Crime Busters (Division B)

Overview

Crime Busters is a lab‑based forensic chemistry event. Given a scenario and evidence, students perform qualitative tests and analyze physical evidence to infer plausible conclusions. The emphasis is on systematic observation, safe technique, and clear, defensible reasoning.

Qualitative analysis (unknowns)

Common solids (examples)

Sodium acetate (anhydrous), NaCl, sugar, flour, gypsum (CaSO4·2H2O), cornstarch, baking soda, CaCO3 (powdered limestone), gelatin powder, Alka‑Seltzer® (powdered), sand (white), vitamin C (ascorbic acid), yeast.

Typical cues

Solubility in water; pH (aqueous); reaction with HCl (carbonates fizz); iodine color (starch → blue/black); heating behavior (sugar caramelizes); effervescence with acids (baking soda). Record consistent multiple traits before concluding.

Non‑powdered metals

Al, Fe, Zn, Mg, Cu, Sn. Note magnetism (Fe), color (Cu), reactivity with acids (Mg effervesces strongly), density/appearance.

Liquids

Lemon juice, vinegar (acidic); ammonia (basic, pungent), rubbing alcohol (isopropyl, odor, miscibility), water (neutral), hydrogen peroxide (3%). Use odor cautiously per rules, rely on pH and reactions.

Polymers and natural/man‑made substances

Hair: human vs common animals (cat, dog). Examine medulla patterns, scale patterns, and diameter under available magnification.
Fibers: animal vs vegetable vs synthetic; burn results may be provided; look for morphology and simple microscopy traits.
Plastics: PETE, HDPE, PS (non‑expanded), LDPE, PP, PVC, PMMA. Burn tests are not performed by participants (but results may be provided). Density and other provided diagnostics may be used.

Paper chromatography

Separate ink/dye components. Compute Rf = distance solute / distance solvent front. Compare unknown to references under the same solvent conditions. Collect chromatograms with the score sheet when instructed.

Physical evidence

Fingerprints: recognize loops, whorls, arches. Note minutiae qualitatively as scope allows.
DNA chromatograms/electropherograms: compare allele bands/peaks of scene samples to suspects (pattern matching, not population statistics).
Shoeprints/tire treads: compare patterns and features; infer direction/speed qualitatively.
Soil: compare composition/texture provided; link plausibly to locations/suspects.
Spatters: infer direction and relative speed from shape/elongation; no calculations required.

Analysis and reporting

Clearly explain which pieces of evidence implicate a suspect and why others are excluded. Tie each conclusion to two or more consistent observations/tests. Note uncertainties and alternative explanations when present.

Strategy and lab hygiene

Safety first: goggles on; apron/lab coat; tie back hair; follow supervisor guidance.
Systematic workflow: one tray per unknown; label containers; fresh sticks/droppers as needed to avoid cross‑contamination.
Record‑keeping: tabulate tests (reagent, observation, inference); keep a clean bench; dispose of waste properly.
Time management: prioritize high‑discrimination tests first (e.g., acid fizzing, starch/iodine), then confirmatory tests.

Quick reference tables

Carbonates (e.g., CaCO3, NaHCO3): effervescence with acid; CaCO3 less soluble; baking soda basic in solution.
Starch (cornstarch, flour): blue‑black with iodine; cornstarch finer/talc‑like; flour off‑white and clumps; behavior with water differs.
Sugars: soluble, sweet odor; caramelize on heating; Benedict’s may appear conceptually (if provided) but is not guaranteed.
Common plastics (provided data): PVC (chlorinated) vs PP/PE (hydrophobic, low density) vs PMMA (clear, high clarity) — rely on provided clues.

Practice prompts

Given four solids (unknown A–D), design a minimum‑test sequence to identify each using water, 1 M HCl, iodine, and heating. Explain decision points.
Compare a suspect’s loop fingerprint to a scene print; state a similarity/difference and whether it supports inclusion.
Compute Rf values for two inks and determine a match or mixture.
From a DNA electropherogram, identify which suspect pattern matches the scene sample at all loci.

Common pitfalls

Cross‑contaminating reagents and unknowns; mislabeling tubes.
Overreliance on a single ambiguous test; failing to record pH or solubility before reactions.
Ignoring provided burn results for plastics (participants do not perform burns).
Concluding guilt from a single matching evidence type without considering alternative suspects.

References

SciOly Wiki: https://scioly.org/wiki/index.php/Crime_Busters
Division B Chemistry Events Lab Equipment List (for what participants may bring)

Crime Busters