A “win-win” lab exam

I have a suggestion that I hope will please you — and your students. It concerns a special lab exam.

Let me explain — in broad strokes — what I do and how I do it. Sometime before the end of the course I post on our class web-page a list of possible questions — I call them scenarios — for the lab exam. Each scenario is based on a lab that was performed, and comes with pre- and post-lab questions. Last year I had:

• determination of %(m/m) of NaOH in crystal Drāno, by acid-base microtitration, given a known g/L concentration of (aluminum-free) Drāno solution
• visible spectrophotometry using a Spec 20 (solutions provided may be Kool-Aid or food colouring)
• determination of the molar mass of butane (from a disposable lighter) by downward displacement of water
• determination of % (m/m) of calcium carbide in impure CaC₂ by downward displacement of water
• identification of unknown(s) from a given list

Currently, I am researching a scenario involving acid-base titration — micro and macro (buret) versions — of ascorbic acid from a vitamin C tablet.

I allow each student to bring one sheet of notebook paper into the exam, in his own handwriting (no computers), as a “security blanket”. Students report that they really don’t use this sheet —the value lies in the processing required to prepare it.

Students are told that they will be randomly assigned¹ one scenario as they enter the lab.

Here are some important nuts and bolts:

• I commandeer all the science labs to have enough space.
• Before the exam I cover the windows to the lab, so snoopy students can’t see inside.
• I ask for science teachers as co-invigilators; I’m the roaming trouble-shooter.
• At each station is a data sheet and a lab exam sheet, complete with the student’s name on it, to ensure that everyone completes his assigned scenario.
• Students work alone — each station has everything required for a given scenario, except maybe an electronic balance. This pretty much eliminates traffic.
• Marks are also awarded for attention to safety (read: goggles) and clean-up.
• Scenarios are chosen for ease of set-up and for the fact that they can be completed in under 30 minutes.²

The advantages of a lab exam of this type extend to teachers and students: Teachers are happy that students have studied and learned a bunch of important labs. Students appreciate that they get the questions up-front — this helps them earn a good grade. That said, grades of 100% are rare, owing to the fact that teenagers³ typically over-rate their preparation and sometimes forget the importance of safety goggles, a spit-polish clean-up and the proper use of units and significant figures.

Teachers need not present all scenarios in the lab exam. One year, when time was tight, everyone did the micro-titration. But students didn’t know that — they studied everything (Suckers!).

Finally, a separate laboratory exam allows the teacher to remove lab-related material, except maybe the “design an experiment”-type question from the final exam. This can allow for more thorough questions on theory-related topics.

Welcome to win-win, my brothers and sisters in chemistry education.

Notes

1. I typically assign the scenarios beforehand, but I don’t tell the students.
2. I allow one hour, though.
3. Especially at an all-boys school, like Crescent.