Trace Evidence Examination Questions and Answers

Answer: A

The correct matching is a-ii, b-i, c-iv, d-iii.

• Oils in oil-based paints, such as linseed oil, are triglycerides of fatty acids (ii).
• Pigments are mostly inorganic oxides, such as titanium dioxide and iron oxides (i).
• Solvents are organic liquids that dissolve the binder and evaporate as the paint dries (iv).
• Latex paints use binders of acrylic or vinyl polymers dispersed in water (iii).

Only the first code gives (ii), (i), (iv), (iii). The last code swaps solvent and latex.

Answer: A

The chloroform test is used for the specific gravity of cement.

Chloroform is a heavy liquid with a density of about 1.48 g/cm³. Portland cement particles, with a specific gravity of about 3.1, sink in it, while lighter adulterants or foreign materials float. Shaking a cement sample with chloroform and observing what sinks and what floats is therefore a simple density (specific gravity) separation test used in the forensic examination of cement and in detecting adulteration.

Cement does not melt in any test of this kind, refractive index is measured on individual particles under the microscope, and elasticity is not a property tested with chloroform.

Answer: D

The rock that is not sedimentary is granite.

Granite is an igneous rock, formed by the slow cooling and crystallisation of magma deep underground. It consists of interlocking crystals of quartz, feldspar and mica, which is why it is coarse-grained and hard.

Sandstone (cemented sand grains), mudstone (compacted mud and clay) and conglomerate (rounded pebbles in a finer matrix) are all clastic sedimentary rocks, formed by the deposition and cementation of particles eroded from older rocks. In forensic geology, the mineral grains of soil reflect such parent rocks, which helps link soil samples to a place.

Answer: A

The liquid described is varnish.

A varnish is a transparent solution of a resin (natural, such as shellac or copal, or synthetic) in a drying oil or solvent. It dries to a hard, smooth, glossy, transparent film that protects the surface beneath, usually wood, from moisture and air, without hiding its grain.

Distemper is an opaque, water-based paint with chalk and glue. A drier is a metallic compound added to paint to speed drying, and the vehicle is the liquid portion of a paint that carries the pigment. Neither drier nor vehicle is a finish in its own right.

Answer: C

The solution is useful for restoring erased numbers on lead.

Restoration works by etching the metal: the stamping left a zone of deformed, strained metal below the surface, and an etchant attacks that zone faster than the undisturbed metal around it, so the number reappears. Each metal needs its own etchant. For lead, such as a battery casing, the reagent given in forensic practice guides is glacial acetic acid with hydrogen peroxide.

The other surfaces use other reagents. Steel and cast iron are etched with acid mixtures such as Fry's reagent (cupric chloride in hydrochloric acid) or acidified dichromate, and zinc alloys with an alkaline or chromic acid solution. Stainless steel needs its own acid etchant.

Answer: B

Assertion is true but Reason is false.

High alumina cement resists high temperatures, frost and chemical attack, including acids and sulphates, so it is used in refractory concrete, furnaces and works exposed to aggressive conditions. The assertion is true.

The reason is false: high alumina cement is a rapid-hardening cement that develops very high early strength, gaining in a day what ordinary Portland cement gains in weeks. Its known weakness is loss of strength over time through 'conversion' in warm, wet conditions, not low strength as such.

Answer: C

Trichology is the study of hair.

The word comes from the Greek thrix (genitive trichos), hair. In forensic science, the examination of hair for its species, body area, colour, medulla and scale pattern is an important part of trace evidence.

The study of bones is osteology, of nails onychology, and of ear prints otoscopy or ear identification. None of these is trichology.

Answer: C

The key gives Perlon as the trade name of nylon 6.

Perlon is the German trade name for nylon 6 (polycaprolactam, made from caprolactam), and it is the name textbooks and trade-name lists give for it. Nylon 6,6, made from hexamethylenediamine and adipic acid, is the other common nylon and is a different polymer.

Nylfrance and ICI-nylon were not found in the lists of nylon 6 trade names consulted, so they are not taken as nylon 6 names here.

Amilan, a Toray brand, is a weaker candidate. It is a brand name covering several polyamide grades, nylon 6 among them, so a reader could argue for it. Perlon is the standard answer and is the one the key accepts.

Answer: C

The body hair of the human foetus is non-medullated and non-pigmented.

Foetal body hair, called lanugo, is fine, soft and short. It has no medulla (central core) and little or no pigment, which makes it different from the coarser, pigmented terminal hair of adults. Its presence helps in estimating foetal age.

The single-feature options are incomplete, since lanugo lacks both a medulla and pigment, and 'medullated and pigmented' describes adult terminal hair.

Answer: C

The key gives soil.

Coning and quartering is a way to reduce a bulk granular sample to a small, representative portion. The material is heaped into a cone, flattened, divided into four, and two opposite quarters are rejected; the rest is mixed and the process repeated. Soil samples collected at a scene or in the field are reduced this way before analysis.

Glass and paint are examined as fragments or flakes and are not reduced by this method.

What could not be confirmed: the same method is used for ores and mineral powders, so 'minerals' is also a fair answer in general chemistry. In a forensic setting soil is the usual example, and that is the reading the key takes.