Forensics

Forensics is a lab-and-logic event. A team of two receives a mock crime scene, a list of suspects and a bench of unknowns. In about 50 minutes you run tests on powders, plastics, fibers and chromatograms, examine physical evidence such as prints, glass and spatter, and then write an analysis that says which suspect the evidence points to and why the others are excluded. The points are spread across bench work and writing, so the habit that wins is simple: record what you actually observed, run the tests in a fixed order, and only then reason from the results in complete sentences.

Event format

Check the lab equipment list early, since teams without those items may be at a disadvantage. Two details are new in the 2027 rules compared with last season: writing utensils are now listed as an allowed item, and the iodine reagent may be either iodine in potassium iodide solution or a 2% tincture of iodine. The content outline (the substance list, the polymer lists, the nine physical-evidence topics, the level table and the score weights) is the same as last season.

What the rules cover

The competition has five parts. Parts 3.b through 3.e are evidence stations; part 3.f is the written analysis. The number of samples and topics depends on the level of the tournament.

The rules list nine physical-evidence topics for part 3.e: fingerprints, DNA, glass, entomology, spatters, seeds and pollen, tracks and soil, blood, and bullet striations. A Regional uses only one or two of them, a National three to five, so you cannot predict which will appear and should prepare all nine. Mass spectra are part of 3.d at every level, including Regionals. The mock scene is built only from the topics a tournament uses, and questions and analysis can only draw on evidence that appeared in that competition. The approximate score weights are part 3.b about 20%, part 3.c about 20%, part 3.d about 15%, part 3.e about 15%, and part 3.f about 30%. Ties are broken by the score on the crime-scene analysis, and the analysis includes why suspects were eliminated or kept in the pool. The write-up outweighs any single bench station and decides close contests.

Strategy

Splitting the work and the clock

Give each partner a block of tests and share the write-up. Partner A takes salts and polymers (about 40% of the score), Partner B chromatography and physical evidence (about 30%), and both log results in one shared table.

A rough 50 minutes: about 5 reading the scenario and suspect list, 25 on the bench (write every observation at once, including negatives), 15 building the analysis from an evidence-by-suspect table, and 5 checking the write-up and cleaning the station, since the 10% cleanup penalty is easy to avoid.

What goes on the sheet

Use the sheet for what you cannot recall under pressure: the flame-color list with the sodium-masking note, a reagent table with positive results, the Benedict's scale as your Supervisor gives it, a plastics table (sinks or floats), a fiber burn table, the formulas for Rf, the medullary index, the angle of impact and the height of origin with one worked example each, and sketches of the eight fingerprint patterns. With two sheets, give one to chemistry and one to physical evidence.

Qualitative analysis of salts (Part 3.b)

The salts and sugars in part 3.b fall into a few groups, and each group has a signature test. Fifteen candidate solids are in play, drawn from sodium, lithium, potassium, calcium and magnesium salts plus two sugars, a starch, boric acid and an ammonium salt. None is a mixture, and every team sorts the same set. The identification is therefore a sorting problem: each unknown goes into a group by one test, then to a single member by a second test.

Solubility, pH and heating

Start with the cheap tests. Most of the solids dissolve readily in water, but several do not: cornstarch stays as a suspension, calcium carbonate is essentially insoluble, calcium sulfate is only slightly soluble, and boric acid dissolves modestly. Calcium nitrate, by contrast, dissolves very freely, which helps separate the three calcium salts. The pH of each solution also helps. Carbonate solutions are strongly basic, a bicarbonate solution is mildly basic, and salts of strong acids and strong bases (such as sodium chloride) stay close to neutral. Sodium acetate is weakly basic because acetate reacts slightly with water. Boric acid, magnesium sulfate and ammonium chloride give weakly acidic to near-neutral solutions (ammonium is a weak acid). Report pH as a range; your indicator sets the resolution.

Heating is the next cheap test. Sodium hydrogen carbonate decomposes when heated strongly, releasing carbon dioxide and water and leaving sodium carbonate behind:

Sugars char with a burnt-sugar smell. Ammonium chloride breaks into NH₃ and HCl gases that recombine as a white solid on cooler glass, so it seems to vanish and reappear. Point heated tubes away from people.

