H13
Classifying and Identifying Substances
The forensic chemist has the job of identifying and tracing the origins of many
substances. The classification of substances is a very important part of this process.
Contamination of
samples
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Inadvertent addition of extraneous material in the
collection, handling and transporting of the sample.
An unclean laboratory.
A careless analyst
Accuracy in forensic chemistry is particularly important as
faulty evidence could convict an innocent person or result in a
person or company paying a massive fine.
Organic compounds
Compounds of carbon (with the exception of carbon
monoxide, carbon dioxide and carbonates, hydrogen
carbonates and cyanides of metals).
Most compounds of living organisms are organic.
Organic compounds burn or react with air or decompose when
heated to leave no residue.
Analysis of soil
Soil is a complex mixture of inorganic material (clay, silt and
gravel) decaying organic material (humus), water, air and
living organisms.
The following tests are performed in the analysis of soil:
 Colour
 Texture
 Appearance when shaken with water
 pH
 Atomic emission or atomic absorption spectroscopy is
used to determine the presence and amounts of the less
common elements.
Simple carbohydrates
Compounds of C, H and O, having the general formula
Cx(H2O)y where x and y may be the same or different.
Pentose sugars
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Have 5 carbon atoms per molecule.
Forms a ring structure with the H, OH or CH2OH above
or below the ring
Ribose
Hexose sugars
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Monosaccharides
Have 6 carbon atoms per molecule
Forms a ring structure with the H, OH or CH2OH above
or below the ring
Examples include:
o Glucose
o Fructose
o Galactose
Carbohydrates that cannot be hydrolysed into two or more
simpler sugars (pentose and hexose sugars).
Monosaccharides exist as a ring and an open-chain form and
these forms are in equilibrium with each other. This
equilibrium leads to  isomerism.
Disaccharides
Carbohydrates that can be hydrolysed into simpler forms.
Disaccharides are dimers in which the monomer is a
monosaccharide.
Condensation
reactions
Polysaccharides
Reaction in which two molecules join together with the
elimination of a small molecule, such as water.
glucose + fructose
sucrose + water
glucose + glucose
maltose + water
Carbohydrates that consist of a large number of
monosaccharide molecules joined together in a chain.
Examples of polysaccharides include
 Cellulose an insoluble plant polysaccharide made of glucose monomers formed into unbranched chains.
 Amylose a soluble plant polysaccharide made of glucose molecules formed into unbranched chains.
 Amylopectin an insoluble plant polysaccharide made of
-glucose monomers formed into branch-chains.
 Glycogen a soluble animal polysaccharide made of glucose monomers formed into unbranched chains.
Reducing sugars
Have an OH attached to the same C as a ring O atom and are
easily oxidised.
The presence of a reducing sugar can be demonstrated by tests
using the following reagents
 Tollens’ reagent (colourless solution of silver nitrate in
aqueous ammonia).
 Benedict’s solution (deep blue solution of copper sulfate
in alkaline citrate).
 Fehling’s solution (deep blue solution of copper sulfate in
alkaline tartrate).
Test for carbohydrate
Develops an intense brownish red colour when aqueous
phenol is added to an aqueous solution of the sample and
concentrated sulfuric acid is added.
Test for starch
Deep blue complex with iodine.
Test for cellulose
Calcofluor fluoresces in the presence of starch.
Test for glycogen
Pale pink colour with starch solution.
Lipids
Naturally occurring organic compounds that are soluble in
non-polar organic solvents.
The two classes of lipids are
 Esters of glycerol (fats and oils)
 Compounds that have a cholesterol-like structure and
including the steroids.
Fats and oils
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Insoluble in water
Ester functional group
Will dissolve certain dyes such as Sudan III.
Hydrolysed by heating with sodium hydroxide solution to
form glycerol and the sodium salt of the fatty acid (the
saponification reaction).
Fats are solid lipids, formed from saturated fatty acids
and predominate in animals.
Oils are liquid lipids, formed from unsaturated fatty acids
and predominate in plants.