B7 Engineering and the Water Cycle

Transcription

1 Collaborative Learning This investigation is Exploros-enabled for tablets. See page xiii for details. Key Question: Is it possible to transform ocean water into drinkable fresh water? TECHNOLOGY SCIENCE M ATH ENGINEERING In this investigation, students will set up a model of the water cycle in the form of a solar still. The investigation will span over several days while water from the evaporates, condenses, and drips into the collection container. The starting water will be a 3.5 percent saltwater solution. After water is collected over several days, students will design their own mini-experiment to find out if the collected water is salty or fresh. Toward the end of the investigation, they will try designing a device that could produce 1 gallon of fresh water per day. Learning Goals Make a 3.5% salt water solution Set up a working model of the water cycle Design an experiment to detect salt in a water sample and design a device that could produce a gallon of fresh water per day GETTING STARTED Time One class period for setup, one class period for analysis (there is a 3 4 day wait in between). Setup and Materials 1. Make copies of investigation sheets for students. 2. Watch the equipment video. 3. Review all safety procedures with students. 4. Have students work in small groups of three to five. Materials for each group y y Water* y Table salt (NaCl)* y Balance* y Large container for mixing water and salt* *provided by the teacher Online Resources Available at curiosityplace.com y Equipment Video: y Skill and Practice Sheets y Whiteboard Resources y Animation: Watershed Impact y 1-L Beaker or graduated cylinder* y Collection container (a short glass jar or beaker)* y 1 Weight* y Student Reading: The Water Cycle y Plastic wrap* y Masking tape* Vocabulary condensation the process by which atmospheric water vapor changes phase from gas to liquid desalination the process of separating salts and minerals from saline water to produce fresh water evaporation the process by which water changes phase from a liquid to a gas salinity a term that describes the saltiness of water water cycle a set of processes energized by the Sun that keep water moving from place to place on Earth NGSS Connection This investigation builds conceptual understanding and skills for the following performance expectation. 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. Science and Engineering Practices Disciplinary Core Ideas Crosscutting Concepts Developing and Using Models ESS2.A: Earth Materials and Systems Systems and System Models 101

2 Engineering and the Water Cycle BACKGROUND In this investigation, students construct a solar still. Solar stills are simple devices that capture fresh water that has been evaporated from salt water. Salinity is a term that describes the saltiness of water. In the past, solar stills were used in emergency survival kits for pilots who flew over the ocean and for anyone in danger of being stranded at sea or in a desert. However, solar stills are not an efficient way to obtain fresh water. The amount of potable (fresh, drinkable) water they generate is far less than our average daily need: A person needs about 1 gallon of fresh water per day, but even the most efficient stills only produce 8 ounces per day. Because of this inefficiency, manual devices that use reverse osmosis have replaced solar stills in survival kits. Reverse osmosis uses pressure to filter salt water through a special membrane. Ions of salts are held back on the pressurized side of the membrane, while water molecules are able to move through the membrane. During osmosis, water moves to the area that has the most solute. During reverse osmosis, water moves away from the area that has the most solute. A solar still works similarly to the water cycle. Both are driven by energy from the Sun. In the water cycle, energy from the Sun causes evaporation to occur. Evaporation is a process in which liquid water gains energy and enters the gas phase. Eventually, the water in the gas phase loses energy, causing condensation to occur, and the water once again enters the liquid phase. To produce fresh water from seawater on a large scale that will serve whole societies, various processes are currently being tried and in development. The key to sustainable desalination is to reduce the amount of energy needed for the process. As a result, desalination is an area of active engineering effort with new technologies being developed all the time. The engineering cycle helps guide this process. For example, to make the reverse-osmosis process more energy efficient, scientists and engineers have created membranes that do not get clogged as quickly. 102

