Lecture Notes:

Chapter Two, Evidence Collection and Preservation

• Case Study: Grave Evidence
  • Why did assistant US Attorney release the information that Wilfred Johnson was an FBI informant against John Gotti? It was an attempt to force him to testify openly against Gotti in court. He was offered protection by the government.
  • Why did Johnson ultimately refuse to testify? He thought that this would prevent the Gambino family from arranging for his death.
  • How did the FBI plan to hold Thomas Pitera responsible for the death of Johnson even though Pitera was acquitted in the trial for Johnson’s murder? They had evidence of other crimes against Pitera, some of which depended on a soil sample.
• Section 2.1 Preserving Evidence: Reactions, Properties, and Changes
  • What is a physical change? A physical change is one in which the chemical identity of the substance involved is not changed. Examples include phase changes between solid, liquid and gas.
  • What is a physical property of a substance? A physical property is one that can be measured without changing the chemical identity of the substance. Examples include mass, volume, color, shape, solubility, freezing and boiling points, luster, conductivity, magnetic properties, electric properties, odor, refractive index, hardness, texture, and density.
  • What is the refractive index of a substance? When a substance is transparent it allows light to pass through it. When light passes from one substance to another (as from air to glass) its path is bent. The refractive index is a measure of how much its path is bent. A demonstration of the way a laser beam is bent upon passing through a prism or a beaker of water may be useful here.
  • What is a chemical change? A chemical change is one in which the chemicals involved become different substances. For example, two elements may react to form a compound or two compounds may react to form two new compounds plus an element.
  • What is a chemical property of a substance? A chemical property is a way of describing the types of chemical reactions a substance can undergo. For example, some substances are flammable and some are not. Some substances are corrosive to some materials but not others (acid will dissolve many metals but do not affect glass). A demonstration of a chemical change is appropriate at this point. Some good and simples ones are burning magnesium or a precipitation reaction (CaCl₂ + Na₂CO₃ works well) or an acid plus a carbonate (vinegar and baking soda works well). Part of the demonstration could include measuring mass before and after the reaction as a foreshadowing of chapter 4.
• Section 2.2 Physical Evidence Collection: Mass, Weight, and Units
  • Measurements without units are useless!
  • What is mass? Mass is a measure of how much matter is present in a sample. It is independent of the strength of the gravitational pull on the substance.
  • What units are used for measuring mass? The gram (g) is the usual unit of mass in the lab. The SI unit is the kilogram (kg), which is 1,000 grams. Other units include the milligram (mg, 1,000 mg = 1 g) and the metric ton (1 ton = 1,000 kg). Different units are used depending on the size of what is being measured. Drugs recovered in law enforcement actions are usually measured in grams but in the pharmacy dosages are given in milligrams.
  • What is weight? Weight is a measure of the strength of the gravitational pull on a substance. Weight depends on location. Objects on the surface of the Moon weigh 1/6 of what they do on the surface of the Earth. On the International Space Station objects have effectively no weight due to the nature of the orbital motion of the station. This does not mean they have no mass.
  • At this point it may be useful to practice the Explanations Framework using the Mass and Weight activity.
• Section 2.3 Mathematics of Unit Conversions
  Coverage of this in the text is minimal and these are very important skills. For that reason the following supplemental activities are to be used:
  • Dimensional Analysis Activity
    Dimensional Analysis Homework 1
    Dimensional Analysis Homework 2
    Dimensional Analysis Addt'l Problems
  • Metrics Part I
    Metrics Part II
    Metrics Homework
  • Square and Cubic Units
    Square and Cubic Units Homework
• Section 2.4 Errors and Estimates in Laboratory Measurements: Significant Figures
  • For this section students should do the Measurement and Significant Figures Lab (if they have not already done so)
  • How to count significant figures:
    1. Nonzero integers are always significant. 7 (1 s.f.) 675 (3 s.f.) 6.75 (3 s.f.)
    2. Zeros are sometimes significant:
       1. Leading zeros in fractions (numbers less than 1) are never significant figures since they only convey information about place value. The number 0.0042 has only two significant figures.
       2. Captive zeros are zeros between nonzero integers. These are always significant figures. The number 1.004 has four significant figures.
       3. Trailing zeros are significant only if they are after a decimal point. The number 4,000 has one significant figure. The number 3.00 × 10⁸ has three significant figures.
    3. Exact numbers have an infinite number of significant figures. Exact numbers include things like numbers that come from counting: 14 beakers is an exact number. Other exact numbers come about because of definitions: one inch is defined as exactly 2.54 cm.
  • Scientific notation is covered in a separate activity:
    Activity: Scientific Notation
    Scientific Notation Homework
    Additional Examples for Scientific Notation
    (typically this is done before beginning work with the book)
• Section 2.5 Mathematics of Significant Figures Calculations
  • Rules for calculations with significant figures are a way to avoid overstating the precision of a calculated result.
    1. For Multiplication and Division the result of a calculation may have no more digits in it than the least precise measurement in the calculation.
       For example:
       2.48 ÷ 5.4 = 0.459259 must be rounded to 0.46 because the number 5.4 has just two significant figures.
    2. For Addition and Subtraction the result has the same number of digits after the decimal as the least precise measurement.
       For example:
       14.56 + 12.1 + 1.019 = 27.679 must be rounded to 27.7 because the number 12.1 has only one digit past the decimal point.
• Section 2.6 Experimental Results: Accuracy and Precision
  • A separate handout about Accuracy and Precision is available.
  • This subject has also already been covered in the Measurement and Significant Figures Lab.
• Section 2.7 How to Analyze Evidence: Density Measurements
  • What is density? Density is the amount of matter (the mass) per unit volume. Often the density of a sample can be used to narrow down the possibilities about the chemical identity of the sample. For this reason, density is important to scientists and crime investigators.
  • How is density different from mass or weight? Density is an intensive property, that is, it does not depend on how much material you have. It is a ratio of mass to volume and that ratio is constant for a substance no matter the size of the sample of it.
  • Is lead (Pb) heavy and is paper light? No, one pound of paper has the same weight as one pound of lead. The difference is that lead has high density and paper has low density. A useful aid to this part of the discussion is to hand around a whole ream of paper (mass = 2.27 kg or about 5 pounds) and a small piece of lead.
  • How can you tell whether something will float in water without putting it in water to find out? Measure its density. Water has a density of 1.00 g/mL and if a substance is more dense than this it will sink when placed in water. If a substance has a density less than the density of water it will float.
  • A styrofoam coffee cup has a mass of only a few grams and floats easily on water. If you had a metric ton of styrofoam in one big block, would it float? Yes! Styrofoam has the same density even if you have metric ton of it.
  • Modern ships are made of steel, which is mostly iron. Iron has a density of 7.9 g/mL. How can they possibly float? They are hollowed out so that the density of the ship is actually less than 1.0 g/mL even though the hull of the ship is far more dense.
• Section 2.8 Mathematics of Density Measurements
  • Density is defined as the ratio of mass to volume: D = m/V where m is usually given in grams and V is usually given in mL (or equivalently in cm³ or cc).
  • To find the mass of a sample whose volume and density are known rearrange the formula to solve for m by multiplying both sides of the definition equation by V:

