Teacher's Guide Techniques Lab A2: Practice with Viscous Solutions
Time of lab
Best to allow part of one period for this lab. Either precede LabA1 or as a review
before Lab C loading step.
Preparation before lab (see also Lab A1)
Check that colored glycerol solutions have been aliquoted in 1.5 mL reaction tubes.
Set out supplies. Tips and tubes need NOT
What to demonstrate/explain
Encourage students to really observe the volume of liquid in the pipet at each step
-- also to note if there is liquid on the outside of the tip.
Plan to demonstrate the use of a micropipet. Use a full page illustration (in plastic
page protector). A set of 20 are in a notebook in Teacher's Box 1.
Watch students' micropipeting technique, and lend assistance as needed. See micropipet
cautions of Techniques Lab A1. The two most common mistakes are:
depressing the plunger to the second stop before filling the pipettor (they must
stop at the first)
releasing the plunger after dispensing the solution while removing the pipettor
from the tube (they must keep the plunger depressed until free of the tube or they
will suck liquid back into the tip.)
Some teachers require that each student demonstrate his/her proficiency at each
step before going on...No air in the tip and no liquid left in the tube...Repeat
until they get it right!
A grading rubric: C for proper use of P20; B for accurately guessing amount of
unknown liquid in a pipet tip; A accurately guessing amount of unknown sample on
waxed paper; A+ for accurately guessing amount of sample in tube!
Have a student refill food coloring solutions for the next class.