In the Liquid Tower Density lesson, students learned that they could change the density of water by adding sugar. Adding sugar increased the mass and changed the behavior of the water. Students were able to layer the colored sugar water with the denser liquid on the bottom and the least dense liquid on the top.
This lab is another concrete learning experience for students intended to build the background knowledge necessary to understand the concept of density and apply the formula - density = mass / volume with concrete examples.
In this lab we further explore changing mass to change density. The volume of the film canister remains constant. We compare the heaviness or mass of identical film canisters. Specifically students will be challenged to change the density of the film canister so that it can float in water, suspend in the water or sink in the water.
Students collect quantitative data and apply math geometry skills as a communication tool. Students see math applied in the science classroom as an opportunity for real-world application of math. Using the appropriate tools (Mathematical Practice 5) and using precision in language and measurement (Mathematical Practice 6) are important to the success of students in the lesson.
Investigation Summary and Standards
Students change the heaviness or mass of the film canister in order to make concrete observations of floating, suspension and sinking. They prove their results mathematically by applying the formula for density -- where they will see that that the more mass that same object has, the greater the density. The ability to follow precisely step-by-step instructions is a desired outcome for students (CCSS.ELA-LITERACY.RST.6-8.3 Follow precisely a multistep procedure when carrying out experiments, taking measurements, or performing technical tasks.).
Students also continue to develop habits of work as a scientist by accurately recording their observations as required by CCSS literacy standards for science content and respond to a prompt at the end of the lesson design for their reflection (CCSS.ELA-LITERACY.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.
Students in Action
For this lab, I begin by reviewing with students how to use the triple-beam balance. My students usually need to be reminded about which slide to move first, how to add the numbers from each beam to find the total mass and what to do if the final slide rests between two numbers.
Video - Why Teach Mass?
We measure the canister and calculate the volume of the cylinder together. Depending on where students are in their math curriculum this may or may not be a new formula for them. Students most likely will need to be reminded about the order of operations. Using math outside of math class is a novel concept for students! It is really important that we use metric measurements for this portion of the lab. The Density of Water lab uses gram centimeter cubes and I do not want to confuse students by changing the units we use.
As a class we will work out a definition of suspend. "What criteria will we use to say that the film canister is suspended in the water?" We agree, with my direction, that no part of the film canister can be above the water.
I circulate around the room to help students find the mass of the film canister. Since film canisters are identically produced, it is easy to spot who needs assistance by inquiring about the mass of the canister alone.
Students find that the empty film canister is the one that will float. I check their calculations by making sure that the density the students calculated is less than 1 g/cm3.
The student results may vary slightly but since the density of water is 1 g/cm3, I can use this as a benchmark to check work in progress. I do not share the density of water with students today. The next lab will be the Density of Water.
Most of my students elect to find the density of the canister that sinks next. Again, I circulate around the room checking for densities greater than the density of water.
Finally, students tackle making the canister suspend in water. I make sure that I have a selection of very small, light objects such as pony beads and small washers in their supply kits. These small, light objects allow students to make minor adjustments in the mass to meet the criteria of suspension.
Once again I travel around the room, looking for densities that are pretty close to 1 g/cm3.
Any time I see student calculations that are not in line with my expectations, I know who needs help with the tools or calculations. It is important that I not do the thinking, or the work, for these students but instead use questioning, or having them go through their steps, to help them realize any errors, problems, or other ways to solve this.
The students cut out the chart from the lab sheet and place the quantitative measurements in their science journals. The final reflection question is also answered in the journal.
Which example -- Float, Suspend or Sink -- best represents the density of water? Why do you think so?
Traveling around the room as students are working is a great differentiation strategy where I can meet the needs of individual students by showing them how to use the equipment, helping them with their calculations or asking probing questions about their work.
Labs are great formative assessments as they let me know immediately what students know and what needs to be retaught.
Students answers make a pretty powerful connection to the Density of Water lesson I am planning next. They find that they are nearly spot on in their predictions. This is a great opportunity to build confidence in students' ability to work as scientists.