Distinguishing Glass Fragments

Transcription

1 Activity 2 Distinguishing Glass Fragments GOALS In this activity you will: Experimentally determine the density of a solid without a definite shape. Understand the difference between intensive and extensive properties. Use an intensive property of matter to identify an unknown substance. Safety goggles and a lab apron must be worn at all times in a chemistry lab. What Do You Think? A good detective must be able to apply what he or she knows in any situation. In a well-known movie, Raiders of the Lost Ark, the hero replaces a gold statue with a bag of sand. How would the hero of the movie know how much sand to put in the bag to replace the gold statue? Record your ideas about this question in your log. Be prepared to discuss your responses with your group and the class. Investigate Many crimes result in broken glass fragments. Breaking a window, shattering a bottle, crashing through a glass table the possibilities are endless because so many products are made of glass. In this activity you are going to learn one way a forensic detective matches a piece of glass from the crime scene. Part A: Collecting the Data 1. Obtain two samples of glass, labeled A and B, from your teacher. Be careful not to mix the two samples. Examine each sample through a hand lens or stereomicroscope. a) Record your observations in your log. 599

2 CSI Chemistry Trial 1 (20 pieces of glass) Mass of sample (g) Initial volume (ml) Final volume (ml) Volume of sample (ml) 2 (25 pieces) 3 (30 pieces) 4 (35 pieces) 2. Physical examination may not be enough evidence to distinguish one piece of glass from another. A second method is to compare the masses and volumes of each piece of glass. Your teacher will assign you one of the two glass samples. a) You may want to use a data table like the one shown to record your data in your log. 3. Using tweezers, obtain approximately 20 pieces of glass from your sample to use in Trial 1. Find the mass of this set of glass to the nearest 0.01 g. a) Record the value in your data table. 4. Fill a 100-mL graduated cylinder to the 50-mL mark with water. a) Record this volume to the nearest 0.1 ml in the initial volume column of your data table. This will be the initial volume for all four trials. A diagram illustrating the proper way to read a graduated cylinder is shown. 5. Using tweezers, carefully place 20 pieces of glass into the graduated cylinder. Tilt the cylinder so that the glass does not cause a splash. Record the new volume in the final volume column for Trial 1. This method of measuring volume is called water displacement. The diagram illustrates this method volume of water and glass sample volume of water volume of glass sample

3 Activity 2 Distinguishing Glass Fragments 6. To calculate the total volume of the sample of 20 pieces of glass, subtract the initial volume from the final volume. a) Record the volume of the Trial 1 sample in your log. 7. Find the mass of five more pieces of glass using your laboratory balance. Add this mass to the mass of the 20 pieces from the previous trial. a) Record the sum as the total mass for Trial Using tweezers, place the five new pieces into the cylinder containing the original 20 pieces. a) Record the final volume for Trial 2. b) Subtract the initial volume from the final volume of Trial 2 and record your answer in the column labeled Volume of sample. This volume should be greater than the final volume for Trial 1, because there are now 25 pieces of glass in the cylinder instead of 20 pieces of glass. 9. Repeat Steps 7 and 8 for two more trials. The glass sample in each trial will contain five more pieces. 10. When you are done, empty the graduated cylinder and pour the glass pieces onto a paper towel. Dry the cylinder and put it away. Your instructor may have you put the glass in a beaker and place it in a drying oven so it will be dry for the next class. Clean up your workstation. 11. Find a group that collected the same data for the other glass sample. a) Record their information in your log in a new data table. Part B: Analyzing the Data 1. You will now graph the data you collected. Before you do, examine the following graphs to determine which graph represents the bestfit line. A best-fit line has two characteristics: It is a straight line and it is close to as many points as possible (as many points are above and below the line). a) Which graph shows a best-fit line? Write your answer and reason in your log. 2. Discuss your choice of graph and your reasoning with a member of your group. Be prepared to discuss your ideas with the class. 3. Graph the data from both glass samples on the same set of graph paper, using different marks or colors for glass sample A and glass sample B. Place volume on the horizontal axis and mass on the vertical axis. The range for the volume axis should be from zero milliliters up to just a few milliliters more than the greatest volume in all your tables. The range for the mass Report sharp edges on the glass and/or broken glass to your teacher. Do not handle broken or sharp glass with your bare hands. Wash your hands and arms thoroughly after the activity. 601

