The activity was developed in 2013 by Carter Smith, Matthew Hoffer and Jason Liebovitz of Crescent School’s Chemistry Education Research Group (CERG). CERG is a small group of chemistry students who meet with me each week for 90 minutes to research the suitability of the activities we use in our program. Our CERG is not a chemistry club; we're not making ice cream or slime or watching chunks of sodium thrown into a swimming pool. We do legitimate research in service of Chemistry Education at Crescent School. I invite our learned readers to set up their own CERG; feel free to contact me through Chem 13 News (chem13news@uwaterloo.ca).
The lab below was tested to “perfection” by Crescent School’s CERG.
Analysis of a mixture
Someone once told me that she suspected that certain cafeterias “cut” their sugar packets with baking soda, ostensibly to save money. While I didn’t for a second believe this, I thought that it might make an engaging activity to start grade 11 chemistry.
I provide each pair of students with three numbered, but unlabelled, samples:
• 1 g sucrose
• 1 g NaHCO3
• 0.5 g sucrose + 0.5 g NaHCO3
Samples are ground with a mortar and pestle beforehand, so that they appear uniform. This prevents the crystals of sucrose to be easily distinguishable from powdery baking soda.
In a pre-lab discussion students decide what they will need to identify each sample — no tasting. Invariably, they request a conductivity apparatus, phenolphthalein, red litmus paper and vinegar. They are also given standard laboratory glassware, including a Beral pipet and a 10 mL graduated cylinder.
After dissolving each sample in, say 20 mL of distilled water, students carry out qualitative tests on conductivity and pH of aqueous solutions of each sample. This allows the pure sucrose — non-conducting and not basic — to be identified unambiguously.
To differentiate the 50-50 mixture of sucrose and NaHCO3 from the pure NaHCO3, students add vinegar in increments of 5 mL; the 1.0 g sample of baking soda will therefore “absorb” more vinegar. This introduces students to a semi-quantitative test.
Post-lab questions can include:
1. Prepare and complete a table as shown below. Point form is acceptable.
Columns for Sample ID #, Test performed, Observation(s), Interpretation(s), Identity of the sample
2. You’re sitting in a fast-food restaurant with your non-chemist friends. Someone at your table insists that the management “cuts” their packets of sugar with baking soda. Explain how you could test this without tasting anything.
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