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. Mix the contents of the test tube containing reagent A with 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. Mix the contents of the test tube containing reagent B with the other test tube containing distilled water. What do you observe? 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 each of the three liquids down the side of the tube. Observe the three layers.
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 Al Aluminum
Ca Calcium Cu Copper
Fe Iron Ni Nickel
Zn Zinc Sn Tin
Li Lithium Bi Bismuth
Pb Lead Ag Silver
Au Gold U Uranium
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.
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 / initial volume of watermass of paper and unknown / 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).
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.
8. Complete the table using the CRC to find the densities.
ELEMENT / DENSITY / PAGE # OF CRCAluminum
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.)
1