4 CSI Chemistry axis should be from zero grams up to just a few grams more than the mass in Trial 4. a) For each sample, use a ruler to draw a straight line that best fits the five points you graphed. b) Calculate the slope of each of the lines and record it in your Active Chemistry log. Slope y 2 y 1. x 2 x 1 The slope of the line will be measured in g/ml. Since 1.0 ml is equal to 1.0 cm 3, the slope could also have the units g/cm 3. This is the density of the glass. 4. Graphing the data from two samples of glass has provided you with a second way to distinguish between the two samples. Review the table of glass densities provided by your teacher. Using the table of glass densities, identify each sample of glass. a) Write and justify your conclusions in your log. If there are samples that cannot be identified, write what additional information you would need in order to match your sample of glass to a type of glass on the list. 5. You now have two related techniques to use to match glass samples from a crime scene. Given an unknown piece of glass, you can measure its density mathematically (D = m/v) or graphically by finding the slope of the mass vs. volume graph of multiple pieces of the same glass. You can then match the density of the unknown glass with other glass samples that you have investigated. Chem Words physical property: a property of matter that can be measured without causing chemical change or a change in the composition of the material. Density is a physical property of a substance. PROPERTIES OF MATTER Physical and Chemical Properties Forensic detectives use properties of matter to identify an unknown object found at a crime scene. This chapter will focus on using properties of matter and deductive reasoning to construct and solve a crime scene. Size, color, flammability, mass, stability, and temperature are all examples of properties (characteristics) you can use to describe matter. Properties fall into two categories: physical and chemical. Physical properties are characteristics of matter that can be observed without changing the chemical identity of the substance. Examples of physical properties include luster, color, mass, volume, and odor. Glass is brittle, hard, 602

5 Activity 2 Distinguishing Glass Fragments odorless, and has a high melting point. It may also be transparent or opaque, colored or uncolored. These are just some of the physical properties of glass. Chemical properties describe how the substance reacts with other materials. Rusting is a chemical property of iron. It describes what happens to iron when in the presence of oxygen and moisture. Firefighters suits are flame retardant; this is a chemical property. Glass is a fairly unreactive material. However, glass does react with hydrofluoric acid. Reactivity with hydrofluoric acid is a chemical property of glass. Forensic chemists often use a combination of physical and chemical properties to identify materials. In this chapter, you will always reflect on the chemical and physical properties of evidence in order to determine or confirm the identity of the matter. Intensive and Extensive Properties of Materials Chem Words chemical property: a property that is displayed when matter undergoes a change in composition (it undergoes a chemical reaction). The burning of wood is a chemical property. intensive property: a physical property that does not change (has the same measurement) no matter how much of the material is present. extensive property: a physical property that varies depending on the amount of material present. Glass fragments found at the crime scene or on a suspect can be large enough to be pieced back together into the original object, such as a broken glass vase.then this can be examined for additional evidence such as fingerprints. Could you imagine trying to put the pieces you used today back together! It would probably be a hopeless task because the pieces are so small! In these cases, the properties of the glass itself must be used for identification. Since glass is a fairly unreactive material, scientists rely on the physical properties of glass fragments to match to a crime scene or a suspect. However, not all physical properties are equally useful. A physical property used in identifying a material must not depend on the quantity of the material. In other words, the property must have the same measurement no matter what amount of material is tested, as long as it is the same material. This kind of physical property is called an intensive property. You can think of an intensive property as one that doesn t change regardless of how small or how large the sample of a particular substance is. In fact, intensive properties are often called characteristic properties, because they can be used to characterize a material. Examples of intensive properties include melting point and freezing point. Length and mass are not intensive properties of a material because they depend on the amount of substance in a given sample. Properties like length and mass, which vary depending on the amount of material present, are called extensive properties. Extensive properties are not used to identify or match matter at a crime scene. 603

6 CSI Chemistry Chem Words mass: a measure of the amount of matter an object contains. volume: a measure of the space occupied by matter. density: the mass of a substance per unit volume. Checking Up 1. Give two physical and two chemical properties of glass. 2. In your own words, describe the difference between an intensive and extensive property. 3. What equation can be used to calculate the density of a material? 4. What does the slope of a mass versus volume graph represent? 5. If two unknown materials have exactly the same density, can you deduce that the two materials are the same? 6. If the density of glass fragments found on a suspect matches the glass fragments at the scene of a crime, can you prove without a doubt that the suspect was at the scene of the crime? Density In this experiment, you determined the mass and the volume of four samples of the same glass. Each sample was a different size. Mass is a measure of the amount of matter an object contains. It does not depend on gravity (like weight) and will be the same regardless of where it is measured. Since mass depends on the amount of matter present, it is an extensive property. Likewise, volume, the amount of space occupied by matter, is also an extensive property. But, you may have noticed that when the mass of the glass sample was plotted against its volume, there was a linear (straight-line) relationship. The ratio of the mass to the volume was constant and did not depend on the sample size. Therefore, the relationship between the mass of a substance and its volume is an intensive property. And since it is an intensive property, you can use the mass versus volume relationship to identify a substance. The relationship between the mass of an object and its volume is called the object s density. Density is defined as the mass per unit volume. Density can be calculated using the equation: mass (g) Density volume (ml or cm 3 ) Density is reported in units of g/cm 3 or g/ml. For example, if the mass of a solid object is 15.0 g and its volume is 25.3 cm 3, the density of the object is equal to: Density m 15.0 g g V 25.3 cm cm3 or g /ml Calculating Density from a Mass and Volume Data Set The density of a substance can be found with a single calculation or by graphing a data set of mass versus volume. In this activity, you used a graph to determine the density of your glass sample. Using this procedure for a sample X, a data table below was created. Sample X Mass (g) Volume (cm 3 ) The next step is to plot the mass versus the volume for all four samples on a graph. After plotting the data, a straight-line fit for the data is drawn. This line may not go through all of the data points. The slope of this line will be equal to the density of the substance. That s because the slope is equivalent to 604