Flame tests

Heat excites electrons in metal ions; as they fall back they emit photons of characteristic energies, hence colors. The colors you need for the 2027 list are sodium (intense yellow), lithium (crimson), potassium (pale lilac) and calcium (orange-red). Sodium's yellow is so intense that it hides other colors.

The fix is cobalt blue glass. Cobalt glass absorbs much of the yellow light, so a potassium flame that is hidden by sodium can be seen through the glass as lilac.

Boric acid can show a green-tinged flame from boron, and ammonium chloride gives no metal color at all. Magnesium salts give no strong characteristic color. The flame cannot separate, say, sodium chloride from sodium acetate; a second test must.

Sodium hydroxide and the decision order

The Supervisor provides 1 M NaOH, which separates three cations that the flame test handles poorly. Magnesium ions form a white precipitate of magnesium hydroxide. Calcium ions also give a white cloudiness or precipitate (calcium hydroxide is only sparingly soluble), usually less than magnesium; use the orange-red calcium flame to tell them apart. Ammonium ions react with hydroxide to release ammonia gas, which you can smell and which turns damp pH paper blue when warmed gently.

Use the table as a decision aid; still run two independent tests on every unknown.

Benedict's and iodine tests

Iodine detects starch. Iodine and iodide combine into polyiodide chains (mainly I₃⁻ and I₅⁻) that fit inside the coiled amylose chains of starch and form a blue-black complex. Cornstarch gives a strong blue-black color; glucose, sucrose, and most salts do not. A drop or two of iodine is enough; excess iodine makes the color hard to read. The 2027 rules allow the Supervisor to supply either iodine in KI solution (Lugol's-type, yellow-brown) or a 2% tincture of iodine (iodine in alcohol, darker brown). Both give the same blue-black result with starch, so read the color change, not the starting shade, and run a known cornstarch sample beside the unknown if you are unsure.

Benedict's solution detects reducing sugars. The solution contains copper(II) ions in an alkaline citrate mixture. When a reducing sugar is heated with it, the copper is reduced and a colored precipitate forms. The color runs from blue (no reduction) through green, yellow and orange to brick red as more sugar is present. The commonly taught semi-quantitative scale puts green at roughly 0.5-1% reducing sugar, yellow at about 1-1.5%, orange at about 1.5-2%, and brick red above about 2%. Use the Supervisor's chart if one is given.

Glucose is a reducing sugar, so it gives a positive Benedict's result. Sucrose is a disaccharide made of glucose and fructose joined so that neither end is free to reduce copper. Sucrose is non-reducing and gives a negative result unless it is first broken into its monosaccharides by acid and heat.

Carbonates and a worked identification

Carbonates fizz with dilute acid because they release carbon dioxide. The three balanced equations you need are:

Polymers, fibers and hair (Part 3.c)

The 2027 lists are exact. The eight plastics are PETE, HDPE, LDPE, PP, PVC, PMMA, PC and solid (non-expanded) polystyrene; you never burn them, but the Supervisor may hand you burn results. Fibers: cotton, wool, silk, linen, nylon, spandex and polyester; burn tests are allowed. Hair: human, bat, cow, squirrel and horse, plus hair structure and scale classification.

Plastics and density

Density is the most useful simple test for plastics. A plastic floats in water if its density is below 1.0 g/cm³ and sinks if it is above that.

A density column is a more precise version of the water test. A sample that floats in a solution of known density has a density lower than that solution; a sample that sinks has a density higher than it. A sample that sinks in water but floats in a solution of density 1.25 g/cm³ must therefore have a density between 1.00 and 1.25 g/cm³.

PVC and PETE are both well above water in density and their ranges overlap, so a density bracket rarely separates them. Here the Supervisor's burn results matter: PVC releases hydrogen chloride when burned, giving sharp, acrid smoke, and it tends to self-extinguish. You never burn the plastics yourself, but you must be able to read burn data you are handed.

To separate the plastics that float from one another, or the ones that sink from one another, you need density solutions that the Supervisor may provide. The typical order, from least to most dense, is PP, LDPE, HDPE (all below water), then non-expanded polystyrene, PMMA, polycarbonate, PVC and PETE (all above water, with polystyrene only slightly denser than water). Typical densities are approximately 0.90, 0.92 and 0.95 g/cm³ for PP, LDPE and HDPE, about 1.05 g/cm³ for polystyrene, and roughly 1.2 to 1.4 g/cm³ for the others. Additives shift these, so trust what the samples do.

