There is much more to learning the water cycle

Transcription

1 A water cycle of many paths by Matthew Vick There is much more to learning the water cycle than memorizing the definitions of evaporation, condensation, and precipitation. The following activity allows students to explore the complexity of the water cycle, create their own model of it, and connect it to the processes that drive it. This activity works best after students have reviewed the basic water cycle and how states of matter change. They should understand how the Sun provides energy to change states of matter and how gravity pulls objects, including water, toward the center of the Earth. This activity addresses the Next Generation Science Standard (NGSS) MS-ESS2-4, which requires students to create models of the hydrologic cycle and identify water in different states of matter as it moves through various pathways driven by the Sun and gravity (NGSS Lead States 2013). Additionally, this activity engages students in the science and engineering practice Developing and Using Models as they describe phenomena and unobservable mechanisms. This activity also addresses disciplinary core idea ESS2.C: The Roles of Water in Earth s Surface Processes: Water continually cycles among land, ocean, and atmosphere via transpiration, evaporation, condensation and crystallization, and precipitation, as well as downhill flows on land (NGSS Lead States 2013). The crosscutting concept Energy and Matter: Flows, cycles, and conservation is also linked to this activity (see standards sidebar on p. 63). This complex water-cycle activity follows a 5E/7E format (Bybee 1997; Eisenkraft 2003). This game and reflection have some similarities with the FOSS water-cycle activity used in the FOSS Weather and Water Middle School Course and the FOSS Weather on Earth Grade 5 Third Edition Module, which FOSS in turn adapted from an activity created by Patty W. Watts and Eric A. Pani (1999). The activity described in this paper was developed independently and does have differences (as confirmed by FOSS staff). Elicit/Engage Begin by asking students to draw a diagram of the water cycle. Most draw a simple diagram with three or four steps, such as a lake with an arrow pointing upward toward a cloud and rain falling from the cloud back to Earth. Typically, they label the arrow as evaporation, the cloud as condensation, and the rain as precipitation. Occasionally, they include water seeping into the ground (and possibly include groundwater). When asked what the state of matter of the water is in while in the cloud, students most often describe it as being a gas or water vapor, a common misconception identified by Keeley, Eberle, and Dorsey (2008, p. 159). Ask students to share the parts of their diagrams and compare their diagrams to each other s. These initial diagrams often look similar to the public-domain graphic shown in Figure 1, which depicts a single cycle involving only evaporation, condensation, and precipitation. You can now challenge this conception. Explore In this activity, students travel through the life of a drop of water in a complex manner that shows them water molecules do not simply flow through a single, predictable cycle of evaporation, condensation, and precipitation. While any number of stations can be created by the teacher, I chose to set up 11 stations around the classroom. Each station represents a different location where water exists and is symbolized by a different color: an ocean (dark blue), the air (yellow), a cloud (white), a lake (light blue), an animal (orange), the ground (green), groundwater (brown), human use (black), a factory (gray), the human body (red), and a plant (violet). I chose stations that helped students realize how water is used by a variety of organisms (animals, plants, and humans). A summary of the stations and station directions, which are written on cards placed at each, can be found in Figure 2. Students keep track of the stations through which they progress by either coloring a strip of paper or adding beads to a string, using colors that correspond to the stations they visit. The beads are slightly more efficient in terms of time, but these must be bought, as opposed to crayons and paper, which are readily available in classrooms. You can re-collect and re-sort the beads to use in the future, if desired. If you are using beads, put about 20 of the corresponding color in a cup at each station. If you are using crayons, place a few of the correspond- 58

