Using Phase Changes to Remove Contaminants from Water
By Ellen Raco
Water, water, everywhere…nor any drop to drink!
(adapted from Samuel Taylor Coleridge, 1798)
Most of our communities provide us easy and safe access to water. We can easily grab it off of the shelf of a store or turn on the faucet and there it is – perfectly perfect water. The idea that water can contain contaminants and/or pathogens and that it is a limited resource is a new concept for our students. Teachers recently grappled with this timely real world phenomenon and the ways they might help their students wrap their heads around possible solutions.
A team of about 25 excited 2nd and 5th teachers in the Galt/Tracy NGSS Early Implementer institute last summer learned to ask questions about the phenomenon through the lens of the crosscutting concept (CCC) of energy and matter, (e.g., What is the role of matter in this system? How does it change? How does it enter and exit the system?). Teachers then proposed solutions by using the science and engineering practices (SEP) of planning and carrying out investigations and developing and using models.
The CCC and SEP provided us ways to think about and the tools to become familiar with core idea PS1: Matter and its Interactions. We focused on disciplinary core idea (DCI) of PS1.A for our mixed grade-level group.
• Different kinds of matter exist and many of them can be either solid or liquid, depending on temperature. Matter can be described and classified by its observable properties. (Grade 2)
• Matter of any type can be subdivided into particles that are too small to see, but even then the matter still exists and can be detected by other means. A model showing that gases are made from matter particles that are too small to see and are moving freely around in space can explain many observations, including the inflation and shape of a balloon and the effects of air on larger particles or objects. (Grade 5)
Use of a unit conceptual flow as a planning document allowed teachers to see how all three dimensions, the CCC, SEP’s, and DCI’s, were intertwined.
On the first day of the institute the teachers were presented with the specific phenomenon of water quality issues in California due to the drought. The focus was on the increasing amounts of contaminants in ground water as water levels drop. The teachers were introduced to this issue, by watching a short video that focused on global water shortage issues. Teachers were then asked to brainstorm, “What are the sources of drinkable water in the world?” The results of the brainstorm were recorded on chart paper. Then, the teachers were asked to discuss within their group the sources of drinkable water in California, specifically the Central Valley, and how the drought has affected these sources of water. The teachers were given a link to the water quality reports for Tracy and Galt (these reports can be found on city websites). A whole group discussion occurred about the source of water for each city and the possible contaminants that could be found in those sources.
Conceptual Flow for the Grades 2 and 5 Galt/Tracy NGSS Early Implementer Institute[/caption]
The question for the week was then posed to the teachers, “How can we separate the contaminants in ground water so that it can be used for human consumption?”
The teachers were given an Erlenmeyer flask of water that contained impurities (dirt and salt) and were asked to design an experiment to purify the water. Each group was provided with coffee filters, funnels, and 1 liter plastic soda bottles. The teachers drew a design of their filtration experiment in their lab notebooks and then attempted to purify the water and record their results.
After testing, the different groups of teachers continued to refine their design, which was altering the number of filters in the funnel, in order to remove all impurities and clarify the water. Each time the design was altered, the teachers recorded the change, why they made that change, and the results. They also recorded any questions they had (keeping the CCC energy and matter in mind) while performing the experiment as well as about the results they were obtaining.
At the end, the different groups were able to clarify the water to different extents, but some particles did remain. The different groups were then asked to share what they did, what their results were, and any questions they had to the whole group.
The teachers then asked to test their solution using conductivity meters to identify if the water had any other impurities. They compared the conductivity of their filtered water with the conductivity of tap water, and salt water. Their results were recorded in their lab notebooks. What the teachers discovered was that the conductivity of the filtered water was similar to that of the salt water.
To understand what the conductivity results meant, the teachers were directed to the PhET simulation, Sugars and Salt Solutions. This simulation allowed them to determine how adding sugar or salt to water, or evaporating water affects concentration and conductivity. They were also able to observe what happened at the molecular level when compounds dissolve in water. The teachers worked with the simulation and recorded down their results as they added sugar or salt or removed water.