7 Activity 2 Distinguishing Glass Fragments the mass divided by the volume, which is the definition of density. The slope of a line 20 is found by choosing two different points that fall on the best-fit straight line. The 15 points should be widely spaced apart on the 10 graph. The difference between the y-values (x 5 1,y 1 ) of the two points chosen is divided by the difference between the x-values of those two points. The result is the slope of the line. The graph with the best-fit straight line and the slope calculations for the sample data are shown. Once the density of an unknown substance is determined, the calculated density can be compared to the known densities of substances. The known densities can be found in a number of reference materials, including the table provided by your teacher. If the unknown substance s density is close to one of the known densities, it provides evidence of the unknown substance s identity. Density is not only used to identify glass samples, but also metals and other materials whose density can be calculated. In the case of forensic analysis of glass fragments, the density of the fragments found on a suspect can be compared to the glass fragments left at the scene. If the densities are identical, then there is a strong chance that the suspect was at the scene. However, a scientist must be very careful when drawing conclusions based on data. For example, if the glass in question is automobile glass from a particular make of vehicle, the forensic scientist should keep in mind that there are thousands of these vehicles on the road in this country. There may be hundreds in the area of the crime scene. Therefore, if a suspect has glass fragments that match the glass of the given vehicle with a broken window at a crime scene, that is valuable evidence against the suspect. However, that alone is not enough to prove that the suspect was definitely at the scene. mass (g) Graph of mass vs. volume for Sample X density = slope slope = slope = slope = (x 2,y 2 ) rise run volume (cm 3 ) rise run y 2 y 1 x 2 x g 3.01 g 25.3 cm cm g slope = 20.2 cm 3 slope = g/cm 3 What Do You Think Now? At the beginning of this activity you were asked: How would the hero of the movie know how much sand to put in the bag to replace the gold statue? The density of sand may be 2.3 g/cm 3, while the density of gold is g/cm 3. If the volume of the sand and the gold statue were identical, would that mean that the masses were also equal? 605

8 CSI Chemistry 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. MACRO NANO SYMBOLIC How did increases in the mass of the glass affect the volume and the density of the glass? In two separate diagrams, represent the densities of two different elements at the nanoscopic (molecular) level. Make one element lead, a very dense element, and the other aluminum, a less dense element. What are the units you use to represent density? What is the equation you use to define density? How do you know? Making specific reference to your data, explain how a person can distinguish two types of glass. Why do you believe? Solid gold and gold-plated jewelry may look identical. How can density measurements be used to determine if a piece of jewelry is real gold or just gold-plated? Why should you care? You will be creating and solving a crime scene for your challenge in this unit. Write a short scene using glass fragments to clear one suspect of suspicion and indicate another suspect. In your short scene, explain how the glass fragments were found, analyzed, and interpreted. Reflecting on the Activity and the Challenge You have learned how to experimentally determine the density of a glass sample. Because density is an intensive (or characteristic) property, this measurement can help determine the identity of a material that looks like glass found at the crime scene or on a suspect. If glass fragments remain as evidence of a crime, then you may need to measure the density of the glass to determine if the fragment is from the scene of the crime or if it is unconnected to the crime. 606

9 Activity 2 Distinguishing Glass Fragments 1. Identify the following as physical or chemical properties. a) height b) flammability c) tarnishing d) hardness 2. Calculate the density for an object that has a mass of 2.53 g and a volume of 4.54 cm Give another example of an intensive physical property. Explain your choice. 4. A collection of glass fragments was found in the car of the prime suspect, Bob. The glass found at the crime scene was from a crystal goblet made of leaded glass. Using the data obtained by the forensic chemists and the graph-slope method, determine if the glass in Bob s car could be from the crime scene. Glass Data from Bob's Car Set Mass (g) Volume (cm 3 ) Type of glass Density (g/cm 3 ) quartz glass 2.2 borosilicate glass 2.3 soft glass 2.6 leaded glass If different glass manufacturers use different window glass formulations, density measurements can be used to help identify the manufacturer of a particular glass fragment. How? Explain. 6. Preparing for the Chapter Challenge You will want your data used in the challenge s crime scene to be realistic. Look at the four types of glass presented in Question 4. Using any resource you choose, find an example of how each of these types of glass may be present at a typical crime scene, such as a kitchen, office, or bedroom. In your Active Chemistry log, create a table to organize your findings, including columns for type of glass, density, uses, and examples of crime scenes where you may find that type of glass. Inquiring Further Glass used by car manufacturers Research the density of the glass used by three different automakers. Obtain and test a windshield sample from a local glass shop or car impound lot under the supervision of a responsible adult. Share this information with your classmates by creating a detailed profile document of each of the windshield types, including comparing your experimental results to the known data you researched. 607