Chapter 21 — Exercises

Work these without looking back at the chapter first; then check yourself. Items marked † have full worked solutions in the answers appendix. There are no answers in this file. Mix of recall, applied reasoning, evidence interpretation, "spot the overstatement," ethics, and a cold-case extension.

A. Recall and definitions

  1. Define forensic chemistry in one sentence, and list the three substance families this chapter says the chemistry section most often handles.

  2. † Define a presumptive color test and a confirmatory test. State the single most important difference between them in terms of what each may be reported as.

  3. What is a microcrystalline test, and name two reasons it remains presumptive even though it is more discriminating than a color test.

  4. Define controlled-substance analysis and explain why it is the highest-volume work in the chemistry section.

  5. † Define explosives residue. Why is the absence of detected residue at a blast scene not evidence that no explosive was used?

  6. State the principle of orthogonality in your own words, and explain why it is the reason a confirmatory identification uses an instrument rather than a second chemical screen.

  7. Name the two broad chemical families of explosives, and state the kind of residue (inorganic ions vs. organic molecules) most associated with each.

  8. What is an ignitable-liquid residue (previewed here, owned by Chapter 22), and which chapter owns the instrumental method that confirms it?

B. Applied reasoning

  1. † A Marquis color test on a powder turns purple-black. List everything this result does and does not establish. Then write the single defensible sentence a report could contain on the strength of this test alone.

  2. Explain why a presumptive negative (no color develops) is often a more reliable result than a presumptive positive. Tie your answer to the "two-stage funnel" and to Theme 1 (exclusion over proof).

  3. A new fentanyl analog appears in the drug supply. Give two distinct reasons a presumptive color test designed for older opioids may fail to flag it correctly, and explain why this makes instrumental confirmation more important, not less.

  4. † An officer's roadside kit "turns positive" for cocaine during a traffic stop. Walk through the "chain of harm" from §21.6 and identify the two specific points at which confirmatory laboratory testing would have prevented a wrongful conviction.

  5. Explain why fire-debris samples are sealed in clean, unused metal cans or nylon bags rather than ordinary plastic bags. What confound does the control sample of unburned material guard against?

  6. A trained accelerant-detection canine alerts to fire debris. State, at its true strength, what the alert establishes — and what must happen before an expert may testify that gasoline was present.

  7. Why does a single negative result on one explosives-analysis method (say, ion chromatography for nitrate) prove very little about whether an organic high explosive was present? Use the two-families distinction.

C. Evidence interpretation

  1. † Re-read Figure 21.1 ("A swab from the crater rim"). Explain precisely why the three control swabs carry as much evidentiary weight as the positive nitrate result. What would the positive alone, with no controls, be worth?

  2. A drug-chemistry report concludes: "Item 3 tested presumptively positive for amphetamine-type substances by color test; confirmatory analysis by GC-MS identified methamphetamine." Identify three things this report does well (where it is appropriately honest about the two stages).

  3. An arson report reads: "The pour patterns and crazed glass establish that an accelerant was used." Using §21.5, name the specific error and state what would have to be done instead to support an accelerant finding chemically.

  4. In the cold case, the status after this chapter is "accelerant indicated — gasoline," not "gasoline confirmed." State exactly what evidence is still missing, which chapter supplies it, and why the chemistry alone cannot yet call the fire "arson."

D. Spot the overstatement / junk-science alert

  1. † A prosecutor's slide reads: "The field test proves the substance was heroin." Identify two distinct problems with this statement and rewrite it honestly.

  2. A bomb-case analyst testifies: "We detected nitrate near the blast, so the defendant handled the device." Name the overstatement and give the defensible version, referencing both the contamination problem and the substance-vs-person distinction.

  3. A television scene shows an analyst drop a powder into a vial, watch it turn blue, and announce "It's cocaine — book him." Using §21.2–21.3, give two reasons this is backward from how a real identification is made.

  4. "The roadside kit is junk science." Is this statement correct? Explain the distinction between junk chemistry and junk use, and state precisely when the kit is valid and when it is not.

E. Ethics and reasoning

  1. † A high-profile bombing investigation is under intense public pressure, and the analyst is told which explosive the investigators "expect." Using the Cognitive-Bias Watch in §21.4 (and previewing Chapter 31), explain the risk to the residue interpretation and describe the safeguard. Why do the stakes make the safeguard more important, not less?

  2. A jurisdiction convicts thousands of people of drug possession on roadside field tests alone, with confirmatory testing skipped to save money and time. You are asked to advise on policy. Argue, using this chapter, why "confirm before conviction" is a scientific requirement and not merely a procedural nicety.

  3. You are asked to report a substance as "identified" on the basis of a color test plus a microcrystalline test, with no instrument available. Explain why you should decline, what you can honestly write, and what principle (from §21.3) your refusal rests on.

  4. Two analysts disagree about whether a faint color in a spot test is a "positive." Is this the same kind of disagreement as two DNA analysts disagreeing about a clean single-source profile? Explain what the disagreement reveals about presumptive tests and why it argues for confirmation.

F. Synthesis and validity spectrum

  1. † Place these on the NAS 2009 / PCAST 2016 validity spectrum, justifying each: a confirmed GC-MS identification of cocaine (Chapter 23); a roadside color test used as the sole identification; a roadside color test used as a screen directing lab work; a Willingham-style burn-pattern "accelerant" claim with no chemistry. Explain why the same color test appears at two different places depending on its use.

  2. Explain how this chapter's presumptive/confirmatory logic is the same logic you met in the serology funnel of Chapter 10. Name one domain in Chapter 10 and one in this chapter where a presumptive negative is the more reliable result.

  3. In one paragraph, explain how this chapter advances at least two of the book's four themes (exclusion over proof; the validity spectrum; cognitive bias; the CSI effect cutting both ways). Name which themes and how.

G. Cold-case extension

  1. † Cold Case. Write the evidence-log entry (Appendix I) for the Mill Creek fire-debris finding. State (a) the defensible status at its true strength, (b) the honest verb, (c) at least three things this evidence specifically does not establish, and (d) why you decline to write "gasoline confirmed" or "arson" at this stage.

  2. Cold Case extension. The field indication is "accelerant indicated — gasoline." Propose the two concrete next steps (think: which instrument, which chapter, what control comparison) that would move the finding from indicated to identified, and state exactly what each step adds.

  3. Cold Case, integrative. The chemistry indicates an accelerant but cannot, by itself, call the fire "arson." List two other analyses — one from this part of the book, one from an earlier chapter — that, combined with a confirmed accelerant, would build toward an incendiary-fire conclusion, and explain why the accelerant indication alone is insufficient.

H. Short writing

  1. In 150–200 words, explain to a juror why a "positive" roadside drug test is genuinely useful to an investigation and why it should never, by itself, convict anyone.

  2. † In 150–200 words, contrast the validity foundations of instrumental forensic chemistry (e.g., a confirmed GC-MS drug identification) with those of a presumptive color test: what does each rest on, where is each strong, and where is each most easily overstated in court?