6th Grade: Great Salt Lake is Salty

Transcription

1 Curriculum written by Megan Black in partnership with The Great Salt Lake Institute at Westminster College. 6th Grade: Great Salt Lake is Salty Lesson Description: In this lesson students will compare water from a fresh lake to water from Great Salt Lake to begin to build a conceptual model for how salt dissolves in water. The lesson focuses on students using and developing models of molecules to build a better understanding of the particle nature of matter. Standard(s): Develop models to show that molecules are made of different kinds, proportions and quantities of atoms. Emphasize understanding that there are differences between atoms and molecules, and that certain combinations of atoms form specific molecules. Examples of simple molecules could include water (H 2O), atmospheric oxygen (O 2), and carbon dioxide (CO 2) Develop a model to predict the effect of heat energy on states of matter and density. Emphasize the arrangement of particles in states of matter (solid, liquid, or gas) and during phase changes (melting, freezing, condensing, and evaporating). Practice(s) Describe how students are engaged in one or two practices. Develop and use a model to describe phenomena. Construct an explanation using models or representations. Crosscutting Concept(s) Explain how crosscutting concept(s) provide a lens for the students Proportion and quantitytime, space, and energy phenomena can be observed at various scales using models to study systems that are too large or too small. Cause and effect relationships may be used to predict phenomena in natural or designed systems. Disciplinary Core Idea(s) State the big ideas students will use to explain the phenomenon. Substances are made from different types of atoms, which combine with one another in various ways. Atoms form molecules that range in size from two to thousands of atoms. Solids, liquids, and gases are made of molecules or inert atoms that are moving about relative to each other. Widely spaced (gas), closely spaced (liquid), or vibrating in place (solid).

2 Lesson Time Frame This lesson will take several class periods. Suggested scheduling is as follows. Day 1 Engage with samples of water and Explore with investigating dissolving salt Day 2 Explain 2 with molecular models of dissolving and evaporating Day 3 Elaborate with where the salt in GSL comes from Day 4 Evaluate with models of salt in GSL and individual explanations Lesson Materials Engage: Per group: samples of GSL and fresh water in clear cups, petri dish (optional) Explore: Per student: Investigating Saltwater Handout Per group: cup, salt, spoon, water, tablespoon, 100 ml beaker, 250 ml beaker, electronic kitchen scale, towel Explain: Per group: evaporated water from Engage section of lesson, color pencils or crayons, scissors, 2 pieces of construction paper, Salt and Water Molecules Handout Elaborate: Per group: Maps of GSL and Utah Lake, Articles - Where does the salt come from?, Salinity Levels in GSL Handout Evaluate: Per group: makers and poster paper or whiteboards

3 Great Salt Lake Is Salty Storyline Overview Driving Question / Anchor Phenomenon Water from GSL and water from a fresh lake look the same, but are very different. Expected Student Explanation Even though it looks like freshwater, GSL is a saline lake. This means that salt is dissolved in the water. The water breaks apart the salt crystals. You can t see the salt because the sodium (Na) and chlorine (Cl) atoms that make up salt are surrounded by water molecules (H 20). You can only see the salt when the water evaporates, because the salt is left behind. The reason GSL is a salt lake is because it is a terminal lake. The freshwater that enters the lake has a small amount of dissolved minerals, such as salt, in it. The lake has no outlets, so the only way water can leave the lake is through evaporation. When the water evaporates, it leaves behind salt. The more water that evaporates, the higher the salinity of the lake. When the lake level is low, the salinity of the lake is higher. This process makes GSL a saline lake. Phenomenon-driven question* What question will students try to figure out in this episode? Engage: How can you tell water is salt water? Explore: Where does salt go when you add it to water? Explain: Why can t you see salt when it is dissolved in water? Episode Description How will students engage in the science and engineering practices? Students ask questions and make observations in order to distinguish between two samples of water, freshwater and saltwater without tasting them. Students explore dissolving by investigating changes in mass and volume when salt is added to water. Students then develop an initial model based on evidence to show where salt goes when it is dissolved in water. Students build models of water and salt molecules and model dissolving. After watching a simulation, they revise their model of dissolving to show what happens to salt when it dissolves at a microscopic scale. Students then extend their model by showing what happens when water evaporates. Conceptual understanding(s) What concept will students figure out in this episode? When saltwater evaporates it leaves the salt behind. When salt dissolves in water it must break into small pieces because you can t see it. When salt dissolves in water, the sodium and chlorine atoms that make salt are surrounded by water molecules. When water evaporates the water molecules become water vapor and leave behind salt crystals.

