Asking your science students to plan an experiment? Grab the Advil because this science and engineering practice, planning and carrying out an investigation, can cause some major headaches. In the past few years, I’ve added teacher scaffolding to help kids brainstorm experimental procedures. Students cannot go from cookbook labs to independent procedure-writing without teacher support. Teachers adapting their curriculum to include this skill must teach kids how to plan investigations.
The scaffolding technique that I’m about to describe aligns nicely with work done by Ambitious Science Teaching (AST) out of the University of Washington. Science teachers that are unfamiliar with the research-based practices outlined by AST can finally read about them in Windschitl, Thompson, and Braaten’s book, Ambitious Science Teaching, due out in early 2018.
AST describes scaffolding as a temporary form of assistance for learners who are facing instructional tasks that are slightly beyond their instructional abilities. Think of it this way, you wouldn’t throw a 2-year old in a swimming pool and expect the child to swim laps. So many little things lead up to successful swimming (breathing correctly, arm positions, coordinating strokes and leg kicks). In the same way, teachers need to help students develop the skills that go into successfully planning a science experiment. Before kids can write a procedure, they must fully understand the purpose of the lab, be able to visualize what must be physically done, learn some unfamiliar lab techniques, and foresee what data might need to be collected to meet the goal. The teacher must guide them through these steps.
Early in the school year I use a gas density lab to help my first year chemistry students develop a procedure. In this experiment, students must plan and carry out an investigation in which they generate carbon dioxide gas and use lab data to calculate its density. Finding the density of a gas via experimentation challenges students to brainstorm an unfamiliar procedure. Unlike a solid, a gas can’t be plopped on a balance to find its mass. In addition, several misconceptions surfaced when my students thought about how to find the volume of a gas.
After introducing the goal of the lab, I share the chemical reaction and explain how to generate the carbon dioxide. Students observe as an Alka-Seltzer tablet is placed in a beaker of water and bubbles begin to form. Then I ask my students to predict which scenario would have a greater mass:
- Scenario 1: Beaker with 100 mL water and Alka-Seltzer tablet sitting next to it (before reacting)
- Scenario 2: Beaker with 100 mL water and Alka-Seltzer tablet thrown in (after the reaction stops bubbling)
Students understand the Law of Conservation of Mass at this point. Most say Scenario 2 will have less mass because the carbon dioxide gas escapes from the beaker of water. To determine if they’re correct, I compare the two scenarios under my overhead camera. The beaker sits on an electronic balance. Students watch the mass of the system decrease as the bubbles of CO2 form and then escape from the beaker. This prediction/discussion helps them develop their procedure for finding the mass of the CO2 in their own experiment. Students realize the loss of mass equals the mass of the carbon dioxide.
Finding the volume of the generated carbon dioxide is more challenging than the mass. At this point I ask kids to brainstorm procedural ideas in their lab teams. In my instructions I always emphasize drawing a visual. (Sometimes pictures help kids realize flaws in their procedures.) After teams brainstorm, they share out with the class and we determine the idea’s feasibility. The teacher’s role is to help the kids visualize the suggested procedures. Basically, I let my students boss me around! To implement this strategy successfully, consider every possible crazy idea that kids might suggest. Then stock your lab drawers with the supplies that might be brought up. It takes me about 30 minutes to help the class vet their ideas to determine if they will work.
Students typically start with simple ideas. They’ve seen the chemical reaction with water and the Alka-Seltzer tablet. They realize they need to capture the bubbles of carbon dioxide. To collect the gas they often suggest “put a Ziploc bag over the beaker” or “stretch a balloon over the top of the beaker.” It’s performance time! Get ready to act out all of these suggestions. I waste at least eight Alka-Seltzer tablets as I physically create the scenarios suggested by my kids. When I put the Ziploc bag over the beaker, they immediately realize the mistake. “Maybe use a rubber band” one student suggests. I place a rubber band around the Ziploc and attach the plastic bag to the beaker. Someone shares, “It’s still going to leak out the sides.” They realize the mouth of the beaker is too big. They ask me if I have a container with a smaller neck, something akin to the neck of a water bottle. I pull out a small Erlenmeyer flask and they yell “Yes!! Like that!”
Some groups will immediately generate the idea of putting a balloon on top of the Erlenmeyer flask. I walk around the room during their brainstorming session to identify these groups and I avoid calling on them until now. I like to flush out all the simplistic ideas first. Eventually I call on a group that brainstormed the balloon idea. To demonstrate their idea I fill the Erlenmeyer flask half-way with water, add the Alka-Seltzer tablet, and quickly place the balloon on top. At first this seems like a great idea. But I challenge the kids to watch carefully. In the beginning the balloon inflates, but as more bubbles are produced, the balloon hits a limit. It stops growing. More bubbles do not equal greater volume. Why? I ask kids if they’ve ever struggled to blow up a balloon. They all say yes. We discuss the limitations of the balloon. It seems to compress the gas and prevents the carbon dioxide from taking up its expected volume. As proof of this problem, I pull off the balloon and they can hear the pressure released as the gas escapes. Herein lies the teacher’s challenge. You need to help kids understand that under normal atmospheric pressure, the carbon dioxide gas takes up a very specific volume of gas. Even though it can be compressed, we want to collect the gas in a way that doesn’t build up pressure.
