In this two day lesson, students build on their understanding of how adding thermal energy affects physical properties by placing a bar of Ivory soap in a microwave and watching it expand to fill the microwave! Then, they work through a set of Density Discussion Questions that ask them to demonstrate both density calculations and conceptual understanding. Last, students develop models of Shape Vocabulary to represent mass, volume, and density.
MS-PS1-2 Analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction has occurred.
MS-PS1-4 Develop a model that predicts and describes changes in particle motion, temperature, and state of a pure substance when thermal energy is added or removed.
CCSS.ELA-LITERACY.WHST.6-8.1.B Support claim(s) with logical reasoning and relevant, accurate data and evidence that demonstrate an understanding of the topic or text, using credible sources.
CCSS.ELA-LITERACY.RST.6-8.7 Integrate quantitative or technical information expressed in words in a text with a version of that information expressed visually (e.g., in a flowchart, diagram, model, graph, or table).
Science and Engineering Practices:
In this lesson, students utilize mathematics and conceptual thinking as they calculate density and answer questions that cause them to think conceptually about density (SP5). Students also answer lab questions that ask them to engage in argument based on evidence (SP7).
Relating to the NGSS, students look for the relationship of adding and removing heat and the physical properties of mass, volume, and density. They also recognize patterns in the relationships between the variables mass, volume, and density. Thus, this connects to the Cross Cutting Concept of “Patterns” which explains that macroscopic patterns are related to the nature of microscopic and atomic-level structure. Last, students develop their understanding that density is proportional. Thus, there is a connection to the Cross Cutting Concept or “Scale, Proportion, and Quantity”.
Ask the students, “What are you going to learn today?”. Students should respond with the Essential Question, “How can the arrangement and motion of the particles in a substance affect its physical properties and how can they be measured?” I keep this EQ posted on the board and students also have a copy of it in their Density Unit Plan.
Let students know that today they are going to be furthering their undersanding of Skill 1:
D.1 I can provide evidence explaining how temperature affects the arrangement and motion of the particles in a substance.
Students have taking part in multiple lessons on density and have already self-assessed on all skills of the Unit Plan on a scale of 1 to 4 (4 being mastery). Ask students to take a moment and re-asses their level of understanding and change the mastery score if they feel it is necessary on any of the four skills.
For some insight into what my students have completed at this point in the unit, you could check out these lessons:
Provide each table with a white board and a dry erase marker. If you do not have small white boards white paper and markers would work as well. I ask each person to write the words mass, volume and density in the shape of its definition. I tell the students that they can add small pictures, but the way that the word is written should be in a shape that shows meaning. For example, students during our waves unit wrote the word "absorption" before and after a sun shining on sun glasses. They explain that they wrote the "ABS" larger on the sun side of the glasses and the "orption" smaller on the other side because some of the rays were absorbed by the glasses. (Here are a couple of other lessons that use Shape Vocabulary: Heat Transfer, Frequency, Wavelength, and Amplitude and Analog vs. Digital Signals)
In the model above, the student explained that mass is the amount of matter in an object. So, she wrote mass twice in the 20 g container showing that there was twice as much matter in the container.
For this model, the student explained that they wrote volume in the shape of a beaker because the beaker measures how much space a liquid takes up.
In this picture, the student explained that in the lead, they wrote density over and over and had the letters of the word represented the molecules. Thus, the lead's molecules are closer together than the feather's making it more dense.
I have completed this demonstration in the past as both a discussion based experience as well as a demonstration in which the students complete a lab document and answer questions as the demo progresses. I have found success in both formats! Just in case you are interested in having the students complete questions related to "The Amazing Ivory Soap Bar", I have included a lab document and answer key in the resources!
Show students that you have a variety of soap brands such as Zest, Caress, Irish Springs, and Ivory. Place each of the soap brands in an aquarium or separate beakers of water one at a time (save Ivory for last). After placing each in the water (all but Ivory will sink), ask questions such as, “How would you compare the density of the water to the density of this brand of soap?” and “How would you compare the arrangement of the molecules in the brand of soap to those in the water?”. After placing the Ivory soap in, say, “Ivory soap actually has a marketing slogan founded in science. It is “Ivory soap….the soap that floats!”. Show the students the picture of this old advertisement. I say, “I love this advertisement, but I would love it even more if they had said, “Ivory soap…..the soap whose mass to volume ratio is less than that of water’s!” (Wink, wink)
Next, ask, “Why do you think Ivory soap is the only soap that floats? What is it about its composition that allows it to do this?”. Students will have lots of ideas, but eventually students typically say, “There might be air pockets.” Explain that that is correct! Ask, “Why would having a gas inside cause it to become less dense?”. Students should reply, “Because the molecules of a gas are more spread out than those of a solid.”
Place the Ivory soap in the microwave for a couple minutes (It cannot be an old bar, it must be new!). As it is in the microwave, ask the students, “What do you predict is happening to the molecules of the soap as we add thermal energy?”. Students might offer suggestions such as, “the molecules move faster and spread apart”. Then, ask, “What physical properties might be affected by that?”. Students might respond, “the volume increases”, or “the density decreases”.
Open the microwave door and get ready for the “Ooohs!” and “Ahhs!”. The soap will have filled the microwave! Let students know that if they were going to try this at home they need to keep two things in mind. One, ask their parents first! Two, it will not work with any other brand of soap. Other brands will burn in the microwave. Allow the soap to cool a little bit. While it cools, it will shrink in size slightly. Ask students, “Why is it shrinking a little?”. Students should identify that because it is getting colder, the molecules are getting closer together so the volume is decreasing.
Break of little pieces of the soap and let students touch it. They love this! Before doing this, I prep the students with a couple of requirements:
After clean up, allow students time to complete the lab sheet.
Below is a video of Steve Spangler modeling the Ivory soap bar demonstration:
Next, I have students begin working through this Density Discussion Questions packet. I remind students they should use all of their resources and think deeply about their understanding of density as they answer. I emphasize the need to show work and include units in their density calculations.
These questions really get at the students' conceptual understanding of density. They will have questions! I always encourage group discourse at their tables before asking me for help. The student document as well as student work is included in the resource section. In addition, the video below goes through a couple of the key points from the student work.
As a class, discuss the answers to the Density Discussion questions. Throughout the discussion, allow students to make any corrections or alterations to their answers that they need. This discussion is important. It's not about being right or wrong, it is about developing a conceptual understanding. Many students need this discussion to develop this!