         m            m            
    D = ---    V·D = --- ·V       so m = DV
         V            V
    

  • To find the volume of a sample whose volume and density are known rearrange the formula to solve for V as follows:

         m                m    DV
    D = ---     m = DV    - = ----     so V = m/D
         V                D    D
    

  • Work through the worked examples 15 - 17 on pp. 41 - 42.
  • Supplemental Activities:
    Activity: Density
    Homework: Density
    Graphing for Density
    Spreadsheet for Graphing for Density
    Lab: Density
    
• Section 2.9 How to Analyze Glass and Soil: Using Physical Properties
  • How is glass evidence analyzed? In part by measuring density and comparing the density to a table of types of glass listing their different densities. The sink-float method is one way of finding the density of a sample and it uses liquids of known density in which the sample may float, sink, or remain suspended.
  • How is soil evidence analyzed? In part by the density gradient method in which soil particles are separated using a column of layered liquids of different densities. Comparing two samples in this way provides evidence that the two samples may have a common origin. It is not conclusive because different minerals may have nearly identical densities. Other methods include color comparison, soil texture, and chemical composition.
• Section 2.10 Case Study Finale: Grave Evidence
  • How did investigators nail Tommy Pitera? They collected evidence against him including recorded phone calls, the testimony of an associate, and physical evidence in the form of soil from a shovel found in his home. They showed that the soil around the mass grave site they discovered (and had already linked to Pitera) was unique and that soil on Pitera’s shovel matched it.

Based on the book Investigating Chemistry: A Forensic Science Perspective, second edition by Matthew E. Johll, W.H. Freeman, © 2009