4 Elaborate: Where does the salt in GSL come from? Evaluate: Why does water from GSL look like freshwater even though it is salt water? Students obtain information from maps and reading about where the salt in GSL comes from. Then students construct an explanation based on evidence from data for why water has the highest salinity in the late summer or early fall. Students use what they have learned to develop a group model and then write an individual explanation for why the water in GSL is salty. Great Salt Lake is salty because it is a terminal lake. Rivers carry a small amount of salt into the lake, and water leaves the lake by evaporation, leaving behind the salt. GSL is salty because salt molecules are dissolved in the water. The salt molecules are too small to see. The salt comes from dissolved minerals in the rivers that enter the lake. The only way water leaves the lake is through evaporation. This leaves behind salt and makes GSL salty. * Note: These questions may need to be modified based on the questions students develop throughout the unit.

5 Great Salt Lake Is Salty Lesson Details Engage: How can you tell water is saltwater? In the Engage section of the lesson small groups discuss how to tell the difference between a sample of water from Great Salt Lake and a sample from a freshwater lake. Provide small groups with two cups of water, one from a fresh lake and one from GSL. If you are unable to collect samples from lakes, make two samples. To make water that represents water from the North Arm of GSL, heat 1 liter of water in the microwave for two minutes. Then add 1 cup of salt to the water. If some of the salt does not dissolve, heat the water again and stir. To make water from a freshwater lake, use tap water. Provide small groups with the two samples of water. Tell them that they can observe the water using their sense of sight, smell, or touch. Remind them that they cannot taste the water drinking untreated water can make you sick. Have groups discuss how they can tell the two water samples apart. Give students time to determine which sample is from GSL and which sample is from a freshwater lake. Have students write a claim / evidence t-chart stating which sample is which, and how their evidence from the observations of the samples supports their claim. Invite groups to share their claims and evidence. Discuss how salt can be the in water, even though we can t see it. Use students ideas to emphasize that one way we know salt is in the water is that it is left behind when the water evaporates. Ask students to share anything they wonder about regarding the two samples of water. Connect the next episodes to student questions, by telling students that they will be exploring where the salt goes when you add it to water, and why GSL is so salty during the storyline. Have students leave their samples of fresh and salt water on a window sill in the classroom to observe what happens to the fresh and saltwater over time. If each group has large cups of water, you may want groups to pour samples into labeled petri dishes to allow evaporation to occur more quickly. Explore: Where does salt go when you add it to water? In the Explore section of the lesson small groups will investigate what happens to salt when it is dissolved in water. Begin this episode by dissolving about 1 T of salt in a cup of water do this either as a demonstration, or have groups add a spoonful of salt to water. Ask them to observe what happens before and after they stir the salt. Have a few groups share their observations, and use this short experience to reinforce that the salt does not melt, which is a common student misconception, but dissolves in the water. Explain that dissolves means that one substance mixes uniformly in another substance. Usually a solid, like salt, dissolves in a liquid, like water. Sometimes gases are dissolved in water, sometimes another liquid is dissolved in water. Explain to students that they are going to investigate further to try to figure out what happens to salt when it is dissolved in water. In small groups students will compare the mass of salt and mass of water to the total mass when both are mixed together. Then they will compare the volume of salt and the volume of water to the total volume when mixed together. Depending on students familiarity with measuring mass and volume, the investigation can be more student led or teacher led. For students that may need more support, an investigation worksheet is available in the lesson folder. Provide groups with an electronic scale, a cup full of salt, a water bottle, a stirrer, a towel, a 100 ml beaker, a tablespoon, and a 250 ml beaker. For the mass measurements, be sure to explain to students how to tare the electronic scale. To do this, students should place the empty container (beaker or paper for salt) on the scale and then press the tare button. They can then add water or salt and find only the mass of the water and salt. Have students find the mass of about 1 tablespoon of salt and the mass of about 100 ml of water. Students should record the mass in their science notebooks. Then have students combine the salt and water in a new container, being sure to tare the

