Prior to this lesson, students would have developed a working definition of density with the lessons in the Density Unit. In this Earth Science Unit we are discovering how our planet changes with plate movement. Students will apply their understanding of density to plate tectonics, discovering that convection currents in the mantle beneath the earth's crust cause the tectonic plates to move. Temperature changes cause these changes in density, creating convection currents.
Investigation Summary & Standards
In this investigation, students will develop an understanding of how heating and cooling causes movement in the form of convection currents in the Earth's mantle by observing how liquids of different temperatures interact. (SP3 Planning and Carrying Out Investigations) They will use this evidence to infer that the convection currents are thought to be responsible for changes in the Earth's surface over time (MS-ESS2-2 Construct an explanation based on evidence how geoscience processes have changed Earth's surface at varying time and spatial scales) and (SP2 Developing and Using Models)
Students working as scientists will record their observations, as drawings, reflecting what they observed and explaining why the movement happened based upon their understanding of density. 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.)
The key to success in this lab is organization. We are creating a model of convection currents by observing how warm and cold water interact in a closed system. The students must have a clear understanding of where and when to obtain the water samples so they have the best possible opportunity to observe the interactions.
If the water is not cool out of the tap, I prepare a tub of water with some ice ahead of time for the students. I also prepare the vials of warm water colored with red food coloring.
If the water is not warm out of the tap, I prepare a tub of water that has been heated ahead of time for the students. I also prepare the vials of cool water colored with blue food coloring.
I prepared a short video explaining how I prepare vials for multiple classes throughout the day. Preparing the vials in advance of the students coming to class allows this lesson to flow smoothly. You can watch the video by clicking here or look for the video in the resource section of this lesson.
Students in Action
In this lab, the colors red and blue are an important connection for students in keeping a front of mine awareness of temperature differences (warm and cool). Scientific drawings use red to represent warm and blue to represent cool.
There are no formally written instructions for the students. I explain the procedures for obtaining materials from me before we begin and demonstrate how to place the vial in the cup. The materials list is included in the resources along with a tip on making this lab easy to manage.
First the students fill one of the 9oz clear plastic cups with cool tap water. The cup will be filled to within a fingers width from the top. Once students have their cups of cool water, the vial of red warm water retrieved and placed upright in the cup. The students make and record their observations.
Image of the 9oz clear plastic cups with vials inserted
Student observations should include that the red colored water begins to rise from the vial because it is less dense. Warm water is less dense than cool water. After a short period of time, student observations should note that the red water begins to sink to the bottom. The red water has cooled and will sink.
Students complete drawings and a description of their observations using what they know about density before beginning the next observation.
Next students will fill one of the 9oz clear plastic cups with warm tap water. Once students have their cups of warm water, the vial of cool blue water is retrieved and placed upright in the cup. Once again the students make and record their observations.
Student observations should include that the vial of blue colored water appears to do nothing when it is placed into the cup of warm water. Patience is needed here. Soon students should see the water in the blue vial mix with the water in the cup. The blue, cool water in the vial has warmed to the same temperature as the water in the cup.
Students complete drawings and a description of their observations using what they know about density.
We begin connecting the learning with a whole class discussion. Students share what they observed and how density can be used to explain their observations. Warm liquids are less dense than cool liquids. When the density of the liquids is different, students observe movement.
Next we will make the connection between what we observed and boiling water. What do you think recipes mean when they say the bring the water to a rolling boil? What is happening in the pot of water on the stove? Where is the water the warmest in the pot? Where is the water the coolest in the pot? How can we use density to explain the rolling boil? I will continue this discussion with students until it is clear that they understand the the water in the pot is warmer near the heat source and the water becomes cooler as it moves away from the heat source.
Students now have concrete experiences that they can apply to a convection current in the Earth's mantle. Scientists have estimated that the temperature of the Earth's core may be as warm as 10,800 degrees Fahrenheit - how is the Earth's mantle which lies just above the core affected by the temperature of the Earth's core? What happens as the mantle is heated? What happens as it becomes less dense? What happens to the mantle as the heated material rises? We call the circular motion created by the heating and cooling of fluids a convection current. How might this convection current cause tectonic plate movement?
This short little video provides a really nice animation of the convection current in the mantle.
This video is from a BBC documentary film titled Earth: The Power Of The Planet.
I will show this video then ask students how the pot of boiling water is like the convection current in the Earth's mantle.