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