3 5E LESSON PLAN Engage Review with students that the water cycle has four main processes: evaporation, condensation, precipitation, and transpiration. They will model evaporation, condensation, and precipitation in this investigation. What about transpiration? What s that? Transpiration is the process by which plants lose water through tiny pores on their leaves. To model this phase of the water cycle, we will turn one into a terrarium. Find out if students know what a terrarium is; they should be familiar with this concept. You can set up a terrarium by adding some small potted plants to the. Use pots that do not have holes in the bottom for water to run out. As in the investigation, create an airtight seal using plastic wrap and masking tape. Create the seal after you have watered the plants! Over the next few days, students will observe that condensation forms on the plastic wrap. However, point out that it is hard to say whether this water came from the soil or the plants! To prove that at least some came from the plant, you can use a large tropical plant or a houseplant like a geranium for a demonstration. Tie a plastic bag over a few leaves and secure it to the stem with tape or a twist tie so that it is airtight. Depending on the conditions (sunny vs. shady; watered or not), the bag will collect a lot or a small amount of moisture. Plants transpire more when they are warm and when they are taking in water. If you have access to a microscope, you can create a slide of a piece of leaf that shows the stomata or pores on the underside of the leaf. Water exits a plant through these pores. Have students complete Investigation, Engineering and the Water Cycle. Students learn about the processes in the water cycle and its importance. They will model three of the processes by demonstrating how ocean water can be transformed into fresh water. They will then devise an experiment to test the salinity of the water they collect. Finally, students will design a device for collecting 1 gallon of fresh water a day. Explain Revisit the Key Question to give students an opportunity to reflect on their learning experience and verbalize understandings about the science concepts explored in the investigation. Curiosityplace.com resources, including student readings, videos, animations, and whiteboard resources, as well as readings from your current science textbook, are other tools to facilitate student communication about new ideas. Elaborate Animation Watershed Impact Have students study the geography of Saudi Arabia and North Africa. These desert regions depend on desalination to boost their stores of fresh water. Desalination plants are also in use in California. Like Saudi Arabia and North Africa, California is very dry. Yet it is the number one farming state in the United States and earned $44.7 billion in 2013 growing food. For these reasons, 80 percent of the water California consumes goes to agriculture. Unfortunately, in recent years California has suffered a drought and is looking to desalination as an option despite the fact that it is not an environmentally friendly process. Desalination uses a lot of energy and the salty waste that is dumped back into the ocean is harmful to marine life. Research and write a newspaper article about desalination in Saudi Arabia, North Africa, or California. Evaluate y During the investigation, use the checkpoint questions as opportunities for ongoing assessment. After y completing the investigation, have students answer the assessment questions on the Evaluate student sheet to check understanding of the concepts presented. 103

4 Engineering and the Water Cycle Is it possible to transform ocean water into drinkable fresh water? The short answer to this key question is yes. In nature, this happens all the time thanks to Earth s water cycle. More than three-fourths of Earth s surface is covered by water and 97 percent of this surface water is held in oceans. The Sun s energy evaporates ocean water and other surface water into the atmosphere. This water is then recycled as it cools and condenses into clouds and then precipitates back to Earth s surface as fresh water in the form of rain or snow. Evaporation and condensation are two major processes of the water cycle. For human beings to transform ocean water into drinkable fresh water, more work is involved. You will explore how this happens on small scale in this investigation. In effect, you will be mimicking the water cycle. Fresh water is a necessary resource, and being able to rely on the oceans as a source for fresh water may become more and more important in the future. As you work, think about how you might engineer a solution for transforming ocean water into fresh water on a large scale. Name Date Creating a 3.5 percent salt water solution You will build a model of the water cycle using a solution that has a salinity of 3.5 percent, like the oceans. You will need to figure out how much salt to add to the water in the so your salty water has a salinity of about 3.5 percent. The goal is to pour a 3.5 percent salt water solution into the so the water depth is about 3 centimeters. 1. You will need about 2 liters of the 3.5 percent salt water solution to get a depth of approximately 3 centimeters. A 3.5 percent solution of salt water has 3.5 grams of salt for every 100 grams of water. Set up a proportion to figure out how many grams of salt you will need to make 2 liters of salt water solution. Show your work and answer below. 3.5 g salt 100 g water = x g salt 2,000 g water 3.5 2,000 = 100 x 7,000 = 100 x x = 70g Materials: Water Table salt (NaCl) Balance Large container for mixing water and salt 1-L Beaker or graduated cylinder Collection container (a short glass jar or beaker) 1 Weight Plastic wrap Masking tape 2. Have your teacher check your work. Then, obtain the amount of salt you need. Because the salt will add volume, subtract this value from the total volume of water. For example, if you needed 5 grams of salt to make a 5 percent solution, you would add the 5 grams to 95 milliliters of water, giving you a total volume of 100 milliliters. 3. Mix the salt thoroughly. Setting up the water cycle model The illustration shows what the water cycle model will look like. 1. Pour the 3.5 percent salt water solution into the. It should be about 3 centimeters deep. 2. Put a collection container that is shorter than the height of the in the middle of the. A short glass jar or beaker works well. Masking tape 3. Cover the with plastic wrap. Do not stretch the wrap tightly. Seal the edges with masking tape all the way around the box rim. 4. Place a weight in the middle of the plastic wrap so the lowest part is centered over the collection container (see illustration). A CPO Science weight is fine to use. Doing the experiment 1. Place the in a warm, sunny window where it can sit undisturbed. Check the water cycle model each day for 3 4 days. 2. Record observations and what you think is happening in Table 1. Do not take the model apart, and do not discard any of the contents until you have answered all questions in the investigation. 3 cm Weight Plastic wrap Collection container Salt water 1 of 6 2 of 6 Guiding the Creating a 3.5 percent salt water solution Setting up the water cycle model The goal is to have students create salt water with salinity that is approximately the same as ocean water. The average salinity of ocean water is 3.5 percent. To achieve this, have students determine the volume of water in the using V= lwh, where h is the 3-centimeter height of water added. Then they will convert cubic centimeters to ml, where 1 cubic centimeter is equal to 1 milliliter. Also, point out to your students that 1 ml of fresh water has a mass of 1 gram. The volume of the water in the is approximately L (or 2,160 ml). The density of water is 1 g/ml, so we can assume the mass of the water is 2,160 grams. In order to obtain water that is as salty as ocean water (3.5 percent), add 78.3 grams of salt to this volume of water. Your total mass will be 2, = 2,238.3 g [78.3/ = 3.5 percent]. This calculation is based on the following formula: mass of salt mass of salt + mass of water = salinity 104

