DENSITY
A material's density is defined as its mass per unit volume.
It is, essentially, a measuremement of how tightly matter is crammed together.
The principle of density was discovered by the Greek scientist Archimedes.
To calculate the density (usually represented by the Greek
letter "rho") of an object, take the mass (m) and divide by the
volume (v):
rho(ρ)
= m / v
The SI unit of density is Kg/cubic meter (kg/m 3 ). It is
also frequently represented in the cgs unit of grams per cubic centimeter (g/cm
3 ).
Using Density
One of the most common uses of density is in how different
materials interact when mixed together. Wood floats in water because it has a
lower density, while an anchor sinks because the metal has a higher density.
Helium balloons float because the density of the helium is lower than the
density of the air.
When in automotive service station tests the antifreeze,
they will pour some into a hydrometer. The hydrometer has several calibrated
objects, some of which float in the liquid. By observing which of the objects
float, it can be determined what the density of the liquid is ... and, in the
case of antifreeze, if the chemicals are still strong enough to retain the
necessary freezing point.
Another important consequence of density is that it allows
you to solve for mass and volume, if given the other quantity. Since the
density of common substances is known, this calculation is fairly
straightforward, in the form:
v x (ρ)rho = m
or
m / (ρ)rho = v
The change in density can also be useful in analyzing some
situations, such as whenever a chemical conversion is taking place and energy
is being released. The charge in a storage battery, for example, is an acidic
solution. As the battery discharges electricity, the acid combines with lead in
the battery to form a new chemical, which results in a decrease in the density
of the solution. This density can be measured to determine the battery's level
of remaining charge.
Density is a key concept in analyzing how materials interact
in fluid mechanics, weather, geology, material sciences, engineering, and other
fields of physics.
Specific Gravity
A concept related to density is the specific gravity (or,
even more appropriate, relative density ) of a material, which is the ratio of
the material's density to the density of water . An object with a specific
gravity less than 1 will float in water, while a specific gravity greater than
1 means it will sink.
Our experiment
The same amount of two different liquids will have different
weights because they have different masses. The liquids that weigh more (have a
higher density) will sink below the liquids that weigh less (have a lower
density).
To test this, you might want to set up a scale and measure
each of the liquids that you poured into your column. Make sure that you
measure the weights of equal portions of each liquid. You should find that the
weights of the liquids correspond to each different layer of liquid. For
example, the honey will weigh more than the Karo syrup. By weighing these
liquids, you will find that density and weight are closely related.
Material ..................Density
Rubbing Alcohol ......0,79
Lamp Oil ..................0,80
Baby Oil ...................0,83
Vegetable Oil .............0,92
Ice Cube ...................0,92
Water .......................1.00
Milk .........................1.03
Dawn Dish Soap.......1.06
Light Corn Syrup..... 1.33
Maple Syrup............ 1.37
Honey...................... 1.42
** NOTE:
The numbers in the table are based on data from
manufacturers for each item. Since each manufacturer has its secret formula,
the densities may vary from brand to brand. You’ll notice that according to the
number, rubbing alcohol should float on top of the lamp oil, but we know from
our experiment that the lamp oil is the top layer. Chemically speaking, lamp
oil is nothing more than refined kerosene with coloring and fragrance added.
Does every brand of lamp oil exhibit the same characteristics? Sounds like the
foundation of a great science fair project.
The table shows the densities of the liquids used in the
column as well as other common liquids (measured in g/cm3 or g/mL).
Density is basically how much "stuff" is smashed
into a particular area... or a comparison between an object's mass and volume.
Remember the all-important equation:
Density = Mass divided by Volume. Based on this equation, if the weight
(or mass) of something increases but the volume stays the same, the density has
to go up. Likewise, if the mass decreases but the volume stays the same, the
density has to go down. Lighter liquids (like water or rubbing alcohol) are
less dense than heavy liquids (like honey or Karo syrup) and so float on top of
the more dense layers.
