What Do You Think? Part A: Stamping a Serial Number

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1 CSI Chemistry Activity 7 Serial-Number Etching GOALS In this activity you will: Apply the chemical and physical properties of metals to serialnumber stamping and recovery. Explain what happens at the nanoscopic level when serial numbers are stamped and recovered. Use and create models to represent the nanoscopic physical and chemical changes of metals. What Do You Think? All matter is made of atoms. Metals are matter, so logically they are also made of atoms. Keep this in mind as you respond to the next two statements. What do you think happens to the atoms in a piece of metal when a serial number is stamped into the metal? Without referencing any sources, draw a picture to illustrate your idea. Record your ideas about these questions and draw the picture in your log. Be prepared to discuss your responses with your small group and the class. Investigate In this activity you will use the chemical and physical properties of metals to investigate the science behind serial-number recovery. Part A: Stamping a Serial Number 1. Obtain a rectangular piece of aluminum metal approximately 3 inches long, 1/2 inch wide, and 1/8 inch thick. 650

2 Activity 7 Serial-Number Etching 2. Before proceeding consider what will happen if you hit the die too hard. If you hit the die too hard it will take a long time to obliterate the number. If you hit the die too softly, it will not penetrate far enough under the metal s surface to allow the number to be restored later. Your instructor may have you practice stamping a scrap piece of aluminum a few times. 3. Using a hammer and dies, carefully stamp eight different numbers into the aluminum bar to create a serial number like the one in the diagram. Do not use the same number more than twice. Also, make sure to hit the die evenly. Important: Record the serial number you have stamped and its position on the aluminum bar in your log because you may not remember these things exactly later on. 6. Turn your sample in to your teacher so that the serial numbers can be ground off. Part B: Etching to Recover Serial Numbers 1. Examine the front of the aluminum piece under a hand lens or stereoscope. a) Sketch your observations in your log. 2. You must first prepare the surface for the etching process by sanding the surface until it is perfectly smooth. Any rough surface will cause the powerful etching solutions to pool, giving you poor results. Sand the surface first using 100-grit sandpaper, followed by 150-grit, and finally 220-grit, wiping the surface with a damp towel between each of the grits. Sand the bar in an up and down motion as shown in the diagram. Safety goggles and a lab apron must be worn at all times in a chemistry lab. Report any spills of the etching solution to your teacher immediately. 4. In the upper left corner, stamp your initials or your group s identification number by hitting it extremely hard with the hammer. Look at the back side of the metal where you put your initials. What do you see? Try to explain why this has happened. a) Record your observations and your answers in your log. 5. Record the number or combination of letters written on the back of your piece of aluminum using a black marker. This number is there for your instructor to match and for you to know which side formerly had the serial number! 3. Again examine the front of the aluminum piece under a hand lens or stereoscope. Even at this early stage, you may start to see the beginnings of a serial number. a) Sketch your observations in your log. 651

3 CSI Chemistry The etching solution contains a concentrated strong acid, so you must be careful not to get any on your skin or clothing. 4. Using gloves, wipe the metal thoroughly with a cotton ball soaked in ethanol. This removes all of the oils from the surface. Oil and the aqueous solutions you ll use do not mix. A layer of oil will keep the etching solution from reacting with the surface of the metal evenly, so it is very important to keep the surface oil free. 5. Still using gloves (keep them on throughout the first portion of this activity), use a cotton ball to evenly apply the etching solution of hydrochloric acid and iron (III) chloride. 6. Repeat Step 5 every two minutes. 7. This process is not quick, so be patient. While working on the applications (which may take up to 40 minutes), begin the next portion of the laboratory activity. Occasionally you may want to look at the metal with a hand lens or under the stereoscope to see what is happening. Be sure that the gloved person uses only the etching solution and touches the bar. The gloved person should not touch the material in the next portion of the lab. 8. When you are finished, rinse the bar in a solution of sodium bicarbonate. a) Sketch your observations in your log. b) What happened? Were you able to discern the serial numbers? How long did it take? Record your answers in your log. c) A metal is made of atoms packed in a regular repeating pattern, called a lattice. Based on the figure and your observations, what do you think happened to the atoms when you hit the die to create a serial number? What do you think the solution did to restore the serial number? Part C: Making a Model 1. Obtain two different colored blocks of modeling clay. Working on a piece of wax paper, cut 20 thin layers, approximately 1 mm thick (about the thickness of a penny), from the rectangular end of each block of clay. Each of these layers will represent a layer of metal atoms in the sample. 2. Using ten layers of each color, create two stacks of alternating colors. 3. Take the wooden dowel and carefully roll out each stack so that it is about 3 4 of its original thickness. Carefully trim the edges so that you can see each layer and so it is rectangular again. 652