5 Table 1: Observations Ocean water cycle Day Observation What you think is happening 1 There are tiny droplets forming on the bottom surface of the plastic wrap. Water is condensing on the bottom surface of the plastic wrap. Thinking about what you observed a. What happened to the water level in the? What process was responsible for this change? The water level in the went down. Evaporation was responsible for the decrease in water level. 2 The droplets are much bigger and seem to be largest near the middle. The condensing water droplets are combining together to form larger droplets. b. What happened to the water level in the collection container? What process was responsible for this change? The water level in the collection container increased because the condensed water droplets rained into the cup. 3 Droplets continue to gather and there is water in the collection container. The condensed water is gathering on the plastic wrap and dripping into the glass in the form of precipitation. c. Using a diagram of the water cycle for reference, label and add details to the illustration below to show each stage of the water cycle as it occurs in your model. Masking tape Weight Plastic wrap 4 The water level in the is clearly lower, more droplets, and more water in the glass, there is a salty residue left behind on the walls of the. The condensed water is gathering on the plastic wrap and dripping into the glass, but the salt is left behind. 3 cm Collection container Salt water See 4c sample labeled diagram. 3 of 6 4 of 6 Guiding the Thinking about what you observed When asked if they notice evidence of evaporation in the, students may report noticing a decrease in water level. However, if only a small amount of evaporation occurred, the level might appear unchanged. Have students refer to their observations from Table 1 to speak more specifically about what they witnessed (or suspect happened) on days 1 4. When asked about evidence of condensation, they should indicate that they have seen water droplets on the plastic wrap over the course of the four days. Provide everyday examples of condensation. For example, evaporated water condenses on the outside of a cold glass. After a hot shower, water vapor condenses on the bathroom mirror. Explain that water dripped from the plastic wrap in a process similar to how precipitation falls to Earth. Make an overhead of the provided whiteboard resource, The Water Cycle, and use it as you discuss how the solar still works. 4c sample labeled diagram Masking tape Condensation Evaporation 3 cm Weight Precipitation Plastic wrap Condensation Evaporation Collection container 105