Fiber burn tests

Fibers are the one polymer group you may burn. Each of the three families burns in a characteristic way.

The most reliable distinction is between protein fibers and cellulose fibers. Protein fibers contain nitrogen from amino acids, so they smell like burning hair, and they stop burning when the flame is removed. Cellulose fibers are carbohydrates; they keep glowing and leave a soft ash. Synthetics melt and shrink from the flame, so watch the melt more than the smell, and burn a known reference beside the unknown.

Hair structure

Human hair has a root (the part in the follicle, ending in the bulb) and a shaft (the part you see) with three layers from the outside in. The cuticle is a shell of overlapping scales that protects the shaft. The cortex is the thick middle layer that holds pigment and gives the hair its strength. The medulla is a central core that may be continuous, fragmented or absent. The scale pattern differs between species and is one of the ways to tell animal hair from human hair. Three patterns are usually taught: imbricate scales are flattened and overlap like roof shingles (typical of human hair); coronal scales are stacked like a crown or a stack of cups (common in bats and many small rodents); and spinous scales are triangular and project outward like petals (seen in some furbearers such as mink). The medulla also differs. Human hair has a thin medulla that may be absent or broken into fragments, while many animal hairs show a wide, continuous medulla. Among the 2027 species, bat hair is fine with prominent coronal scales; squirrel hair has a wide medulla filling much of the shaft; cow and horse hairs are coarse with a continuous medulla and an index often above one half. Use scale pattern and medulla together against the reference images.

The medullary index compares the width of the medulla to the width of the whole hair:

Growth cycle and root. Know the three growth phases, because they decide what the root looks like. In anagen, the growing phase, the follicle is active and the root bulb is rounded, often with a sheath of tissue attached. In catagen, a short transition, growth stops and the root shrinks. In telogen, the resting phase, the root is a hard club shape, and the hair is eventually shed. A pulled anagen hair carries root tissue with nuclear DNA; a shed telogen hair or bare shaft usually yields only mitochondrial DNA.

Chromatography and spectroscopy (Part 3.d)

In paper chromatography, the paper is the stationary phase: water held in the cellulose fibers. The solvent creeping up the paper is the mobile phase. Each compound partitions between the two phases. Compounds that bind the paper more strongly (usually more polar) move less. Draw the origin line in pencil, because pen ink would itself run, and start with the line above the solvent level. A lane that splits into several spots is a mixture.

Rf values

The retention factor compares how far a spot travels with how far the solvent travels:

Record the solvent with each chromatogram, since a different solvent changes every Rf value.

Thin-layer chromatography (TLC) uses the same Rf ratio on a plate coated with silica. Silica is polar and the solvent is usually less polar, so the more polar compounds stick and travel shorter distances, which gives them lower Rf values.

Mass spectra

A mass spectrometer ionizes molecules, sorts the resulting ions by their mass-to-charge ratio (m/z), and records how many ions reach the detector at each m/z. The plot shows m/z on the horizontal axis and relative abundance on the vertical axis. Three features matter for this event:

• The molecular ion peak (M⁺) is the ion of the intact molecule. For a singly charged ion, its m/z equals the molar mass. It is usually the highest-m/z peak with a meaningful intensity.
• The base peak is the tallest peak and is assigned 100% relative abundance. It is the most stable fragment, not necessarily the molecule itself.
• The fragmentation pattern is the set of smaller peaks. Matching it to a reference spectrum identifies the compound.

Useful hints: a loss of 15 from M⁺ suggests a methyl group, a loss of 18 suggests water, and a small peak at M+1 is usually the carbon-13 isotope, not a heavier molecule.

Physical evidence (Part 3.e)

Part 3.e draws on the nine topics below. Questions ask you to recognize a pattern, explain a mechanism, or judge whether an observation supports a conclusion.

