"When are we going to do an experiment?" This is a common refrain in middle school science. An important feature of science instruction is to help students understand that there is a distinct difference between "messing around in the lab", "randomly mixing chemicals together", "blowing things up" and actually planning an investigation that results in meaningful and accurate data. The trick is that messing around just sounds like so much fun!
This lesson on Exploring Controlled Experiments is an introduction to how to plan an investigation individually and collaboratively, and in the experimental design to:
In this practice, students also explore the nature of scientific concepts such as: "Scientific Investigations Use a Variety of Methods" and "Scientific Knowledge is Based on Empirical Evidence". Without controlled experiments and the various approaches to design those experiments, students' data become less empirical and more muddled, making if more difficult to answer "Questions About the Natural and Material World". The basis for controlled experiments rests on the cross-cutting concept of Cause and Effect. By exploring variables, students recognize that cause and effect relationships may be used to predict phenomena.
The Exploring Controlled Experiments series of lessons is an investigation that including lessons taught over the span of 2 or more days or equivalent block periods. To help manage the magnitude of this activity, you will find the project split into 2 parts. This lesson works in conjunction with Exploring Scientific Methods as well.
In order to ENGAGE students in this lesson, I announce that we are going to analyze an experiment conducted on children. The idea of conducting experiments on children is enough to catch everyone's attention! I ask students to watch this video, while thinking about three prompts:
1) What does the teacher change about the students or their environment?
2) What does the teacher measure to see if the change affects the students?
3) What does the teacher discover in her experiment?
After viewing the video, I re-frame the prompts to lead students toward the idea of independent and dependent variables and the cross-cutting concept of cause and effect:
1) What does the teacher change about the students or their environment? What causes the outcome?
2) What does the teacher measure to see if the change affects the students? What is the effect resulting from the cause?
3) What does the teacher discover in her experiment?
During discussion of students' answers to these questions, I introduce the scientific vocabulary of:
Independent Variable and Dependent Variable
Teacher Note: For many students, this is the first introduction to variables; it will also not be the last time we revisit the topic. It is very much worth the time to build understanding of the word variable as a factor that varies or changes and to continue to ask these types of questions repeatedly throughout the year. The questions posed in the section as well as in the upcoming "Explore" section are examples of questions I use while introducing every investigation or experiment that we do. While the concept of an independent variable as a "cause" and a dependent variable as an "effect" seems simple enough, in my experience, it is not a concept the "sticks" easily for many students.
Once students understand the basic concept that controlled experiments have measurable factors that change, it is time to move on to further exploration of the concept.
The EXPLORE stage of the lesson is to get students involved in the topic so that they start to build their own understanding. To help students explore controlled experiments, I conduct a mini-lesson using slide 4 of Controlled Experiments Mini Lessons while students take notes on a copy of the Controlled Experiments Student Handout. A student example can be viewed here: Controlled Experiments Student Work.
Now that students understand what a variable is, we build on that knowledge to differentiate between independent, dependent and controlled variables. Using the plant diagram as an example, students generate possible independent, dependent and controlled variables. This is a great opportunity to probe student thinking about the "why" of controlled experiments. Questions like these help this build conceptual understanding:
Why is it important to design controlled experiments?
What happens when an experiment is not controlled?
Why is it important to change/test one variable at a time?
If you are mixing a bunch of chemicals together to see what happens, how will you know which chemical explodes when mixed another?
At this point, we revisit the Blue Eyes/Brown Eyes Experiment (for more on Jane Elliott, visit this site: Jane Elliott's Blue Eyes Brown Eyes Exercise) Together, we analyze her experiment to identify the variables and explore the idea of control and experimental groups. To help understand this concept, I split the class in half a run a simulation to illustrate the exercise and the purpose of the control and experimental groups. For additional help with the strategy of using a simulation to engage students as active participants, check out this section's reflection: Role Playing in Science - Stimulating Simulations or view this video explanation:
Teacher Note: This famous experiment offers many great departure points for discussion. The social implications are many. Students love discussing: human experimentation, experimentation on children, psychological effects of experimentation, how this experiment relates to stereotypes and discrimination. Even without a big discussion, it is important to mention to students the positive and negative aspects and ethical components of experimentation with human subjects.
After completion of the role-playing simulation, continue on to Part 2, which includes the EXPLAIN, EXTEND and EVALUATE components of the lesson; Time: 1 - 2 50-minute lessons or equivalent block periods.