6 Engineering and the Water Cycle What is the composition of the collected water? You started with a 3.5 percent salt water solution in the. Now, after several days, you have liquid in the collection container. Is this liquid salty or fresh? Design a way to test your hypothesis. a. State your hypothesis as to whether the liquid in the collection container is fresh water or salty water. Explain your reasoning. I think the liquid in the collection container is fresh water, because I only saw salt residue on the sides of the, not in the collection container or on the underside of the plastic wrap. d. Engineering problem: Imagine you have to transform ocean water into a gallon of fresh water a day for yourself. How might you do this? Your only criteria are (a) you need a gallon of fresh water and (b) you need this volume of water produced on a daily basis. What are your constraints? List those at the bottom of this sheet. Then sketch a device that will achieve your goal of 1 gallon of fresh water per day. Come up with a name for your device. b. Design a simple experiment to determine whether the water in the container is fresh or salty. Don t forget to include a control. Write the steps of your procedure here, and obtain permission from your teacher before performing the experiment. Answers will vary. Students will probably want to evaporate the water to see if any salt is left behind. They should use some of the salt water and some fresh water as controls for comparison with the experimental water. Using the set of three aluminum dishes that comes with the will work well. Not much water is needed, but each dish should get the same amount of water. c. Discuss the results of your experiment. Is the water fresh or salty? How does the result compare to your hypothesis? Students will find that the collected water is fresh. Possible constraints that students will come up with might be access to enough ocean water, financial constraints, the time limit of 24 hours, and the fact that the Sun only shines for a limited amount of time each day. If the students thinking of using just the, the container is a constraint. After they have thought a while, you may eliminate a few constraints such as ocean water is limitless and money and access to special equipment is not a problem. Have students present their ideas during the next class period. 5 of 6 6 of 6 TEACHING TIP Explain to students that when they placed the es near the window, they created a solar still. A solar still uses energy from the Sun to distill water. In other words, the solar energy causes pure vapor to condense on the plastic wrap and then drip into the collection container. Ask students to think of some situations where this might be helpful. Students may need prompting. During a natural disaster, power outages are common and residents are often without potable water. A solar still could be used to obtain clean, drinkable water. SCIENCE AND LANGUAGE Based on what you learned about desalination in this investigation and on your own research, write a position paper supporting either greater desalination efforts or stricter water conservation rules. 106

7 Evaluate Name Date 1. Research the term desalination. Summarize what you discover. Desalination is the removal of dissolved salt from ocean water or from salty groundwater. Two major types of desalination processes are reverse osmosis and distillation. In reverse osmosis, salty water is pumped at high pressure through a special membrane that allows the water to pass through but traps the salt molecules and removes them. The distillation process is similar to the model we set up in this investigation. In distillation, salt water is heated until it evaporates and condenses as fresh water, leaving the salt behind. 2. Research and find at least two different places in the world that operate desalination plants. Where are these plants? What are the advantages and disadvantages of using desalination to get fresh drinking water? Saudi Arabia and North Africa produce most of the world s desalinated water. The big disadvantage to producing fresh water through desalination is the cost. It is very expensive to produce fresh water by removing the salt from salty water. Saudi Arabia and North Africa are very dry, with very few fresh water resources. Desalination, although costly, is one of the only options these places have for obtaining fresh water. Evaluate Name Date 3. You observed the water cycle model over a period of three to four days. Suggest something you could have done to speed up the process. Perhaps I could have used a light source so the evaporation would have continued throughout the night. Also, the warmth from the light might speed up the daytime evaporation. I would have to be careful, though, not to melt the plastic! 4. What might have happened to the results of your water cycle investigation if the plastic wrap was not carefully sealed all the way around the box? If the box was not carefully sealed, some of the evaporated water could have escaped into the classroom air, and less would be available for condensation and precipitation into the collection container. In addition, the air inside the box might not have gotten warm enough for sufficient evaporation. 5. Describe a situation in which this investigation s apparatus setup would be useful to someone. This apparatus, sometimes called a solar still, would be useful for someone stranded in the ocean or in a desert without fresh water to drink. SCIENCE AND MATH Solve these problems. Set up proportions as needed. 1. Calculate how much salt you would need to make 100 grams of a 5 percent solution. Answer: 5 grams 2. You have 2 liters of a 12 percent salt solution. How much salt was required to make this solution? Answer: 240 grams 3. What volume of water do you need to make a 2 percent salt solution using 10 grams of salt? Answer: 500 ml or 500 grams WRAPPING UP Have your students reflect on what they learned from the investigation by answering the following questions: 1. What are the four main processes of the water cycle? 2. Write the recipe for making a 3.5 percent salt water solution. 3. Why is getting fresh water from the water cycle so much cheaper than using the process of desalination? 107

8 Engineering and the Water Cycle Notes and Reflections 108