4 Activity 7 Serial-Number Etching 4. Cut each rectangle into two squares. You have now created four copies of a simple model of a metal sample. You will use three of the squares. The fourth square is an extra, in case you make a mistake. a) Take one of the square models and make a drawing of what it looks like in your Active Chemistry log. Label this model the unstamped model. b) What do you think will happen to the layers of metal atoms when the square is stamped? Record your prediction in your log. Explain the reasoning behind your prediction. 5. Carefully press the end of an unsharpened pencil 2 3 mm into one square of the clay model. This will simulate stamping the metal with a blunt stamp. Set the model aside. 6. Take a second square model. Using a sharpened pencil, press the sharpened end 2 3 mm into the surface of the metal. a) Create two new drawings in your log. The first drawing should illustrate what you think the layers will look like under the blunt stamp. The second drawing should illustrate what you think the layers will look like underneath the sharp stamp. Leave space underneath each drawing. 7. Once you have finished your drawings, cut each square in half using a thin blade. Make sure you cut through the middle of the stamped areas. a) Underneath the drawings you made previously, draw what the layers actually look like now that you can see the middle of the stamped areas. How did your prediction compare to the actual results? What happens to the metal atoms underneath the stamped areas? Record your answers in your log. b) Compare the results of the blunt stamp and sharp stamp. Which process affects the metal layers the most? Explain your reasoning. Record your answers in your log. 8. Dispose of the materials as directed by your teacher. Clean up your workstation. Wash your hands and arms thoroughly after the activity. 653

5 CSI Chemistry Chem Words lustrous: a characteristic property of how much a metal shines. ductile: a characteristic property of how easy it is to pull metal into the form of a wire. malleable: a characteristic property of how easy it is to flatten a piece of metal. grain: a small piece of metal that shows how the atoms are arranged. Identification Numbers Many expensive personal items are stamped with an identification or serial number. Automobiles also have a series of letters and numbers, a Vehicle Identification Number (VIN), to identify each car. All registered vehicles have their own unique number. Many of these VIN numbers are stamped on aluminum plates and mounted in different parts of the car. Thieves work to hide this identification number by scratching out the aluminum plates on the engine and front of car. Clever forensics student scientists, like you, can apply your knowledge of the structure of a metal and the activity series to recover the scratched-out serial number. The Nanoscopic View of Metals Everyone is familiar with metals. They are an integral part of society. Metals are used in cars, airplanes, skyscrapers, and many common household items such as kitchen stoves, utensils, and pots and pans. What classifies a material as being metallic? Metals are a class of elements that have many common properties. Most metals are lustrous (shiny), they feel cool to the touch because of their ability to conduct heat, they conduct electricity, they are ductile (they can be drawn into wires), and they are malleable (they can be hammered into thins sheets). And as you learned earlier, metals are located on the left side of the stair-step line in the Periodic Table of the Elements. But why does serial-number etching work? To understand this you need to take a much closer look at the metal, zooming in on its nanoscopic structure. In a metal, the atoms are arranged in tiny regions called grains. These grains range in size from 0.01 mm to 0.1 mm. 654

6 Activity 7 Serial-Number Etching Within each grain, the metal s atoms are arranged in an orderly or crystalline arrangement. Within the crystalline structure of the grain, however, there are tiny irregularities called dislocations. When a metal is bent repeatedly, the dislocation moves inside the crystal creating other dislocations. This increases the stress inside the metal until the metal becomes brittle and breaks. You can try this yourself by bending a paper clip or a coat hanger back and forth numerous times. Eventually, the paper clip or the coat hanger will break. If you were to heat the metal and allow it to cool slowly, the metal will anneal, that is, the metal atoms will rearrange themselves to relieve the stress and the metal will be malleable (bendable) again. High temperatures and slow cooling are needed for annealing to give the metal atoms sufficient energy to move and rearrange. The Chemistry behind Serial Numbers in Metals Serial numbers are usually stamped into metal with hard, blunt metal dies. When this is done at a fairly low temperature, such as room temperature, the metal is said to be cold-worked or work-hardened. The die strikes the surface of the metal with a strong force, causing the die to penetrate the metal. The atoms just below the surface of the metal undergo a plastic (permanent) deformation. This creates a huge number of dislocations in the metal beneath the surface of the stamp. As you saw in your model, the layers of the clay were compressed directly underneath the stamped area, with the blunt stamp compressing more area than the sharp stamp. The compression in your clay model is an example of plastic deformation. The further away from the stamp, the less compressed the layers were. The same kind of physical deformation happens in the metal when it is stamped. In metals there are grains instead of layers like in the clay. The dislocations force the electrons of the atoms in the stamped grains to get closer together. This creates a lot of stress in the crystalline structure of the grain and hardens the material in that region. Chem Words dislocations: atoms misplaced in a crystalline structure. plastic deformation: a permanent dislocation of atoms in an area of a metal that has been struck with a strong force. 655