Fingerprints

Skin layers and how prints form. Skin has an outer epidermis over a deeper dermis, which sits on a fatty layer beneath. The boundary between epidermis and dermis is not flat: the dermis pushes up into the epidermis in small bumps called dermal papillae. Their arrangement sets the friction ridges that you see at the surface, and it is established in the womb. Because the pattern is set at the dermal-epidermal boundary, surface damage that heals without reaching the dermis does not alter it, while a deep wound can leave a permanent scar in the print. Identical twins still have different prints. Pores of the eccrine sweat glands open along the tops of the ridges, so a touch leaves a deposit of water, salts and amino acids, often mixed with skin oils (lipids) picked up from the face and hair. Prints recovered at a scene come in three types:

• Patent prints are visible, left in a substance such as blood or ink.
• Latent prints are invisible residue of skin oils, sweat and other material. They must be developed.
• Plastic prints are three-dimensional impressions left in a soft material such as wax, putty or soap. They can often be photographed without development.

The eight pattern types are the classification the rules name. Arches typically have no delta, loops have one, and whorls have two or more. Loops are the most common pattern (roughly 60 to 65% of people), whorls next (about a third), and arches the least (around 5%). The arch types are the plain arch and the tented arch. The loop types are the radial loop and the ulnar loop, which are named for the side of the hand toward which the loop opens: toward the thumb for radial, toward the little finger for ulnar. The whorl types are the plain whorl, the central pocket whorl, the accidental whorl and the double loop whorl. A pattern is a class characteristic. Many people share the same pattern type, so a pattern match narrows the pool but does not identify one person.

Individual features are called minutiae. The terms you need are these:

• Ridge ending: a ridge that stops.
• Bifurcation: a ridge that splits into two.
• Island (dot): a short ridge with no connection to its neighbors.
• Spur: a short ridge branching off a longer one.
• Enclosure (eye): a ridge that splits and rejoins, enclosing a small space.
• Delta: the triangular point where ridges from three directions meet; the core is the approximate center of the pattern.
• Trifurcation: a ridge that splits into three.
• Double bifurcation: two bifurcations in quick succession along one ridge.

A pattern is class evidence; the arrangement of minutiae within the pattern is what supports an individual match. Explain both levels in your write-up: "the pattern is a radial loop, and the three bifurcations in the same position match the suspect's known print" is a stronger statement than "the prints look alike."

Development methods make latent prints visible. The choice depends on the surface. The four methods the rules name are these:

Choose by porosity: paper absorbs the residue, so a reagent that reacts with it (ninhydrin) works; a nonporous surface keeps it on top, so powder or cyanoacrylate is preferred.

DNA and PCR

The competition uses DNA evidence at the level of chromatograms and electropherograms. You compare the pattern of peaks or bands from a scene sample with the patterns from suspects. Matches are judged at each locus shown: a suspect is consistent with the sample if the sample matches at every locus shown, and is excluded if the sample mismatches at any locus. Each peak marks an allele, a repeat count at a short tandem repeat locus. A person has two alleles per locus, one from each parent, so a heterozygote shows two peaks and a homozygote one taller peak. A child shares one allele with each parent at every locus. This is a pattern comparison, not a population-genetic calculation.

DNA is negatively charged because its phosphate backbone is ionized. In gel electrophoresis, the gel is an agarose matrix in a buffer. When voltage is applied, DNA migrates toward the positive electrode (the anode). Smaller fragments pass through the gel's pores more easily and travel farther than larger ones, so the bands are sorted by size.

The polymerase chain reaction (PCR) copies a target region of DNA. Each cycle has three steps. The DNA is heated to separate its strands (denaturation, typically near 95 °C). The temperature is lowered so short primers can bind to the flanking sequences (annealing, which depends on the primers and is often in the range of about 50-65 °C). A heat-stable polymerase extends the primers into new strands (extension, usually near 72 °C). Each cycle roughly doubles the target, so 30 cycles produce on the order of a billion copies in principle. The Nobel Prize's PCR educational game, linked from the rules, walks through the steps interactively.

The annealing temperature of a primer depends on its length and its G/C content, because G-C pairs have three hydrogen bonds and A-T pairs have two. The rough rule Tm ≈ 2 × (A + T) + 4 × (G + C), in °C, works only for short primers (under about 14 bases); for longer primers, use the number of G/C bases together with the length only to rank primers against each other. Adding bases never lowers the melting temperature, so G/C fraction alone can mislead when lengths differ.

