Apologia Chemistry: White Powder Lab Forensics

Update: The conductivity probes were producing inconsistent results.  I got out these conductivity meters instead.  The materials tested may be too low for voltmeter sensors, which are designed to use with batteries.  The conductivity meters are used to test water and work better for this lab.  Sometimes you have to punt.

Update:  The White Powder lab is a good example of qualitative analysis or testing chemicals systematically to identify unknown compounds.

Thursday the class is doing a forensics lab, What is that white powder?  The kids start by testing several household chemicals: table salt, washing soda, baking soda, sugar, cornstarch, cream of tartar,  Epsom salts, washing soda, glucose or corn sugar, chalk, and plaster of Paris.  The kids test each white powder with universal indicator, iodine, water, HCl, Benedict’s reagent, and NaOH.  As the kids perform these tests, they record detailed notes about each chemical.  It is important to use small amounts of each chemical and to test samples separately.  The kids tend to run iodine, water, pH, etc. tests at once.  Step two is to test the same white powders as unknowns.  The kids may retest known powders to verify their identifications.  This lab takes at least one long class period and often two periods.  The kids are graded on correctly identifying all of the unknown powders.

If you have younger students, simplify the lab and use cornstarch, salt, baking soda, sugar, chalk, and Epsom salts.  You will get a variety of results with these chemicals.  Treat it as a CSI lab.  Use plastic art palettes, disposable pipettes, safety glasses, and disposable aprons.  Put the chemicals into small containers or plastic bags.  Label each one with both the chemical and common names, such as NaHCO3 and baking soda.  Put the unknowns in separate bags marked with numbers.  Don’t forget to make yourself a key.  The kids love this lab.  They feel like real scientists.

To prep for this lab, I pulled out my electric kettle.  The copper sulfate reagent is Benedict’s solution. The kids put hot water in a beaker and placed samples in the hot water bath and added the Benedict’s solution.  Only glucose reacts with Benedict’s; I did a demo for the kids.  When they start their unknowns, they will have to test all of the powders with Benedict’s.  To save time, when the kids narrow down their unknowns, glucose and sucrose are similar.  I will have them test just the two to differentiate glucose from sucrose.

I used two types of conductivity meters: Pasco Spark and Vernier with Easylink.  Basically, the kids recorded either a positive or negative charge.  Encourage the kids to stay organized and to write detailed notes about each reaction.  The kids should be able to complete testing on knowns in one hour.  Keep the knowns available.  Sometimes, they need to retest a known to confirm identification of an unknown.

 This is the TI 84 calculator with a Vernier Easylink and Vernier conductivity probe or sensor.

 It helps to stay organized.  The kids labeled craft sticks with the test reagents and the chemicals.

 This is the Pasco Spark unit with a Pasco conductivity probe used to measure the conductivity.

The lab requires a number of supplies, a high level of organization, and detailed notes.