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What is DENSITY ?
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DENSITY is a physical property of matter, as each element
and compound has a unique density associated with it. Density
defined in a qualitative manner as the measure of the relative
"heaviness" of objects with a constant volume.
For example: A rock is obviously more dense than a crumpled
piece of paper of the same size.
A styrofoam cup is less dense than a ceramic cup.
Density may also refer to how closely "packed" or
"crowded" the material appears to be - again refer
to the styrofoam vs. ceramic cup.
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| Density Comparison to Water:
In chemistry, the density of many substances is compared
to the density of water. Does an object float on water or sink
in the water? If an object such as a piece of wood floats on
water it is less dense than water vs. if a rock sinks, it is
more dense than water. |
| Density examples: ***See top
menu bar for more examples*** |
| Oil and vinegar salad dressing:
The oil floats on the vinegar water mixture, while the solids
sink to the bottom. |
| Oil spills: What happens when an oil tanker
leaks on the ocean? The oil floats on the water since it is less
dense, and this provides some opportunity to clean up the oil
spills by skimming the oil from the surface of the water. |
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Ice: Everyone knows that ice floats on water, but did
you know that this is an abnormal physical property of solid/liquid
state of water? The more normal physical property is for the
solid of a compound to sink in its own liquid.
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Demonstrations
with Density |
Mysterious Ice
Layers of Liquids
Egg Densities - sugar water/oil
Smart Eggs - salt water and acid
Floating Eggs - sugar and water
Floating Spheres
Lava Lamp
Underwater Smoke Stack
Floating objects in water |
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Mathematical Definition of Density |
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The formal definition of density is mass per unit volume.
Usually the density is expressed in grams per mL or cc. Mathematically
a "per" statement is translated as a division. cc is
a cubic centimeter and is equal to a mL Therefore,
| Density = |
mass = |
g/mL |
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volume |
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Mass vs. Weight: Although the terms mass and weight
are used almost interchangeably, there is a difference between
them. Mass is a measure of the quantity of matter, which
is constant all over the universe. Weight is proportional
to mass but depends on location in the universe. Weight is the
force exerted on a body by gravitational attraction (usually
by the earth).
Example: The mass of a man is constant. However the
man may weigh: 150 lbs on earth, 25 lbs on the moon (because
the force of gravity on the moon is 1/6 that of the earth), and
be "weightless" in space.
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Densities of Common Elements and Compounds |
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Substance |
Density
grams per mL |
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Pine wood |
0.35 -0.50 |
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Water |
1.00 |
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Salt, NaCl |
2.16 |
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Aluminum, Al |
2.70 |
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Iron, Fe |
7.80 |
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Gold, Au |
19.30 |
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Mercury, Hg |
13.5 |
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In order to determine the density of an object, it is
necessary to know: the mass, the volume of the substance, and
the definition of density.
Density = mass (g)
volume (mL)
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| Example: Calculate the density
in g/mL of aluminum if a 50 mL block weighs135 g. |
Solution:
Apply the definition:
| Density = |
135 g = |
2.70 g/mL |
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50 mL |
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| If the density of a substance and
either mass or volume is known, volume or mass, respectively,
can be calculated using either simple algebra or dimensional
analysis. The density must be translated as a conversion factor. |
| Example: Calculate the mass in a 200
cc block of Titanium with a density of 4.51 g. per cc. |
Solution: The density translated as a conversion factor
is:
4.51 g = 1 cc - "per" is equivalent to an equal sign.
| 200 cc x |
4.51 g = |
902 g |
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1 cc |
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