7 CSI Chemistry Chem Words etching: reproducing an engraving on the surface of a metal plate with acid. In these stressed areas some of the energy used to make the deformation is stored in the stressed (stamped) grains. This stored energy gives the stressed (stamped) grain a higher energy than the unstamped grains. As a result, when an etching solution is applied to the surface, the stamped areas react faster than the unaltered metal. The products of this reaction have a different appearance than does the unreacted metal, so the serial number reappears as impressions, made of the reaction products, in the metal. The etching method works by using single-replacement chemical reactions. As you learned in Activity 6, a single-replacement reaction is a type of oxidation-reduction reaction. The etching solution used for aluminum was made of hydrochloric acid and iron (III) chloride. Aluminum (Al), being higher on the activity series than iron and hydrogen, will donate some electrons to the iron (III) ions and the hydrogen ions in the etching solution. Since the iron (III) ions and the hydrogen ions are gaining electrons from the aluminum, they are being reduced and the aluminum is being oxidized (see equations below). This results in the formation of iron metal atoms, hydrogen gas, and aluminum ions. Remember, the stamped aluminum atoms have a higher energy than the unstamped atoms, so the stamped atoms will react faster with the two compounds than the unstamped aluminum atoms. Checking Up 1. What is a crystalline structure? 2. What are dislocations in a crystalline structure? 3. What type of reaction is the etching reaction? Reaction 1 from the etching solution: Al(s) Fe 3 (aq) Al 3 (aq) Fe(s) Reaction 2 from the etching solution: 2Al(s) 6H (aq) 2Al 3 (aq) 3H 2 (g) If you wanted to etch a metal other than aluminum, such as iron, you would need to consult the activity series for an appropriate replacement for the iron (III) chloride solution. What Do You Think Now? At the beginning of this activity you were asked: What do you think happens to the atoms in a piece of metal when a serial number is stamped into the metal? Without referencing any sources, draw a picture to illustrate your idea. Look back at your answers. Write a paragraph critiquing your original response. Comment on what was correct, what was incorrect, and how you would change your answers and diagram now that you ve completed the lab. 656

8 Activity 7 Serial-Number Etching What does it mean? Chemistry explains a macroscopic phenomenon (what you observe) with a description of what happens at the nanoscopic level (atoms and molecules) using symbolic structures as a way to communicate. Complete the chart below in your log. How do you know? What evidence do you have from this activity that the serial numbers can still be retrieved after they have been sanded off the metal? Why do you believe? MACRO NANO SYMBOLIC Explain what events took place at the macroscopic level when you sanded, etched, and reconstructed the serial numbers. In tracking stolen cars, the police often use the serial numbers to identify the vehicles. What other items can be tracked using serial numbers etched in metal? Why should you care? What oxidation-reduction reaction was able to reconstruct the serial numbers? How can you increase the interest in your crime-scene drama by adding a serial-number component? Draw a structure for the metal and the serial-number etching to explain why you were able to retrieve the numbers once they were gone. Reflecting on the Activity and the Challenge Part of the problem you are facing in creating a crime scene is to understand how to observe something that is not initially visible. To restore an obliterated serial number, you used knowledge developed from a model and from reading about the microscopic structure of metals, combined with knowledge about the Metal Activity Series from the last activity. There are many different metals and you should begin thinking of ways to etch different metal samples. You can now also use the concepts that you have learned to describe the chemistry behind the technique. 1. Describe two ways that the original serial number could be obscured so that the etching method would not be useful to reveal the serial number. Use your model to explain why it would keep the etching process from working. 2. Based on what you learned in the previous activity on the activity series, why is iron (III) chloride used to etch aluminum instead of magnesium chloride? 3. Based on the activity series, what other metals could be etched by a hydrochloric acid and iron (III) chloride mixture? 657

9 CSI Chemistry 4. Why did the etching solution bubble? Explain. Hint: you can start to explain by writing a reaction that occurs. 5. A more complete activity series is: Mg Al Zn Fe Pb H Cu Hg Ag. Answer the following questions using this information. a) Which is the most active of these metals? b) Which is the least active of the metals? c) If a magnesium ion has a 2 charge and a zinc ion has a 2 charge, write the reaction that occurs spontaneously between them when one is a metal and the other is an ion. Be sure to write the aqueous or solid information in parentheses. d) Name two other metal ion solutions, besides Fe 3 (which you used), that could be used to restore a serial number in aluminum. 6. Complete the word equations for the following reactions. If a reaction does not occur, write N.R. a) zinc sodium nitrate b) zinc silver nitrate c) tin zinc nitrate 7. Preparing for the Chapter Challenge Using the chemistry and techniques in this chapter, you must create a crime scene, analyze the evidence, and link the evidence so that it implicates one of the four suspects. Write a paragraph describing how you can use what you learned in this activity to analyze a metal piece of evidence. In addition, write a paragraph that describes how you can link that piece of evidence to one of your characters. Inquiring Further 1. Etching other metals Pick a common metal other than aluminum, such as iron or copper. Using the activity series, predict what might be a good reagent to use with that metal. Research serial-number etching further to determine a method for etching the metal you select. Under the supervision of an adult, test the method. 2. Restoring serial numbers on nonmetallic materials Research serial-number restoration methods that are used on nonmetallic materials. Begin by reading the article titled, A Compilation of Techniques and Chemical Formulae used in the Restoration of Obliterated Markings, by Ernest E. Massiah, in the AFTE Journal, volume 8, number 2, Under the supervision of an adult, experiment with one of the methods you read about. 658