A PENNY FOR YOUR THOUGHTS
The first people to scientifically study nature had no analytical balances, graduated cylinders, or other accurate measuring devices by which they could collect data. Those pioneers had only their five senses and a deep desire to understand the universe around them. Even without sophisticated instruments, however, much was discovered, and today, good scientists still use their five senses to make observations which lead to great advances in understanding the world in which we live. This laboratory experiment will give you experience observing some of the chemical and physical properties of a common object. From your observations, you will be able to draw significant conclusions about the composition of the object.
Before you can truly be successful in this experiment, you must clearly understand the difference between an observation and a conclusion. The two really are quite different things, although they are sometimes expressed in very similar ways. To a scientist, an observation is a record of any event or property which can be detected by either the five senses or a scientific instrument. A conclusion is a judgment about the meaning of one or more observations. An example may help to illustrate the difference:
You are the manager of the House of Pisces, a renowned seafood restaurant in large city. You are on duty on Tuesday evening, when John and Mary come into the restaurant for dinner. You watch as John and Mary are seated. You are pleased to note how elegantly the waiter (who you trained) poured their water and offered them menus. A few minutes later, the waiter returns to John and Mary's table: John has ordered oysters Rockefeller for the appetizer, followed by entrees of baked rockfish for John, and steamed Alaskan King crab (the restaurant's specialty) for Mary. The food soon arrives, and you smile to yourself, as you watch John and Mary laugh and talk with each other as they consume their meal in the candlelight. As you return to more pressing matters, you check the waiter's ticket book, and learn that John and Mary's meal will cost $68.74, Walking away, you say to yourself, "Those two really enjoy a good seafood dinner!"
In the scenario just described, many observations were made: John and Mary walked in, were seated, were given water and menus; both laughed and talked; they ordered a meal costing $68.74. One notable conclusion was drawn; that the two enjoy seafood dinners. That John and Mary enjoyed the seafood dinner was not observed! You made a judgment, based on the good service, the laughing and talking, and the willingness to pay $68.74, that John and Mary enjoyed their meal. Certainly there are other possibilities: John may hate seafood, but, because he wants to impress Mary (who likes seafood), he pretends to enjoy himself. Perhaps Mary has no sense of taste or smell, so that it doesn't much matter what she eats. She appears to enjoy the dinner because she and John are celebrating John's recent promotion.
The point is simple: observations provide the justification for conclusions that are drawn, and a reasonable conclusion can come about only by intelligent interpretation of one or more observations. As you perform this week's experiment, try to identify the conclusions that you make, and try to determine what it is about the conclusions that distinguished them from your observations.
Experimental Procedure
1.Bring to lab an American penny minted after 1982. Observe the penny, Is it solid, liquid, or gas? For a solid, you can ask more questions: Is it hard or soft? Is it dull or shiny? Is it brittle or flexible? What is its color?
2.Using a triangular file, mark the edge of the penny by firmly drawing the file against the edge of the penny at approximately a 45º angle. Consider the face of the penny to represent a clock. The first mark is at twelve o'clock. On the same side of the penny, make a second mark at six o'clock. Turn the penny over and make two additional marks at the three and nine o'clock positions. Now observe the penny. What color do you observe at the file marks? What can you conclude about the composition (makeup) of the penny? It should be silvery. Color, as we know, is an intrinsic property. Hence, we can conclude (not observe!) that a penny consists of two different metals. One metal is brown and the other is silver in color.
3.Weigh the penny on an analytical balance. Record the mass.
4.Place the penny in a clean 50 mL beaker. Label your beaker with your nameIn one of the fume hoods, add 10.0 mL of concentrated HCl (hydrochloric acid) to the beaker. (CAUTION: Acids and bases are dangerous reagents, especially in concentrated form. If you spill a small quantity of these reagents, clean up the area immediately by soaking a paper towel with water and mopping the area with the paper towel. Spills of 50 mL or more should be immediately reported to the instructor. Always wash these reagents off of your skin immediately with lots of tap water. Do not work with these reagents unless you are wearing safety goggles.
What can you say about the HCl reagent? Is it a solid, liquid, or gas? What is its color? Is the solution clear or cloudy? Does it have an odor? (CAUTION: never place a chemical directly under your nose to smell it. Instead, hold the chemical a few inches in front of you with one hand, and gently wave the fumes toward your nose with your other hand.)
Now look at the beaker and its contents. Would the bubbles be classified as solid, liquid, or gas? Did the HCl originally appear to have these bubbles?