Glass and refractive index

The refractive index (n) of a material is the ratio of the speed of light in a vacuum to the speed of light in that material. Ordinary soda-lime glass has an index near 1.5. When light crosses a boundary between materials with different indices, it bends according to Snell's law:

The Becke line test uses this bending to estimate an unknown glass fragment's index. A fragment is immersed in a liquid of known index, and the microscope is focused on the edge. A bright halo, the Becke line, appears at the boundary. When you raise the objective (increase the distance between the objective and the slide), the Becke line moves into the material with the higher refractive index. When the fragment's index equals the liquid's, the edge disappears. Many glasses share an index, so this narrows sources without individualizing.

Ordering the breaks. Rules ask which holes or fractures came first. Two ideas settle most questions. First, a crack stops when it runs into an existing crack, so a crack that terminates at another crack is the later of the two; the earlier crack is the one that runs through uninterrupted. Second, for a single impact, radial cracks (spokes from the impact point) form first and concentric cracks (rings) form afterward. Radial cracks start on the face of the glass opposite the impact, where the glass is stretched, and concentric cracks start on the impact face. A bullet hole is also a clue: the hole is usually wider, with a cone-shaped chip, on the side the bullet exited. With several holes, line them up by asking whose radial cracks stop at whose.

Glass type is found from the refractive index. Typical values are about 1.47 for borosilicate (laboratory) glass and roughly 1.51 to 1.52 for window and bottle glass, with lead crystal higher. Match your fragment's index to the table you are given, and remember that two glasses with similar indices cannot be told apart by this test alone.

Blood spatter

Bloodstain patterns record the physics of the drops that produced them. Four categories are common in the event:

• Passive drips fall under gravity alone and make round stains with a clear edge.
• Cast-off stains come from blood flung off a moving object, such as a swinging weapon, and form lines or arcs of drops. Projected stains, such as arterial spurts, are blood forced out under pressure.
• Transfer or contact stains are shapes left when a wet surface touches another surface.
• Void patterns form where an object blocked the spatter, leaving a blank area with an outline of the object's shape.

The shape of a single drop reflects the angle at which it hit. A drop striking a surface at 90 degrees makes a nearly circular stain. As the angle drops, the stain elongates into an ellipse, and the narrow end is the tail. The tail points in the direction the drop was traveling, so the origin of the spatter lies on the opposite side from the tail.

To locate the area of origin, draw a line along the long axis of each elongated stain, extending it backward from the tail toward the origin. In two dimensions, the lines from several stains should converge on a common point. That intersection is the area of origin. For the height, measure the distance d from a stain to the point of convergence and use the stain's angle:

Spatter size and velocity are related. Commonly cited ranges are low velocity (drops of roughly 4 mm or more, as from dripping), medium velocity (about 1 to 4 mm, as from blunt trauma) and high velocity (mist-like drops under about 1 mm, as from a gunshot). Cutoffs vary by source; use any the Supervisor gives. A stain pattern can also include satellite spatter, small drops around the main stain produced when a drop breaks apart on impact.

Blood typing

The ABO system is the main class-level blood test. Red cells carry A or B antigens or neither, and plasma carries antibodies against the antigens a person lacks. Type A has A antigens and anti-B antibodies; type B has B antigens and anti-A; AB has both antigens and neither antibody; type O has neither antigen and both antibodies. These are class properties: many people share each type, so typing narrows a pool but does not identify a person.

The event asks for two blood tasks. The first is ABO typing with artificial blood, following the Supervisor's instructions. In the usual method, a drop of blood is mixed with anti-A and anti-B serum (and anti-D for Rh), and clumping (agglutination) shows that the matching antigen is present. A sample that clumps with anti-A only is type A, with anti-B only is type B, with both is AB, and with neither is O.

The second task is to decide, from a slide or picture, whether blood is human, avian, mammalian (non-human) or reptilian/amphibian. Mammalian red cells are round, biconcave and have no nucleus, whereas the red cells of birds, reptiles and amphibians are oval and keep a visible nucleus. So a nucleus rules out a mammal. Bird red cells (about 11-16 µm long) are larger than human cells but smaller than reptile cells; amphibian cells are among the largest. A human cell is about 7 to 8 µm across, which is the size to compare against. Human versus other mammal relies on size against a scale.

Time of death: entomology and postmortem changes

Entomology questions here estimate the time since death from the insects found on a body. The method rests on two facts: insects colonize a body in a predictable succession, and their development speed depends on temperature. Blowflies (Calliphoridae) are typically the first insects to find a body. They lay eggs in moist openings and wounds, and the eggs hatch into larvae that feed and grow. The larvae pass through several stages before they pupate and emerge as adults. The life-cycle order is egg, larva, pupa, adult.

