Chemistry/Forensic Analysis

Preparation Procedure for Laboratory Experiment

Forensic Analysis Preparation — 1
Chemicals For a class of 24 students (12 pairs)

Equipment

 * 12 – spatulas
 * 3 – centrifuges
 * 24 – 250 mL beakers
 * 12 – match books
 * 24 – small test tubes
 * 24 – stirring rods
 * 12 – ceramic pads
 * 12 – test tube holders
 * 12 – rubber stoppers to fit test tubes	6–hot plates
 * 12 – evaporating dishes

Forensic Analysis Preparation — 2
Unknowns (Actual  Composition) Note that two unknowns in each set contain two different substances, one of which is soluble in water and one of which is insoluble. Each student will receive three separate unknown samples, labelled as a, b and c.

Note: The chemicals in each sample must be extremely well mixed, grinding them if necessary to guarantee that a “pea–sized” sample contains equal volumes of each substance.

Forensic Analysis Preparation — 3
Student Unknown  Numbers and their Contents

Forensic Analysis Preparation — 4
Note: There are actually only 10 different mixtures, which are distributed as follows.

TEACHER NOTES

 * 1) The tests have been tried exhaustively and appear to work well. This lab is a test of a student’s ability to accurately follow instructions.  A student (or pair of student, if desired) should be able to test 3 samples in one hour IF they have been shown how to use a centrifuge properly in the period previous to the lab.  Also, in the previous period students should be shown how to read the Instructions and how to interpret the results and what actions to follow.  They should be shown how the Flow Chart allows them to keep track of where they are at a given point in the analysis.  They should understand that “Analysis is Complete” should be interpreted as “Now start afresh on the next sample”.
 * 2) IMPORTANT: Students must be instructed that all measurements are approximate, so that “1 mL” can be a bit more or less than 1 mL, for instance, and still give good results. ALSO, to avoid contamination when using 10¬mL graduated cylinders, students MUST wash out a graduated cylinder three times with ordinary tap water before putting a new solution in it.
 * 3) At the start of the lab period, have one student at each bench get a hot plate, fill a 250 mL beaker 3/4 full of water and set the hot plate on “high”. This is needed for students who have to do Step 9.  Warn students that whenever the water boils bown to 1/2 full, they should add extra water using another beaker.
 * 4) The anthocyanin solution should be kept in the refrigerator until ready to be used. (Storage at room temperature for a week may cause decomposition and changes in the colours.)
 * 5) The Fehling’s test may give a false positive if reducing impurities are present. If a solution does not turn yellow–orange within about 30 seconds, the result should be considered to be NEGATIVE.

SUGGESTED MARKING SCHEME
2 marks for each substance correctly identified +  2 marks for quality of written descriptions of results

Total = 12 marks