A chemist could perform tests that would show that the gas is hydrogen, and that it has the chemical formula, H2.
Allow your penny to produce hydrogen for 20 minutes before you proceed to the next step.
5.Add 30 mL of distilled water to the beaker holding the penny. Transfer the solution (leave the penny behind) to a clean 150 mL beaker, and add another 30 mL of distilled water to the solution. Save the solution for Step 7. Make some observations on this solution.
6.Rinse the penny in tap water to remove all of the acid. Dry the penny and weigh it on an analytical balance. Record the mass. How much metal was dissolved by the acid?
The outer surface of the penny should be very flexible at each point where it was filed. Break off this outer layer and place these brown metal chips into a clean 100 mL beaker. In a fume hood add 10 to 15 drops of concentrated HNO3 (nitric acid) to the beaker. Try to dissolve as many of the brown chips as possible. Wait 5 minutes and then add 20 mL distilled water. Save this solution for Step 8.
7.Place 0.5 mL of the solution from Step 5 (the solution in the 150 mL beaker) into each of three 10 x 75 mm test tubes. Label the tubes A, B, and C. The easiest way to measure 0.5 mL is to determine how many drops from a medicine dropper is 1 mL, then count the drops for 0.5 mL.
Test Tube A: Add 0.5 mL of 1 M NH4C2H3O2, ammonium acetate solution*, followed by 0.5 mL of 0.2 M K2CrO4, potassium chromate solution, to the test tube. Record your observations. Formation of a yellow solid (a precipitate) would allow us to conclude that the metal in the penny that reacts with the HCl is lead (Pb).
* “1 M” is an abbreviation for "one molar," a measure of the concentration of the reagent.
Test Tube B: Add dilute (5 M) NH4OH (ammonium hydroxide) until the solution is basic. You can tell if the solution is basic by testing with litmus paper: a basic solution turns red litmus paper blue. Add 10 drops of the NH4OH, then perform the litmus test.
Litmus Test: Wet the red litmus paper with distilled water. Dip your spatula or a glass rod into the solution in the test tube. Touch the wet litmus paper with the tip of the spatula or glass rod. A blue color appears if the solution is basic.
If the solution is not basic, add 5 more drops of NH4OH and test again.
After the solution turns basic, add 2-4 drops of dimethylglyoxime solution to the test tube. The formation of a strawberry-red precipitate would allow us to conclude that the interior metal of the penny is nickel. Record your observations.
Test Tube C: Add 3-4 drops of K4Fe(CN)6 (potassium ferrocyanide) solution to the test tube. The formation of a grayish-white to bluish-green precipitate would indicate that the penny's interior metal is zinc. Record your observations.
Can we now conclude which metal reacted with the HCl and, therefore, which metal made up the interior of the penny?
8.To half of the solution from Step 6 (100 mL beaker), add concentrated (15 M) NH4OH until the solution is basic. Ignoring any undissolved metal chips, record your observations.
Any copper present will react with the NH4OH to form a blue solution. To the other half of the solution, add 2-4 drops of K4Fe(CN)6 solution. The formation of a red to reddish-brown precipitate the presence of copper metal in solution. Record your observations.
9.Wash all of your solutions down the sink drains with large amounts of water and return the cleaned glassware to your drawer.
Report Format
Date ______
Names ______
______
Tape
Penny
Here
Using a transparent tape, tape your penny to this report in the box. Your penny's evidence that you performed the experiment!
Provide the responses requested for each step in the procedure, as indicated.
Step / Substance Observation(s) / Observation(s) / Conclusions(if any)
1 / Penny before filing
2 / Penny after filing
3 / Penny after filing / Mass = g
4 / HCl reagent
4 / Contents of 50 mL beaker
5 / Solution in 150 mL beaker
6 / Dried penny / Mass = g
6 / Contents of 100 mL beaker as chips dissolved
7 / Contents of Test Tube A
7 / Contents of Test Tube B
7 / Contents of Test Tube C
8 / Solution mixed with NH4OH
8 / Solution mixed with K4Fe(CN)6
Final Conclusion:
A penny made in the year ______consists of two types of metal, one of which has a ______color and the other which is a ______color. From the observations gathered in this experiment, we can conclude that a penny has an outer covering ______of and an inner core of ______.
Answer the following questions:
1.In step 8 of the procedure, the solution made in step 6 was divided into two portions, and each portion was tested. Suggest a reason for performing step 8 this way, rather than doing a single test on all of the solution produced in step 6.
Revised Ali/Fall 2006