Developing the procedure becomes more challenging now. The teacher becomes the guide, or facilitator, as students struggle to brainstorm new ideas. Students are venturing into unfamiliar territory. The teacher must help them understand procedures and concepts by drawing on their background knowledge.
After the balloon failure, one student generally suggests that the carbon dioxide be transferred to another container. I push them to describe this technique. They rely on background knowledge for their resources. Someone suggests “use a pipe”(like a PVC pipe) and another fine-tunes this suggestion to “use a hose.” I open a drawer in front of me, pull out plastic tubing, and say “something like this?” They laugh and roll their eyes because they realize this act was way too coincidental. When I put one end of the tube in the open Erlenmeyer flask they realize the gas will escape. More suggestions now… Duct tape the top… a cork… a rubber stopper? I open another drawer and I pull out tubing attached to a glass tube threaded through a one-hole rubber stopper. There is a roar of laughter and “oh my gosh!” The rubber stopper is connected to the Erlenmeyer flask and the other end of the tubing is placed in a beaker of water. We try the experiment again and they immediately notice the carbon dioxide is escaping. Trap it, trap it. How do you trap the gas? Again, the same problem. How do you trap the gas without compressing it into a specific volume of space?
Hang in there because this is the hardest part. Most students hit a roadblock and want to give up. The best way to keep kids involved is to harness their background knowledge. Use their experiences as resources to help them put the pieces of the puzzle together. Now I ask my students a ridiculous question because I know I can build off of it: “How do you know if you fart in a bathtub?”
Yes, I seriously ask this question. And they all know the answer! When you fart in a bathtub you can see the bubbles rise upward in the water. The gas is less dense than the water and it pushes the water downward as it rises upward and takes up space. In a sense, a type of water displacement. Help your students understand that gases can displace water just like solids. When a solid is more dense than water, it will sink. As it sinks in the water, it pushes the water volume upward because the object takes up space. Carbon dioxide is less dense and therefore does the opposite. When carbon dioxide is generated in water, it will push the water downward as it rises to the top and takes up space (volume). In other words, we need to release the carbon dioxide into a container full of water that is closed on the top (so the gas can’t escape) and open on the bottom (so that the water can be pushed out as the gas takes its space). Sometimes they suggest filling an Erlenmeyer flask or a water bottle with water and flipping it upside down. How do I prevent the water from just falling out? I purposely leave a tub or a bucket on a counter that’s within students’ view. Generally they figure out that the water will stay in the flask if it’s sitting in a tub of water. As a final touch I mention that it could be nice if the flask had better increments on it so we could find the exact volume of the gas. They realize a graduated cylinder would work perfectly.
Now you can feel the momentum building in the class. We figured it out right? I grab all the equipment and try the experiment again. I fill the graduated cylinder with water, flip it upside down into a bucket, insert the tubing into the bottom of the 100-mL graduated cylinder, toss the Alka-Seltzer into the Erlenmeyer flask filled with water, cap with rubber stopper, and the bubbles begin!! Everyone is excited as they watch the volume of the gas growing in the cylinder. Then, in horror, they realize that the graduated cylinder is completely filled with gas and the reaction is still creating bubbles. The 100-mL cylinder is too small!!! A rumbling of voices and then someone yells out “Get a bigger one!” I smile and pull a 500-mL graduated cylinder out of another drawer. Laughter, shaking of heads, they are thinking “This woman is crazy.” But in a good way.
This brainstorming process takes my students about 30 minutes. After our class discussion, students are asked to draw the procedure with pictures and to write the procedure out in words step-by-step. This experience is the first of many situations where they develop a procedure for an experiment. After doing this activity for three years, it remains one of the most powerful lessons of the year. I think of it as the first of many situations where they won’t know all the answers and they have to work together to develop a procedure. As the year progresses, and they’ve had time to hone this skill, I shift from class developed procedures to team developed procedures to individually designed experiments.
Final reflections if you try this lesson:
- Be prepared. Be very well-prepared. Think of every possible crazy scenario that your kids might come up with. Then stock your drawers with these materials. Ziploc bags, tubing, rubber stoppers, balloons, beakers, Erlenmeyer flasks, water bottles. The room must be prepared ahead of time in order to act out the ideas they brainstorm. My kids think it’s hilarious that I pull supplies out of drawers like I’m a magician.
- This teaching technique takes more time than cookbook labs. We spend a lot of time brainstorming in my class. I’ve had to cut other activities/curriculum out as a result. I believe developing this skill is worth the sacrifice.