Beetles and other insects arrive later, once the body changes. In rough order, blowflies and flesh flies come while the body is fresh and bloated, other flies and early beetles during active decay, and dermestid beetles and mites when the remains are dry. Because development depends on temperature, the stage of the oldest larvae, combined with the temperature history of the scene, is what estimates the interval. Cold slows development and warm speeds it up, so an estimate must account for temperature. A standard way to use temperature is accumulated degree-hours (ADH): the development a species needs is a fixed total of degree-hours above a base temperature. For illustration only (the numbers are invented), suppose a species needs 1,200 degree-hours above 10 °C to reach pupation and the scene averaged 25 °C. Each hour contributes 15 degree-hours, so the time is 1,200 ÷ 15 = 80 hours. The estimate is a range, and the strongest answers say which evidence sets the minimum and which sets the maximum.

The rules frame this topic around the insects, so the insect evidence should carry your estimate. Body changes such as livor mortis (settled blood), algor mortis (cooling) and rigor mortis (stiffening) only help in the first day or so, and only if the scenario gives them; use them as a cross-check, not as the main method.

Seeds and pollen

You will see pictures, not live samples: seeds or pollen from the scene beside pictures from each suspect, or from different regions. Compare features in a fixed order. For seeds: overall shape, size against the scale bar, surface texture, and attachments such as wings, hooks or hairs (which also hint at how the seed travels and so where it could have been picked up). For pollen: the number and type of apertures (furrows called colpi, round pores, or both), the outline in polar view, and the wall ornamentation (smooth, spiny, netted). Pollen is produced in huge numbers and clings to clothing and hair, so a match links a suspect to a place or season rather than to the exact spot. State it that way: "the pollen on suspect B's jacket matches the flowering shrub at the scene, consistent with B having been near it."

Tracks and soil

Footprints and tire tracks are matched in two stages. First the class features: outsole or tread design, size, and for tires the tread width. These exclude suspects quickly; a different tread pattern is a clean elimination. Then individual features: cuts, nicks, stones lodged in the tread and uneven wear, which can tie a print to one shoe or tire. Compare the scene impression with the suspect's item at the same scale, and remember a print may be partial.

Soil questions give you a composition (color, texture or grain size, mineral content, organic matter, sometimes pH) for the scene and for samples taken from each suspect. Compare row by row. A suspect whose soil differs in a stable property, such as mineral makeup, can be excluded; a match is class evidence that places the suspect in a similar setting. Soil from a shoe can mix layers from several places, so a partial match deserves a cautious sentence, not an exclusion.

Bullet striations

A rifled barrel has spiral lands (raised strips) and grooves cut along it, which spin the bullet and scratch it. First compare class characteristics: caliber, the number of lands and grooves, their widths, and the twist direction (left or right). Any difference excludes that gun. Then compare the fine striations, the tiny scratches from imperfections in the barrel, by lining up the scene bullet beside a bullet test-fired from the suspect gun, as in a comparison microscope image. Matching striation patterns across several lands support the same barrel. On cartridge casings, the firing-pin impression, breech-face marks, and extractor and ejector marks play the same role. Say in the write-up which features were class and which were individual.

Crime-scene analysis (Part 3.f)

The written analysis is the largest part of the score, so structure it. A strong write-up has five parts:

1. A short summary of the scene: where the body or object was, what was found and what question you are answering.
2. An evidence table with one row per evidence item: what it is, what the test showed, whether it is class or individual, and which suspects it affects.
3. Suspect-by-suspect reasoning that says why each suspect is kept or excluded. The rules require you to explain why the chosen suspect was selected and why the others were not, so exclusions count as much as inclusions.
4. The conclusion, stated with its strength: which suspect the evidence most supports, and what evidence would change the answer.
5. Limitations and alternatives: what the evidence cannot rule out, and any alternative explanation the Supervisor's questions raise.

Locard's exchange principle (contact transfers material) is the logic behind it: hair, fibers, soil and blood link a person to a place only as strongly as the evidence is specific.

Never state that a class match identifies a person; say it is "consistent with" the suspect. Build an evidence-by-suspect matrix first, then write one paragraph per suspect from it:

Here B is eliminated twice by class evidence, A is consistent but never individually matched, and C is tied to the scene by an individual feature, so the written analysis names C and explains, item by item, why A and B were not chosen.