6 scale, and find the mass of only the saltwater. Ideally, students measurements should show that mass is conserved. The sum of the mass of salt and water should be the same as the saltwater. For volume measurements, have students measure 100 ml of water. Remind students to read the measurement at eye level, to be sure they have exactly 100 ml of water. Then have students use a tablespoon to measure 15 ml of salt. We usually do not use volume measurements like this for solids, however, measuring the volume this way will make understanding the results more accessible for students. Have students record both of these initial measurements in their notebooks. Then have students mix the salt and water together in a new beaker. Once all of the salt has dissolved, have students find the total volume of the saltwater. Students should see that the total volume is less than the sum of the initial volume of water and salt. Ask small groups to discuss what their results can tell them about salt dissolving in water. Have groups consider both the mass results and the volume results. Through small group and whole class discussion, guide students to think about how the mass is conserved this means the same amount of water and salt is still in the saltwater. Neither the water nor the salt disappeared. However, the saltwater takes up less volume than the salt and water did alone. Encourage students to draw an initial model to show what they think is happening to the salt as it dissolves. Have students share these models with peers. In the next section of the lesson, students will build on these models to show what is happening at a micro-scale, or molecular level when salt dissolves in water. Explain: Why can t you see the salt when it is dissolved in water? In the first explain section of the lesson, students will build on their initial models to show what happens when salt dissolves in water at a molecular level. Start by having students look at one grain of salt under a hand lens or digital microscope. Discuss the shape of the salt crystal as a cube. Explain that a salt crystal is made from molecules that are made of two different atoms, sodium (Na) and chlorine (Cl) that are bonded together. Each grain of salt has more than hundreds of trillions of molecules, more than we could possibly imagine. Tell students that in the activity today, they will add on to their model for salt dissolving in water by showing what happens on a molecular level. (Teacher note: Salt is a compound since it is formed from ionic bonds. In 6 th grade students do not need to know that there are different types of bonds that hold compounds and molecules together. This lesson refers to NaCl as a molecule because the focus is on students understanding particles rather than bonds.) Start by having students draw models of water and salt molecules. Provide students with colored pencils or crayons and the following information: The chemical formula for water is H 2O. The chemical formula for salt is NaCl. Na = sodium (grey circle) Cl = chlorine (green circle) H = hydrogen (white circle) O = oxygen (red circle) Ask students to use this information to draw a model of NaCl and a model of H 2O in their notebooks. Alternatively, have students use the Molview program to build computer models of molecules. Have students select 2D-3D under the Molview menu. Students can then drag and drop atoms, and use lines to show the atoms are bonded to form a molecule. By pushing the 2D to 3D button on the top menu bar, the molecule students create shows up as a 3D model. The model can be a ball and stick model or a sphere model depending on what is selected from the model menu. Students can explore and build molecules, then they can download their 3D models as jpegs to save the images. After students have drawn models or created 3D models online, discuss the two molecules by comparing the number and types of atoms in each molecule. Show students the structure of each molecule, Na Cl and O H O. Help students visualize how these molecule form a substance, ask students to describe the shape of the salt crystal that they looked at during the start of the Explain

7 phase of the lesson. The cubic shape is related to the structure of Na Cl. The individual molecules stack to form cubic crystals. Project an image of a salt crystal to help students visualize how the structure of a salt molecule relates to the crystal they observed in the beginning of the lesson. Provide small groups with the salt and water molecule sheet. Instruct students to divide the sheet amongst their group members by cutting it up and then have each group member color and cut out the molecules based on the colors used in the beginning of the lesson. Give groups 2 sheets of construction paper. Have groups use the molecules they have cut out to create a model that shows how salt dissolves in water. Start by having students make one model that shows the water molecules as a liquid. Then have students create a model of a salt crystal. Finally ask students to add the salt to the water to create a model of salt dissolving in water. Remind students of the results of their investigation, most importantly, the volume of saltwater is less than the sum of the initial volume of the water and salt. After students have made this initial molecular model of salt dissolving in water. Show students an animation of salt dissolving in water. The following animations can help students better conceptualize how salt dissolves. Note: you may not want to include the sound as students watch these animations. Both animations discuss ions and charges. It is more important that students understand that when salt dissolves, the salt crystal is broken apart, and the atoms are surrounded by water molecules. Ask students to revise their groups model to show how salt dissolves in water. Help students make the connection between the smaller volume of saltwater in their investigation and how the atoms that make up salt fit in spaces between the water molecules. Next have students pick up the water samples from the Engage section of the lesson. At this point, the water should have evaporated. Ask students what happened to the water in their samples. Students should notice that the freshwater evaporated, leaving an empty cup or petri dish. The salt water evaporated leaving behind cubic salt crystals. Ask students to use their paper models to show what happens when saltwater evaporates. Students should move their water molecules to the atmosphere, spread the molecules apart, and reform their salt crystal. If students have not yet discussed how the arrangement of water molecules changes as water turns from a liquid to a gas, you may want to let them explore the States of Matter, Basics PhET simulation: Use the states of matter simulation, rather than the phase change simulation. This will help them to visualize what happens to water molecules during evaporation. After sharing and discussing the paper models, have students return to their science notebooks. Students can either add on to their initial model of salt dissolving in water or they can draw a new model at a micro-scale to show what happens when salt dissolves in water at a molecular level. Check for student understanding by making sure the models in students notebooks show water molecules surrounding sodium and chloride. Encourage students add a key or captions to clarify the molecular models they have drawn. Elaborate: Where does the salt in GSL come from? In the elaborate section of the lesson students will use maps and articles to figure out where the salt in GSL comes from. Provide students with the map of GSL and Utah Lake and the articles, Great Salt Lake and All Drains Lead to the Ocean. Ask students to use the maps to figure out how water moves into and out of each lake, and read the articles to make sense of why the inflow and outflow of water controls whether a lake is saline, like Great Salt Lake, or fresh, like Utah Lake.

