Simple Dilution

OBJECTIVE: The students shall demonstrate their understanding of dilution by making dilutions and determining the concentration of an unknown. They will further demonstrate some basic lab skills needed for this lab by 1) reading the meter on a simple homemade spectrophotometer 2) keeping careful laboratory records 3) graphing results on linear graph paper 4) using a colored filter to enhance contrast and sensitivity

PURPOSE: Dilution is an essential skill and understanding for both biology and chemistry. A "simple" or linear dilution is one than will be encountered in everyday life as well, from measuring out bleach for a wash load, adapting a recipe, to mixing a blend of special oils for an auto project.

SET: I went to the movies the other day and I think they watered down my cola. How could I test for this?


MODEL: 1) the spectrum 2) absorption of cola 3) absorption of a blue filter 4) the spectrophotometer 5) the standard curve

STRUCTURED PRACTICE: Instructor models doing the standard curve while students observe and ask questions.

GUIDED PRACTICE: Students do a standard curve with their "spectrophotometers" and use this to determine the concentration of an unknown.

CLOSURE: How could I improve the accuracy of my spectrophotometer? What would happen if the unknown was between the first dilution and the blank (zero concentration)? Could I make up a new standard curve? How would I do that?

INDEPENDENT PRACTICE: The "Unknowns", lab reports and Implication Questions.

NOTES: Unless you want to make up a separate lesson for building the spectrophotometer, the instructor should make up all but one of these ahead of time. The last one can be shown in parts for students to see how it all fits together.