Liquid Tower Density
Lesson 2 of 7
Objective: SWBAT change the density of water and observe how changing the density changes the behavior of water.
This lab is the second in a series about density. Our first lab, the Rainbow Measurement lesson, helped us hone our measurement skills so we could be successful in subsequent labs.
Early in 7th grade, students still need guidance to conduct experiments. While this lab does provide step-by-step instructions, it is leading students to the discovery of a very important idea about density. Density is not simply a comparison of how heavy an object feels to them. We need to make a connection to atoms, specifically how decreasing the space between the atoms will increase the density of the water.
I like to take mini steps in the development of important concepts such as density. With an emphasis on math and language arts in the elementary school, many of my students lack hands-on science experiences. This series of mini labs is designed to make sure that students do not have gaps in their understanding and helps to build their background knowledge. In the lesson, Density of Irregular Objects, students will take what they learn from each of the prior labs and create their own investigation.
We are still working on following directions and making observations. Students will record qualitative observations in their science journal by recording exactly what they see as they create the density tower. Students will also share their thoughts about why the density tower worked.
Investigation Summary and Standards
Can the density of a liquid be changed? Students follow step-by-step instructions to observe how the behavior of water changes when the density is increased. The Density Tower Lab and materials list for this lab are included in the resources.
The ability to follow precisely step-by-step instructions is a desired outcome for students in Science and Engineering Practice 3 -Planning and Carrying Out Investigations. They will also continue to develop habits of work as a scientist by accurately recording their observations as required by CCSS literacy standards for science content (CCSS.ELA-LITERACY.RST.6-8.3) and respond to a prompt at the end of the lesson design for their reflection. Reflection about the learning helps students think deeply about what they are learning.
Students in Action
What is density?
I begin this lesson by asking my students, "What is density?" Many of my students "respond" with a blank look. Some students attempt to describe density as how big or heavy an object seems to them. The students rarely show a clear understanding of density.
One of the materials listed is warm water. Students will need to retrieve the warm water from me. I heat water ahead of time so that it is warm to the touch but not hot enough to burn anyone.
I explain that warm water is important because the sugar mixes more easily with the water. Next I ask students if they have ever added sugar to a cold glass of tea or lemonade. Did the sugar dissolve easily or was some left at the bottom of the glass? I add sugar to a cold glass of tea and we observe that there is indeed sugar sitting at the bottom of the glass.
Next I ask a table group of six to stand up. We are going to model why the water needs to be warm for the sugar to dissolve. First, I ask the group to pretend they are atoms and show how they would be arranged if they were a solid. The group responds by standing close together and moving very little. Next, I ask the students how they would be arranged if they were the atoms in a liquid. This time the group responds by moving a bit farther apart and moving slightly more. Finally, I ask the group to show how they would be arranged if they are the atoms in a gas. The students respond by moving even farther apart and moving freely.
Then, I ask the students to arrange themselves as atoms again -- this time how they think they would appear if the water was cold. The student atoms should be close together but not like a solid. I ask a second table group of six to stand up and be atoms trying fill in the spaces between the atoms in the first group. A couple of students atoms will be left out. I ask the students how this is like the sugar at the bottom of the glass of tea.
I am looking for students to make the connection between how close the atoms are and whether or not the sugar can easily mix with the water. I ask the first group of atoms to arrange themselves as warm water. They expand out just a bit further and all of the atoms in the second group are now able to mix together. I ask students what they think will happen to the sugar in the cold glass of tea if the tea is warmed? I am looking here for understanding that the sugar will dissolve easily in the warmer tea because the sugar will be able to fill in the empty spaces.
I ask students to explain why the warm water is needed for this lab. This concrete experience is important for some of my learners who are still weak in abstract thinking. Answering this question helps students develop the meta-cognitive thinking - what did I observe and what did I learn from this observation? This is the kind of thinking I am trying to encourage through out their 7th grade science experience.
Video of students modeling the behavior of molecules.
Students in Action
First I review the materials list with my students. I ask them to point to or hold up the supplies such as graduated cylinders and pipettes. We are working on learning the proper names for the equipment used by scientists. I do not want students asking me for the plastic thing that sucks up water :-). And I make sure that the students know that the sugar should not be tasted. It may seem obvious but sometimes middle school students need to be reminded that materials in the science classroom are not there to tasted.
I spend a few minutes reviewing how to use the equipment we need for this lab. First I remind students to read the water level at the center of the meniscus. Meniscus is from the Greek, meaning crescent shaped or diminutive moon. I share that a concave meniscus is created when the liquid (water) molecules are more strongly attached to the side of the cylinder than to each other. Students need to be eye level with the meniscus to measure it properly.
Once the warm water is collected by all the students groups, I check in with each group to make certain they are using the equipment properly. I have notes from checking student journal entries from the Rainbow Measurement Lab. The students should have recorded equal or nearly equal volumes using the graduated cylinder in the Rainbow Measurement Lab. If they did not, I make a point to work with these students/groups in this lab to make sure that they are reading the graduated cylinder correctly.
One question I ask students is, "Does the mass of the water appear to change?" I bring a cup of water with me that has the same volume as their first cup and ask them to lift each cup. What is the difference? Why are they different? Which cup is bigger? Which has more volume? What is the only difference between the two cups? Which liquid is more dense? What did they do to change the density? This questioning is to help students think about how the sugar makes the same volume of water heavier and therefore denser.
Before students begin the process of adding the colored sugar water to the test tube, I ask them to make a prediction about what will happen when the second liquid is added, and to record that in their journal.
Connecting the Learning
I ask questions at each lab table as students work. Why not a whole class discussion? In a whole class discussion, one or two students may dominate the conversation or become frustrated if the rest of the class is not answering as quickly as they are thinking. This strategy allows me to give students who do not normally participate an opportunity to answer questions without worrying about the rest of the class or a wrong answer. It also allows me to extend questioning when a group shows a high level of understanding. Asking questions in this manner is a great way to differentiate the instruction without the need to develop separate activities or lab sheets.
I have done this lab in the past and students like to have their photos taken with the results. I build this excitement with a tweet or post on Instagram the students in action. This is another great tool to keep students on task, as I will not take photos of students who are not on task. Also it is a fantastic communication tool for parents - students can take photos themselves and answer the question every parent wants to know - what did you do in school today? Of course you need to have an agreement with parents a well as adhering to school and district policies regarding photos and the use of social media.
This student work sample shows how my students are recording their journey in the understanding of density. We are constructing knowledge. Each lesson will challenge students to refine their definition of density either individually or through a whole class discussion. I encourage my students to try more than once, label their drawings and respond to a prompt. In this lesson the students share their current definition of density as well as a summary of why they think the layering work.