather than 1000 kilograms per meter cube.
If you want you can do the conversion
to prove to yourself that it actually is 1000.
Those are the reference densities
for liquids and solids,
for gases it's generally dry air at 20 degrees C
which is equal to 1.203 kilograms per meter cubed.
In order to find the density of a substance
you multiply the tabulated specific gravity
times whatever your reference density is.
For example, let's look at the specific gravity of mercury
and at four degrees C it's equal to 13.6
and remember that is unitless.
So if we wanna find the density of mercury
in grams per centimeter cubed at four degrees C.
We just multiply 13.6 times the reference density
and again I'm using this one
because we want it in grams per centimeter cubed.
And we find that the density
is 13.6 grams per centimeter cubed.
One thing to be careful of
is that density is a function of temperature.
Therefore it is important to use
the correct specific gravity
for your particular temperature.