The teachers were asked the question, “How can you separate the salt impurities from the water?” The teachers were provided with a close reading on the states of matter and were directed to the PhET simulation, States of Matter: Basics where they observed how matter changed from solids, to liquids, to gases as the substance was heated or cooled. Teachers were also able to observe what happens to the particles in the three different phases. The teachers recorded their observations in their science journals. They were also given puzzle pieces that had the name, a picture of particle arrangement and a description for each phase of matter. The teachers were asked to match the pieces for each phase of matter and glue them in their notebooks. A whole group discussion about the characteristics of each phase of matter and how one phase can change into another followed these activities.
The teachers applied their knowledge of phase changes to design an experiment to separate the salt from the water in their Erlenmeyer flask. They were provided the following materials: plastic tubing, ice, hot plate, bucket, Erlenmeyer flasks, and stopper. The teachers then designed an experiment to separate the salt from the water. The basic design that the groups produced was a distillation process. The salt water in the Erlenmeyer flask was heated on the hot plate. The stopper was placed into the top of the Erlenmeyer flask (this is important to prevent the evaporated water from moving into the atmosphere). The plastic tubing was attached to the Erlenmeyer flask and then was placed in a bucket of ice. The tubing was placed in the second Erlenmeyer flask. As the salt water was heated, the evaporated water flowed into the plastic tubing where it began to condense. Condensation continued as the evaporated water flowed through the tubing that was covered by ice. The liquid water was captured in the second Erlenmeyer flask.
Teachers in the Grades 2 and 5 Galt/Tracy NGSS Early Implementer Institute conducting the distillation experiment.[/caption]
The teachers observed that the volume of water in the second flask was approximately equal to the amount of water lost from the original flask. A discussion of why the amounts were not exactly equal occurred. The conductivity of the water in the original flask and the flask that captured the condensate was measured. All results and observations were recorded in the science journals.
As a culminating activity, each group was asked to record on chart paper how they were able to separate the impurities from the water in the flask they were given. They were asked to include scientific explanations of the separation processes that occurred. The next day we had visitors (administrators) from the different districts and the teachers were given the opportunity to present their experiments and results to them.
Each component of the conceptual flow that we used to develop this week’s lesson was placed on a whiteboard in the classroom as we covered it and we debriefed which Cross Cutting Concepts and Science and Engineering Practices allowing us to make sense of and study our ideas. Modeling was evident when we used the PhET simulations to explore conductivity and substance dissolved in water; and the phases of matter. The CCC of Energy and Matter was linked to the states of matter and their molecular arrangement as well as the changes in the phases of matter. The SEP of Planning and Carrying Out an Investigation was central to this week’s activities as the teachers designed and conducted experiments to separate the impurities from the water. The CCC of Cause and Effect was also evident as the teacher’s used the PhET simulations and observed how heating and cooling causes phase changes to occur and how changing the concentration of a solution alters its conductivity.
The lessons that the teachers experienced can be used in the elementary classroom with some modification. The PhET simulations are a great way for students to experience physical science concepts. It is important for the teacher to provide structure for the students’ learning such as giving the students tables to fill in while experiencing the simulation. This provides students with a focus of what information is important to attend to. Alternatively, students could be asked to draw a model of the arrangement of the molecules in the different phases of matter and what they think will happen to the molecules as they are heated or cooled, and then revise their models after using the PhET simulation.
Further, asking each group to share their experimental design before conducting an experiment is an important modification when asking students to design experiments. This allows the teacher to ask questions, check for safety, and it also allows for groups who are struggling to get ideas of how to design their own experiment. Students can then make modifications once the designs are shared.
Investigation of a real-world phenomenon that is relevant in our communities gave teachers the opportunity to build understanding and to experience how DCI’s, SEP’s, and CCC’s come together to provide a rich experience that is powerful for student learning.
PhET Interactive Simulations are an open educational resource provided by the University of Colorado Boulder, http://phet.colorado.edu.
Ellen Raco is a Science Teacher at Tracy High School, TUSD and a member of CSTA. She can be reached at firstname.lastname@example.org.