TAC Experiment - Background

Teacher Background
Students are most engaged in learning when they can steer the direction of their own learning. In this lesson, students are able to design an original experiment, learn about experimental variables and controls, and present their results to the class. They use their experience with terraqua columns, water quality testing and soil analysis to guide their inquiry in a direction of their choosing.

To get a sense of what students might come up with, these are a few of the experiments my students conducted:

  • add soda to the water
  • add vinegar to the water
  • compact the soil by stomping on it after planting the seeds
  • compare a regular column made from 2 liter soda bottles to a tiny column made from 500 ml water bottles
  • use a wick made from thick cotton twine rather than a strip of towel
  • throw garbage from a school lunch on top of the soil before watering
  • use water from the San Francisco Bay versus tap water
  • use water from the creek versus tap water
  • use garden soil versus school yard soil
  • add fertilizer
  • add compost
  • put one in direct sunlight and the other in shade
  • put one outside and one inside the classroom
  • add ice to the water each day

A key outcome of this project is a basic understanding of the scientific process and of variables in experiments. While your students may not need to know everything that is outlined below, it is essential that the teacher is solidly grounded in the scientific method and experimental design issues. Pick and choose from the content below to decide how much your students need to know.

The Scientific Method
The scientific method is simply the process scientists use to learn about the world. Generally, the steps are

  1. Pose a question.
  2. Research as much as possible about that question.
  3. Based on that research, come up with a hypothesis (an educated guess).
  4. Design and conduct an experiment.
  5. Collect observations (your results).
  6. Draw conclusions that hopefully answer your question and compare your results to your hypothesis.

For example, in this project, a team might start with the question: “If I spilled a soda, how would that pollution affect the plants, water, and soil in an ecosystem?” They would find out what they know about pollution, urban runoff, and the chemical composition of soda. Finally, they might hypothesize: “Pollution will make the plants grow more slowly or not at all, make the water have a low pH because of the acid in soda, and make the soil have a low pH as well.” They would design an experiment: in one terraqua column, pour a soda into the water chamber and in the other, use plain water. They would measure the height of any plants that grew as well as the pH of the water and soil. Finally, they would look at their results over the month and draw some conclusions.

One unique element that makes science different than any other discipline is the controlled experiment. In science, we create theories based on evidence, and the evidence must be something that other people can repeat and observe for themselves. Every step of the scientific method centers upon understanding the need to control variables. So what is a variable and what does it mean to control a variable?

A variable is anything that might change in an experiment. Anything that you as the scientist change is a variable (like whether or not to put soda on your plants). Most everything that you measure and observe at the end of an experiment is a variable (like whether or not the plants grow and how tall they grow). And there are often variables that you don't even notice but can affect the experiments (the temperature in the room, how much time has passed, how damp the room is, whether the spoon you are using to measure materials is clean or not). An experiment has three kinds of variables: independent, dependent, and controlled.

The scientist changes the independent variable. In a good experiment there is only one independent variable. As the scientist changes the independent variable, he or she observes what happens. In our example, the independent variable is whether or not the scientist added soda to the water compartment.

The dependent variable is what you measure and want to know about in your results. The dependent variable changes in response to the independent variable. For example, the dependent variable is the height of the plants, the pH of the soil and the pH of the water.

Experiments also have controlled variables. Controlled variables are quantities that a scientist wants to remain constant, and he must observe them just as carefully as the dependent variables. For example, if we want to measure the difference in the height of plants in soda versus water, it is essential that we use the same kind of seeds in both terraqua columns. If we use radish seeds in one column and carrot seeds in the other, we can't be sure if the carrot seeds with soda grew more slowly because of the soda or because carrot seeds normally grow more slowly than radish seeds. Similarly, we want to make sure that they get the same amount of light, that they have the same type of soil to grow in, and that they stay the same temperature.

In a controlled experiment the scientist has considered all 3 types of variables. She tests a hypothesis by changing the independent variable and noting the effect on the dependent variables, all the while making sure that controlled variables stay the same. Good experiments make it so that the only difference between the control group and the experimental group is the independent variable.

Experimental and Control Groups
In many experiments, such as these terraqua column experiments, the scientist makes a comparison between different groups. The group that does not receive any treatments - the one where the independent variable is not changed - is called the control group. For example, a plant with no fertilizer.  The group or groups that do receive a treatment - the ones where the independent variable is changed - is called the experimental group. It is essential to have a control group in these types of experiments or you will not be able to determine if a plant that got a treatment is any different.

When all of these details are taken care of, and when a difference in the dependent variable exists, then the experimenter can say it was the independent variable that caused the difference. There have been plenty of bad experiments in the real world. So, a smart scientist (and a good students) will look to see exactly how the experiment was designed and conducted in order to determine if the conclusion drawn from the data is really true.

Student Prerequisites
Student should have:

  1. Built and observed a terraqua column (see Terraqua Column lesson)
  2. Tested water quality (see Water Analysis lesson)
  3. Tested soil (see Soil Analysis lesson)