In the Float, Suspend, Sink lesson students changed the density of the film canister by adding mass. In this lab students will change the density of raisins and popcorn kernels by adding volume in the form of carbon dioxide bubbles.
Adding volume that has very little mass (the carbon dioxide bubble has a small mass) will change the density of the raisins and popcorn kernels in the same way that we add volume and very little mass when we put on a PFD - personal flotation device (a/k/a life jacket).
Investigation Summary and Standards
Students change the volume of the raisins and popcorn kernels in order to make concrete observations of density changes. The Raisins And Popcorn Kernels Investigation and materials list are included in the resources.
The ability to follow precisely step-by-step instructions is a desired outcome for students. They also continue to develop habits of work as a scientist by accurately recording their observations as required by CCSS literacy standards for science content (RST.6-8.3 Follow precisely a multistep procedure when carrying out experiments, taking measurements, or performing technical tasks) and respond to a prompt at the end of the lesson design for their reflection (WHST.6-8.2.D Use precise language and domain-specific vocabulary to inform about or explain the topic).
Reflection about the learning helps students think deeply about what they are learning. Production of writing is a critical skill, and the only way to build stamina is to practice writing. Having students routinely write, as part of their scientific habits, is a critical element of instruction (WHST.6-8.10 Write routinely over extended time frames (time for reflection and revision) and shorter time frames (a single sitting or a day or two) for a range of discipline-specific tasks, purposes, and audiences.)
Students in Action
For this lab I begin by reminding students that they will need to keep their goggles on until all cleanup is finished. We discuss all the materials that students have for their lab. I want to make sure my middle school students do not taste anything and know what the procedures are in the classroom if there is a spill. Since we are using vinegar and effervescent tablets I also let the school nurse know that what we are doing before the lab. A complete materials list is included in the resources.
What is the density of water? If the density of an object is > the density of water, what would they observe? If the density of an object is < the density of water what would they observe? It the density of an object is = the density of water what would they observe? These are review questions from previous concrete experiences with density in the Float, Suspend, Sink lesson.
Before we start the lab I activate background knowledge by asking students if they have ever used a life preserver. How does a life preserver keep a person above water? I will be looking for students to make a connection between increasing the volume of a person by adding the life preserver and changing the density of that person. If the students do not make the connection right away, I will continue to prompt them until we find the aha moment. Connecting background knowledge to new learning is a strategy that helps students process and remember new learning.
Tips for this Lesson
I model for students how to create a small scoop by cutting off the end of a straw at an angle. Students use this scoop to add small amounts of baking soda to the cup with water, popcorn kernels and raisins. I also model how to add vinegar to the cup by placing the pipette at the bottom of the cup where the baking soda has settled before I squeeze the bulb. This will keep the vinegar from becoming diluted in the water before it reacts with the baking soda.
This video demonstrates my tips.
I walk around the room asking students questions in small groups. What are the bubbles made of? Why are the raisins and popcorn kernels resting on the bottom of the cup? Why do the raisins and popcorn kernels with bubbles float to the top? What happens to the raisins and popcorn kernels when the bubbles pop? Why does this happen? What does this tell you about density?
Keep some extra effervescent tablets in your pocket to make sure there are plenty of opportunities to see the raisins and popcorn kernels float.
The strategy of small group questioning gives all students an opportunity to answer questions. It is a safe environment for students who are unsure how to answer because they have more confidence to venture a guess. Also, I can do more probing for answers and allow for more wait time with struggling students.
The students add drawings and responses to questions 8 and 9 into their science journal. We are working on carefully representing what we observe, in order to share accurate findings with other scientists. Students are making qualitative observations in this lab.
As a group we discuss whether it is as easy to change the density of other objects in the same way we changed the density of raisins and popcorn kernels. Also we compare this lab to the Float, Suspend, Sink lesson - what did we learn about density in each of these labs? Students should respond that mass and volume can be changed. Changing mass and/or volume will change the density.