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 an explicit number of duckweed fronds which were subjected to added soap. In each classroom, the soap dosed fronds were compared to a control without any added soap. All cups had a certain starting volume of nutrient solution plus either more water for the control or added soap.
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 )