Common pitfalls

How tournaments test this

Scio.ly holds 10,734 archived Forensics tournament questions (10,666 from Division C) and 16,269 question-bank items.

About five in six questions are free response, so practice one-sentence mechanisms and a short justification for every conclusion.

The archive tags questions by subtopic, and tags overlap. Evidence Analysis is tagged on 5,659 archived questions (about 53%), Chemistry Lab on 4,519 (about 42%), Fingerprints on 1,121 (about 10%), Crime Scene on 1,000 (about 9%) and DNA on 755 (about 7%). Tags overlap, so shares exceed 100%.

Among tagged question styles, "explain or why" leads (about 9% of archived questions). Calculations are a small share (about 2%), but the calculations that recur across this guide are Rf, the angle of impact, the height of origin, the medullary index and molar mass from a mass spectrum. Practice all five until each takes under a minute.

Frequent phrases include "crime scene" (1,191 questions), "chemical formula" (793), "flame test" (447) and "burn test" (241), so write a balanced equation for any reaction you cite.

The most frequent correct answers are PETE (51 questions), synthetic (38), cornstarch (36), wool (36), sodium carbonate (34), animal (33), silk (33), cotton (32) and calcium carbonate (30).

Test yourself

These questions come from archived tournament tests with official keys, re-solved against this guide, and limited to topics in the 2027 outline.

Test yourselfTest yourself: qualitative analysis, polymers and chromatography

0 / 0 correct

1. 1

   A polymer floats in water and feels waxy. Identify it.

   Question 1 options

   APVCBPolyethyleneCPolystyreneDPETE

   From: Dick Smith Memorial Invitational - PA, 2025-26, Division C

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Test yourselfTest yourself: fingerprints, glass and DNA

0 / 0 correct

1. 1

   What type of fingerprint development methods would be used to examine latent fingerprints on a paper cup?

   Question 1 options

   AIodineBNinhydrinCCyanoacrylateDWetwop

   From: Brown Invitational, 2022-23, Division C

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Test yourselfTest yourself: entomology and spatters

0 / 0 correct

1. 1

   What insect typically arrives first to the scene of a dead body?

   Question 1 options

   AMitesBBeetlesCMothsDBlowflies

   From: GullSO Invitational, 2023-24, Division C

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Related guides

No forensics subtopic guides exist yet. The guides below cover the chemistry, materials and biology behind this event:

• Chemistry Lab for reactions, indicators and lab technique.
• Disease Detectives for another case-based event that rewards clear reasoning.
• Designer Genes for DNA structure, PCR and electrophoresis.
• Botany for seed structure and dispersal.

References

• Science Olympiad (soinc.org): the 2027 rules manual, Forensics C (pages C35-C36), which sets the level table, substance and polymer lists, physical-evidence topics and score weights; the event page links the chemistry lab equipment list.
• SciOly Wiki, Forensics: https://scioly.org/wiki/index.php/Forensics (linked for navigation only; no text is reproduced here).
• Nobel Prize educational game on PCR, linked from the rules: http://educationalgames.nobelprize.org/educational/chemistry/pcr/
• OpenStax, Chemistry 2e (free online): https://openstax.org/details/books/chemistry-2e. Use for reactions, acids, indicators and solubility.
• OpenStax, Biology 2e (free online): https://openstax.org/details/books/biology-2e. Use for DNA structure, replication, repair and PCR.
• Saferstein, R., Criminalistics: An Introduction to Forensic Science (textbook). Standard reference for fibers, glass, paint and drugs.
• James, S. H. and Nordby, J. J. (eds.), Forensic Science: An Introduction to Scientific and Investigative Techniques (textbook). Covers fingerprint development and crime-scene methods.
• Bevel, T. and Gardner, R., Bloodstain Pattern Analysis with an Introduction to Crime Scene Reconstruction (textbook). Source for stain geometry, angle of impact and area of origin.
• Caddy, B. (ed.), Forensic Examination of Glass and Paint (textbook). Source for refractive index measurement and fracture analysis.
• Byrd, J. H. and Castner, J. L. (eds.), Forensic Entomology: The Utility of Arthropods in Legal Investigations (textbook). Source for insect succession and development.