2 ing color at each station. (Safety note: If using beads, be sure to remind students that they are a choking hazard and should not be played with.) Before starting, students are given a die to carry with them throughout the activity. If using colored beads, also give students a piece of string about 30 cm long. Otherwise, give each student a strip of paper about 30 cm long by about 5 cm wide. Each student is then randomly assigned a station at which to start. I usually have students move to one of their choice, with a limit of no more than three people at a station. Students place a bead the same color as their station on the string or color about 2.5 cm of the strip of paper the same color as the station. Students are now ready to independently begin the activity. They roll their die and then follow the directions at their station that correspond to the die number rolled. For example, if a student who starts at the Orange Station (Animal) rolls a 1 or 2, the animal exhales the water into the air as vapor, and the student proceeds to the Yellow Station (Air) and adds a yellow bead to their string. The student would then roll the die at the Yellow Station and continue in a similar manner. If the student rolls a 3 or 4 at the Orange Station, the water remains in the animal s body and another orange bead is added to the string. Finally, if the student rolls a 5 or 6 at the Orange Station, the water is excreted by the animal and the student proceeds to the Green Station (Ground), where a green bead should be put on the string. The student would then roll the die and follow the directions at the Green Station in the same manner. The beads accumulate as students move from station to station, showing the path that they took. It may take several rolls before the students leave their present stations, as many of the stations directions have options to remain there if certain numbers are rolled. Student should continue rolling and moving from station to station until they find their way back to their original station. At that point, they either tape the strip of paper together or tie off the string to make bracelets representing a water cycle. Usually, 5 10 minutes is long enough for most students to return to their point of origin. If students finish quickly (due to the random chance of their dice rolls), they can perform a second cycle, which will undoubtedly differ from the first. If students do not return to their point of origin by the end of class, they can tie off their string to create a strand, which will indicate that their cycle has yet to come to an end. Both short and long cycles will be examined during the discussion phase. FIGURE 1 The teacher should make the following classroom management considerations when organizing this activity. First, clearly model what to do at a station (take a bead or color the paper, then roll the die, then follow the directions about which station to go to next). Second, remind students about the appropriate volume of voices expected during the activity. Third, move around the room during the activity to make sure students are on task. They may get frustrated if they get stuck at one station due to their die rolls, but let them know that this is modelling what really happens in the water cycle. Explain Simple water cycle similar to the drawings of many students After the Explore Phase, I ask students to answer the following questions in groups of four to guide their thinking: Compare your string/paper to those of the other people in your group. Do you have the same number of beads/markings? Are they in the same order? What does that mean? Did anyone get stuck at one of the stations (does anyone have many of the same-color beads/ markings in a row)? For how long? What does that mean in real life? How does this activity challenge the water cycles you all drew earlier? Which of the station changes in water s state of January

3 matter does the Sun drive in your cycles? [Give students a handout with a list of the stations to refer to.] Which of the station changes does gravity drive in your cycles? Students usually recognize that there are many possible water cycles that a drop of water can follow. They also come to realize that the traditional, three-step water cycle is limited compared to the many different factors that actually affect the cycle. Some students point out that there are smaller cycles within an overall cycle. Often, they remark that they have never thought about how complex the water cycle is. FIGURE 2 Directions for the stations Dark Blue Station: Ocean Yellow Station: The air White Station: Cloud Light Blue Station: Lake Orange Station: Animal Green Station: Ground Brown Station: Groundwater Black Station: Human use Gray Station: Factory Red Station: Human body Violet Station: Plant 1,2,3: You evaporate and go into the air. (Go to Yellow Station.) 4,5,6: You remain a part of the ocean. 1,2,3: You condense (turn into liquid) and become part of a cloud. (Go to White Station.) 4,5,6: You remain water vapor in the air. 1,2: You remain in the cloud. 3,4: You fall as rain onto the ground. (Go to Green Station.) 5: You fall as rain into a lake. (Go to Light Blue Station.) 6: You fall as rain into the ocean. (Go to Dark Blue Station.) 1,2: You evaporate and become part of the air. (Go to Yellow Station.) 3,4: An animal drinks you. (Go to Orange Station.) 5,6: Humans process you into drinking water. (Go to Black Station.) 1,2: The animal exhales you into the air as vapor. (Go to Yellow Station.) 3,4: You remain in the animal s body. 5,6: You are excreted by the animal. (Go to Green Station.) 1: You soak into the ground and become groundwater. (Go to Brown Station.) 2: You run off into a nearby lake. (Go to Light Blue Station.) 3,4: You soak into the ground and are pulled into a plant by its roots. (Go to Violet Station.) 5,6: You evaporate back into the air. (Go to Yellow Station.) 1,2,3: You remain underground. 4,5,6: Humans pump you up. (Go to Black Station.) 1,2,3: You are drunk by humans. (Go to Red Station.) 4,5,6: You are used in a factory to cool machinery. (Go to Gray Station.) 1,2,3: You continue to cycle around the pipes in a factory. 4,5,6: You are released into the air as water vapor (gas) and steam (liquid). (Go to Yellow Station.) 1,2: You are exhaled into the air. (Go to Yellow Station.) 3,4: You remain in a human s body. 5: You are excreted. You go into a lake. (Go to Light Blue Station.) 6: You are excreted. You go into groundwater. (Go to Brown Station.) 1,2: You remain a part of the plant. 3: An animal eats part of the plant. (Go to Orange Station.) 4: A human eats part of the plant. (Go to Red Station.) 5,6: You are evaporated through the plant s leaves. (Go to Yellow Station.) 60

