Density
Density is a property of all substances. It is a relationship between the substance's mass and
size; this relationship is described mathematically by the equation,
d = m/v
where d is the density, m is the mass of the substance, and v is the volume of the substance.
Density is an example of an intrinsic property, that is, a property that relates to the essential
nature or constitution of the substance. Some properties, called extrinsic properties, depend on
the amount of substance present, along with the essential nature or constitution of the
substance. In addition to density, we find that color, temperature, malleability (the ability to be
beaten into a flat sheet), and ductility (the ability to be drawn into a thin wire), are also examples
of intrinsic properties. Mass, volume, and electrical charge are extrinsic properties. Since
intrinsic properties are independent of the amount of the substance, they can be used to
classify or identify the substance.
Procedure:
Part I. Density of Liquids
1.
From your lab drawer, select four clean 10 X 75 mm test tubes. (Note that the 10 mm
and 75 mm are the width and length of the test tubes. The 10 X 75 mm size is the
smallest of the three sizes in your drawer.)
2.
Fill one test tube about 1/3 full with the liquid reagent labeled "A".
3.
Fill two test tubes about 1/3 full with distilled water to which a very small amount of
yellow food coloring has been added to give the water a color. (Unless otherwise
instructed, always use distilled water in your experiments, since tap water contains iron,
scale, and other contaminants which may affect your experiment.)
4.
Fill one test tube about 1/3 full with the liquid reagent labeled "B".
5.
Carefully pour the contents of the test tube containing reagent A into one of the test
tubes containing distilled water. Which liquid, A or the water, is more dense? Save the
contents of the test tube.
6.
Carefully pour the contents of the test tube containing reagent B into the other test tube
containing distilled water. Which liquid, B or the water, is more dense? Save the
contents of this test tube.
7.
In a 20 X 150 mm test tube, carefully (drop by drop) place two full droppers of the most
dense liquid down the side of the tube. Then add two full droppers of water down the
side of the tube. Then add two full droppers of the least dense liquid down the side of
the tube. Do you observe the three layers?
8.
When you are finished, pour the contents of all of the test-tubes into the “Hexane
Waste” bottle in one of the fume hoods.
Density
Part II. Density of Solids
1.
Obtain an unknown (record your unknown number!!!) from the stockroom window and
record the unknown's number.
2.
Determine the mass of the unknown by the following procedure:
a. Weigh a plastic weighing boat (which can be found wrapped in your towel) and
record the weighing boat's mass on the Report page.
b. Place the unknown sample in the weighing boat and record the mass.
c. By subtraction, calculate the mass of the unknown, and record the result.
You might think that it would be easier to simply place the sample directly on the
balance and weigh it. Maybe so for an inert solid, such as your sample, but one usually
weighs powders, crystals, etc., that may corrode an unprotected balance pan. Not only
would this shorten the service life of the balance, but the corrosion would contaminate
your sample. We therefore get into the habit of using a weighing boat or glass container
every time we weigh on the balance.
3.
Place 3 to 5 mL of distilled water into a graduated cylinder (you may need a larger
volume of water and larger graduated cylinder) and record the volume to the nearest
0.05 mL (this requires that you estimate the distance between the 0.2 mL marks on the
cylinder).
4.
Carefully add the unknown to the graduated cylinder, making certain that you do not
splash any water out of the cylinder as you add the unknown. Also ensure that you do
not break the glass cylinder by dropping the unknown directly into it. The unknown must
be completely submerged. Record the new volume.
5.
Calculate the volume of the unknown by doing the necessary subtraction. Record the
result.
6.
Calculate the density of the unknown and record the result.
7.
Use Section B of the CRC Handbook of Chemistry and Physics to identify your unknown
based on the density that you calculated. Your unknown metal should be one of the
following:
Mg Magnesium
Ca Calcium
Fe Iron
Zn Zinc
Li Lithium
Pb Lead
Au Gold
8.
Al Aluminum
Cu Copper
Ni Nickel
Sn Tin
Bi Bismuth
Ag Silver
U Uranium
Return your unknown to the storeroom.
Density
Part III. Penny-Power
In this section, you will be using some of the information and techniques from the previous parts
of the experiment to help you solve a problem. Imagine it is the year 2190, and many available
energy sources have been depleted. However, a new technology has evolved that uses certain
metals to generate the needed energy. Those metals are aluminum, zinc and lead.
In addition, the economic systems of the world have been unified into one currency, known as
the ‘plastic code.’ Bills and coins have not been produced for over 150 years, so copper has
become a ‘precious’ metal and of interest historically. Your team of scientists has discovered an
old bank vault with bags of pennies. You wish to find out if the pennies contain the metals that
are needed for the new technology. Since the coins are also of value as a historical collection,
you decide to use the property of density to identify the metal in the coins.
Procedure
1.
Obtain your bag of coins. You may also be supplied with samples of several metals to
use in your investigation.
2.
You and your partner must design an experimental procedure to solve the problem.
Note that you must not use color to eliminate any metals. It is possible that the pennies
are coated and so the property of color may not be an indication of the metal present.
3.
The design must include a clearly-numbered stepwise procedure and neatly recorded
data.
4.
After your plan has been approved by the instructor, follow it to solve the problem.
Density
Date _________________
Name _______________________________
PART I
1.
Which is more dense, Reagent A or water? _____________
What observation led you to this conclusion?
2.
What did you observe when you mixed reagent B and water?
3.
Rank the three liquids (A, B and water) in order of increasing density. Label the drawing
below as you would see them placed in a graduated cylinder.
_____________
(least dense)
_____________
______________
(most dense)
PART II
4.
Complete the following table. Remember to include units and to report numerical values
with the correct number of significant figures.
UNKNOWN # ____________
mass of weighing paper
mass of paper and
unknown
initial volume of water
volume of water and
unknown
mass of unknown
volume of unknown
5.
Show the setup and calculation for the density of your unknown (use rules for significant
figures).
Density
6.
Identify of your unknown:
Page of the CRC Handbook on which your unknown's density appears:
Record the color from the CRC listed for your unknown:
Does it match your sample?
Do you believe the density you have calculated was accurate enough to identify your
sample? Explain.
7.
Suppose bubbles of air stuck to the side of your unknown when it was submerged in the
water. Would the density of your unknown be more, less or the same as you have
calculated? Explain your answer in complete English sentences.
PART III
Penny bag # ___________
Instructor's initials __________
Write your stepwise procedure below and get your instructor's initials above before proceeding.
Record your data neatly in a table and show your calculations.
Density
8.
Complete the table using the CRC to find the densities.
ELEMENT
Aluminum
DENSITY
PAGE # OF CRC
Copper
Lead
Zinc
9.
Will you take your bag of pennies to the Historical Society or to the Energy Department?
Explain. (Recall that copper is a precious metal and of historical interest, while zinc is a
metal needed to generate energy.)