Forensic Science Crime Busters Identification Techniques
Crime Busters Forensic Science Identification
Chemical Substance Identification
- Iodine Test: Starch turns blue-black or a dark color (e.g., flour and starch).
- Sodium Chloride (NaCl): Table salt. It dissolves in water, is pH neutral, and forms crystals.
- Sugar: Dissolves in water and does not give the starch iodine reaction.
- Cornstarch: A starch that turns blue-black with iodine; it does not behave like a simple salt or sugar solution.
- Flour: A mixture of substances containing starch (iodine reaction); generally does not dissolve completely.
- Sodium Bicarbonate (Baking Soda – NaHCO₃): Basic; reacts with acid to produce CO₂ bubbles.
- Calcium Carbonate (CaCO₃): Reacts with acid to produce CO₂ bubbles; common in limestone and does not dissolve in water.
- Sand (Silicon Dioxide): Does not dissolve in water; mostly silicon dioxide and generally chemically unreactive in these simple tests.
- Gypsum (Calcium Sulfate Dihydrate – CaSO₄·2H₂O): Has relatively low solubility.
- Ascorbic Acid (Vitamin C): Acidic.
- Yeast: Biological agent.
Hair Vocabulary and Comparison
Hair Anatomy
- Cuticle: The outer layer consisting of overlapping scales.
- Cortex: The middle layer containing most of the pigment.
- Medulla: The central portion of the hair shaft.
General Comparison
- Human: Usually has a relatively thin or less prominent medulla and a relatively uniform appearance.
- Dog: Often has a thicker, prominent medulla that can take up much of the shaft.
- Cat: Often has a characteristic ladder-like or patterned medulla.
Fiber Classification and Microscopy
Vegetable Fibers
Made from plants. Examples include cotton and flax/linen.
Animal Fibers
Made from animals. Examples include wool and silk.
Synthetic Fibers
Man-made polymers. Examples include nylon, polyester, and acrylic.
Note: The 2026 rules specifically tested animal vs. vegetable vs. synthetic, rather than requiring identification of specific fiber types.
Microscopy Clues
- Cotton: Twisted or ribbon-like appearance.
- Wool: Scales on the surface.
- Synthetic: Often very smooth and uniform.
Fingerprint Patterns and Identification
There are three major fingerprint patterns:
- Whorl (W): Circular or spiral pattern (Think: Wheel).
- Loop (L): Ridges enter from one side, curve around, and exit the same side (Think: Loops around).
- Arch (A): Ridges enter one side and leave the other, forming a raised arch (Think: Arch shape).
DNA Evidence and Chromatograms
You may have to compare DNA chromatograms or electropherograms. Note: The 2026 rules specifically include comparison of DNA chromatograms/electropherograms.
- Same band/peak pattern: Consistent with the same DNA source.
- Different pattern: Not a match.
When comparing suspects, look for matching patterns between the evidence sample and Suspect 1, Suspect 2, or Suspect 3.
Shoeprints, Tire Treads, and Soil Analysis
Shoeprints and Tire Treads
Look for tread patterns, size, shape, unique markings, cuts, damage, and wear patterns. The shape of the print can help determine the direction of travel and speed.
Soil Comparison
Compare color, texture, composition, and minerals or particles. If soil at the crime scene matches soil associated with a suspect, it can support the connection.
Terminology Tip: Don’t say, “The soil proves the suspect did it.” Instead, say, “The soil evidence is consistent with Suspect B having been at the crime scene.”
Crime Scene Analysis and Strategy
This section accounts for approximately 25% of the score in the 2026 rules. You need to explain:
- Who? Which suspect is implicated?
- What? What evidence connects them?
- Why? Why does that evidence matter?
- Why not the others? This is crucial. The rules state the analysis should explain why the chosen suspect was selected and why the other suspects were not.
Effective Sentence Structure
“Suspect ___ is most strongly implicated because evidence ___ matches ___, while evidence ___ is inconsistent with Suspects ___ and ___. Therefore, Suspect ___ is the most likely source of the evidence.”
The Winning Strategy
Don’t just look for a single test. Follow this mindset: Observation → Test → Result → Identification → Compare to Suspects → Conclusion.
Note: The 2026 scoring breakdown makes qualitative analysis and crime-scene analysis equal to about 75% of the event, so these should be your biggest study priorities.
The Role of the Iodine Test
The iodine test helps identify whether an unknown substance contains starch. 🔎
How the Test Works
Apply a small amount of iodine solution to the unknown substance:
- Iodine stays yellow/brown: Starch is likely not present.
- Turns blue-black: Starch is present.
Application in Forensic Analysis
If a crime scene has an unknown white powder (flour, sugar, salt, or cornstarch), iodine helps narrow it down:
- Blue-black: Starch present (could be cornstarch or flour).
- No blue-black: Likely not a starch-containing substance.
Combine this with other tests like solubility, pH, and appearance for a final identification. Remember: IODINE TEST: starch → blue-black.
Evidence Characteristics
- Material can be distinguished by its physical and chemical behavior.