8 After students have read the resources, ask them to answer the question: where does the salt in Great Salt Lake come from. Discuss students ideas as well as why the salinity level in the lake changes when the lake level changes. At this point, students should be able to explain that rivers carry small amounts of dissolved salts into Great Salt Lake. The only way water leaves GSL is through evaporation, and this process leaves the salts behind, making GSL a saline lake. If lake levels are low, then the salinity will be higher. If lake levels are high than the salinity will be lower. As an extension or further challenge, provide students with the graphs showing salinity in GSL and lake elevation, Salinity Changes in GSL, available in the lesson folder. Ask students to describe the times of year when the lake has a low salinity and the times of year when the lake has a high salinity. Then invite students to work together to claim-evidence-reasoning chart that explains how the salinity level of the lake changes during the year based on data from the graphs and reasoning from their understanding of saltwater. Have groups share their C-E-Rs, be sure to discuss how salinity in the lake is lower in the spring because of the freshwater coming in from snowmelt and higher in the fall because of the evaporation that occurs over the summer. Evaluate: Why does water from GSL look like freshwater even though it is saltwater? For the evaluate section of the lesson, students will construct an explanation for why GSL water looks like freshwater, but is salt water. Remind students of where they started this lesson, by comparing water from a freshwater lake and from Great Salt Lake. The water samples may look the same, but they are very different. To help scaffold student explanations, have small groups of students work together to draw a model that explains why Great Salt Lake is salty. The model should address both where the salt is in the water as well as how the salt gets into the lake. The models can include both pictures and captions. Have students draw these models on poster paper or on whiteboards. After students have finished, invite them to display their models, and have a few groups explain what their model shows. After discussing the models, have students construct an explanation for why Great Salt Lake is salty. Provide students with the following prompt. A sample of water from Great Salt Lake and Utah Lake look the same, but the water from GSL is saline. 1. Use what you know about molecules to explain how the GSL water can look like freshwater even though it is a saline lake. 2. Explain why GSL is a saline lake.

9 Sample Student Response: Even though it looks like freshwater, GSL is a saline lake. This means that salt is dissolved in the water. The water breaks apart the salt crystals. You can t see the salt because the sodium (Na) and chlorine (Cl) atoms that make up salt crystals are surrounded by water molecules (H 20). You can only see the salt when the water evaporates, because the salt is left behind. The reason GSL is a salt lake is because it is a terminal lake. The freshwater that enters the lake has a small amount of dissolved minerals, such as salt, in it. The lake has no outlets, so the only way water can leave the lake is through evaporation. When the water evaporates, it leaves behind salt. The more water that evaporates, the higher the salinity of the lake. When the lake level is low, the salinity of the lake is higher. This process makes GSL a saline lake. Sample Rubric: Explanation for why GSL is salty Explains what happens when salt dissolves at a molecular level, including chemical symbols, formulas and the arrangement of atoms. Describes how salt enters GSL Describes what States that salt is happens when dissolved in the salt dissolves at a water. molecular level. Explains the role of evaporation in making GSL a saline lake. Describes how salt enters GSL. Explains the role of evaporation in making GSL a saline lake. Describes how salt enters GSL. States that water leaves the lake through evaporation, but may not relate this why GSL is a saline lake. States that GSL has salt water. Description of how salt enters the lake or why the lake is saline is incorrect or unclear. Explains how evaporation and/or lake levels affect salinity.