Duckweed is a free floating plant and students make direct observations on plant viability. The plant fronds which look like leaves are actually the stems. The plant obtains its nutrients directly from the water it is floating on, so dosing the samples with water soluble chemicals is simple.
To mimic an actual bioassay where a number of different concentrations are tested on a population of plants separately, the experiment involved similar populations of duckweed in separate cups. Soap at two concentration levels was added to two different cups with duckweed, plus another cup without any added soap served as a control. All cups received the same quantity of nutrient solution for the lab.
After setting up the lab experiment, the 3rd graders made observations over two days to determine if the soap helped or hindered the duckweed.
The formal handout for the third grade students with a summary table for multiple classrooms is provided here. ( Duckweed experiment handout )