4 Elaborate/Evaluate Now ask students to draw a revised water-cycle diagram based on the activity. While each student s activity path, as recorded by the beads or markings, can be a starting point, students should be directed to draw a water cycle that involves all 11 stations. They should be incorporating information about the path they took through the station as well as those of their peers when constructing the diagram. In other words, students should include examples of the stations they visited (such as a factory using water). Additionally, students are asked to identify the processes involved and whether each stage of the water cycle is driven by the Sun or by gravity. They should be able to draw on background knowledge about phase changes. Guiding questions to ask students include: Is the Sun involved when water freezes? When it evaporates? When it condenses? Student diagrams should look more like a web than a simple circle. You can also ask students: Where in the water cycle did transpiration, evaporation, condensation and crystallization, and precipitation occur? Have students label these processes on their water cycle as called for by NGSS ESS2-4. Depending on their background knowledge, you may have to review the definitions of these terms. Some of the stations will be difficult for students to label (such as what draws groundwater into roots or how water is cycled around a factory for cooling). Students should then label the driving forces for each of the transitions. This can be done on their water-cycle diagram or in a separate table. Students can use informational texts or online resources to find ways to describe what is happening. Textbooks usually have sufficient definitions of transpiration, evapo- FIGURE 3 Processes and driving forces for water-cycle stages Starting station Ending station Process Driving force Ocean Air Evaporation Sun Air Cloud Condensation/crystallization Temperature/altitude Cloud Rain Precipitation Gravity Lake Air Evaporation Sun Lake Animal/human Drinking Physiological forces Animal Air Respiration/exhaling Physiological forces Animal Ground Excretion Physiological forces/gravity Ground Groundwater Percolation/filtering Gravity Ground Plant Capillary action/transpiration Water cohesion/osmotic pressure Ground Air Evaporation Sun Ground Lake Runoff Gravity Groundwater Humans Pumping Displacement or gravity Humans Humans Cooling factory equipment Displacement or gravity Human Air Respiration/exhaling Physiological forces Human Lake Excretion Physiological forces Human Groundwater Excretion Physiological forces/gravity Plant Animal/human Digestion Physiological forces Plant Air Transpiration Sun January

