Forensics

Overview

Forensics blends qualitative chemistry with the interpretation of physical evidence. You score best by running a disciplined, reproducible sequence of tests, recording unambiguous observations, and then writing short, defensible conclusions that link at least two independent traits to each identification. Think in workflows: observe, test, confirm, and only then infer how pieces of evidence fit (or do not fit) a scenario.

Qualitative analysis and chromatography

Powders and liquids can be separated quickly with a minimal test set. Begin with appearance and solubility, check pH, then test reactivity—carbonates fizz with acids, starch turns blue‑black with iodine, sugars caramelize on heating. Keep separate tools to avoid cross‑contamination and always test fresh surfaces. Polymers and plastics are often distinguished by density, microscopy, and provided burn results; your task is to explain how the provided traits point to a particular class. Paper chromatography separates mixtures by differential affinity; compute Rf as solute travel divided by solvent front and compare unknowns to references developed under the same solvent conditions.

Physical evidence and reasoning

Fingerprints are class evidence by pattern (loops, whorls, arches) and, where within scope, by simple minutiae. DNA representations in competition are pattern matches rather than population genetic calculations: a scene sample must match a suspect at all shown loci for inclusion. Glass analysis relies on concepts of refractive index and fracture: radial fractures form first on the opposite side of impact and concentric fractures follow; sequencing depends on fracture intersections. Spatter shapes encode direction and relative speed (elongation grows with angle from normal and with impact energy), but most questions remain qualitative. The thread running through all physical evidence is restraint—treat each line as supportive, not conclusive on its own, and explain exclusions as clearly as inclusions.

Worked micro‑examples

• Powder identification sequence: solubility and pH separate salts from organics; 1 M HCl produces CO₂ in carbonates; iodine distinguishes starches; heating behavior differentiates sugars. A baking soda sample is basic, fizzes strongly, and shows no iodine color, while cornstarch is iodine‑positive and insoluble in cold water.
• Paper chromatography: an unknown with two visible bands whose Rf match black and purple reference inks indicates a mixture rather than a single source; collect and label the chromatogram with the score sheet.
• DNA gel reading: if the scene sample matches Suspect B at all loci and differs from Suspect A at one locus, state that B is consistent with the evidence and A is not; add that other evidence should corroborate.

Pitfalls

Common mistakes include reusing droppers and contaminating unknowns, concluding on a single ambiguous test, miscomputing Rf by measuring to the wrong point, and treating class characteristics (plastic type, fingerprint pattern) as if they were individual identifications. Many errors vanish when you tabulate reagent → observation → inference and commit to a fixed test order.

Practice prompts

• Propose a three‑step test plan to separate four white powders and justify each branch point.
• Write a short inclusion/exclusion report combining chromatography, a fingerprint class match, and a simple DNA gel pattern.
• Describe which fracture lines in a provided glass image formed first and in what order subsequent impacts occurred.

References

• SciOly Wiki – Forensics: https://scioly.org/wiki/index.php/Forensics