5 ration, condensation, and precipitation for students to use. The American Chemical Society s online middle school resources also have a good interactive site for phase changes (see Resources). Figure 3 summarizes the processes and driving forces for the changes in water s state of matter during the activity. (The rubric for the student-constructed water cycle is available with the online version this article, located at middleschool/connections.aspx.) Finally, students should write a brief reflection on how their model of the water cycle shows its complexity. A good prompt is: How is this model of the water cycle more complex than the one you previously learned about? Follow up with questions that relate to the NGSS science and engineering practice Developing and Using Models, such as: How does this model accurately describe the real world? What are its limitations? The crosscutting concept Energy and Matter relating to natural systems is addressed when students can describe the processes that drive the changes in water s state of matter. Ask students: How do the Sun and gravity drive the changes in the state of matter of water as it travels through its cycle? Additional assessment tools for water-cycle content knowledge can be found online. The Environmental Protection Association (EPA) provides a free, 25-question quiz with immediate feedback about the water cycle and water usage (see Resources). Students can also construct an argument to answer the questions: How can humans alter part of the water cycle? What effect will this have? Students need to defend their response with research from informational texts or websites. For example, you can ask them how the water cycle modeled in the game would be impacted if humans decreased the amount of water percolating into the soil by paving land for streets and parking lots. Other questions requiring higher-order thinking include: How can humans impact the driving forces for parts of the water cycle? How would a hurricane impact the water cycle? How do severe thunderstorms affect parts of the water cycle? Extension Students are now ready to write their own questions about the water cycle based on the activity and then conduct research to answer their question using books and media resources. For example, after drawing the water cycle and discussing it, students have asked, Is the water cycle the same everywhere on Earth? and Does anything change the water cycle? Students should seek websites and books to answer their questions. Questions can also be posed that connect the water cycle to other science topics. Such questions include: How do ice ages affect the water cycle? How does the water cycle affect water salinity in different regions of the Earth? How is the water cycle used to preserve food? How does the water cycle cleanse water? How is pollution affected by the water cycle? How do specific animals (such as beavers) affect the water cycle? The Drop in My Drink: The Story of Water on Our Planet (Hooper 1998) from the National Science Teachers Association s Outstanding Science Trade Books for Students K 12 (see Resources) is a great picture book that describes the history of water on Earth, beginning with the formation of the planet. Another book from this list, Our World of Water: Children and Water Around the World (Hollyer 2008), is also a good starting point to link the activity described in this article to other subjects. It can help students to analyze how various cultures use water and how its scarcity affects them. Students can link the concepts from these texts to their constructed water cycles by determining the parts of the water cycle that are affected by geography or human activity. More than vocabulary This activity should not be seen as an end unto itself. The most important part of the lesson is in the discussion after the activity, in which students process the way their traditional, simplistic view of the water cycle has been challenged. From there, they create their own visual model to demonstrate their understanding of the multiple pathways present in the water cycle, the processes involved, and the forces that drive them. Like all models, the water cycle is never a complete replica of nature; it can be modeled with different levels of complexity that serve as a good enough model for the question being asked (Gustafson and Mahafy 2011). 62

6 References Bybee, R.W Achieving scientific literacy: From purposes to practices. Portsmouth, NH: Heinemann. Eisenkraft, A Expanding the 5E model: A proposed 7E model emphasizes transfer of learning and the importance of eliciting prior knowledge. The Science Teacher 70 (6): Gustafson, B., and P. Mahaffy The concept of a model: Ways to teach students the meaning behind models and not just what they represent. Science and Children 49 (1): Hollyer, B Our world of water: Children and water around the world. New York: Henry Holt Books for Young Readers. Hooper, M The drop in my drink: The story of water on our planet. New York: Viking. Keeley, P., F. Eberle, and C. Dorsey Uncovering student ideas in science: Another 25 formative assessment probes. Arlington, VA: NSTA Press. NGSS Lead States Next Generation Science Standards: For states, by states. Washington, DC: National Academies Press. next-generation-science-standards. Watts, P.W., and E.A. Pani Water-cycle game: Learning about the hydrologic cycle and global climate change: A demonstration. Fifth International Conference on School and Popular Meteorological and Oceanographic Education: Weather, Ocean, Climate. Australian Meteorological and Oceanographic Society 5 9: Resources EPA water-cycle quiz flash_qagame.html Outstanding Science Trade Books for Students K 12 Phase-change resources com/multimedia/chapter2/lesson3 Matthew Vick (vickm@uww.edu) is an assistant professor at the University of Wisconsin Whitewater in Whitewater, Wisconsin. Connecting to the Next Generation Science Standards (NGSS Lead States 2013) Standard MS-ESS2: Earth s Systems Performance Expectation: The materials/lessons/activities outlined in this article are just one step toward reaching the Perfomance Expectation listed below. MS-ESS2-4. Develop a model to describe the cycling of water through Earth s systems driven by energy from the Sun and the force of gravity. Matching student task or question taken directly Dimension Name or NGSS code/citation from the activity Science and Engineering Practice Disciplinary Core Idea Developing and Using Models ESS2.C: The Roles of Water in Earth s Surface Processes Water continually cycles among land, ocean, and atmosphere via transpiration, evaporation, condenstation and crystallization, adn precipitation as well as downhill flows on land. Global movements of water and its changes in form are propelled by sunlight and gravity. How does this model accurately describe the real world? What are its limitations? Where in the water cycle did transpiration, evaporation, condensation and crystallization, and precipitation occur? Is each stage of the water cycle is driven by the Sun or by gravity? Crosscutting Concept Energy and Matter How do the Sun and gravity drive the changes in the state of matter of water as it travels through its cycle? January