Summary

In this lesson, students will consider their water footprint and means to obtain fresh water from seawater using a solar still. To understand the differences between fresh water and seawater, students will determine the composition of artificial seawater by using qualitative analysis to test for different ions in solution and calculate the molarity of different salts used in the recipe. Students will observe the effects of solutes in aqueous solutions by measuring conductivity and the freezing and boiling points of seawater and deionized water and determine total dissolved solids. In addition, students explore the buffering ability of seawater and the effect of carbon dioxide on its pH.

Grade Level

High School

NGSS Alignment

This lesson will help prepare your students to meet the performance expectations in the following standards:

• HS-PS1-2: Construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.
• HS-PS1-3: Plan and conduct an investigation to gather evidence to compare the structure of substances at the bulk scale to infer the strength of electrical forces between particles.
• HS-ESS2-5: Plan and conduct an investigation of the properties of water and its effects on Earth materials and surface processes.
• Scientific and Engineering Practices:
  • Asking Questions and Defining Problems
  • Analyzing and Interpreting Data
  • Planning and Carrying Out Investigations
  • Constructing Explanations and Designing Solutions

Objectives

By the end of this lesson, students should be able to:

• Calculate molal and molar concentrations.
• Write net-ionic equations and predict precipitates.
• Understand how solutes affect freezing and melting points.
• Explain how solutes affect the properties of a solution.
• Observe how buffers minimize changes in pH when either acid or base is added.

Chemistry Topics

This lesson supports students’ understanding of:

• Solutions
• Molarity, Molality
• Concentration
• Colligative Properties
• Ionic Compounds
• Chemical Reactions
• Net Ionic Equations
• Buffers
• Solubility Rules and Solubility
• Precipitates and Predicting Products

Time

Teacher Preparation: 120-180 minutes (2-3 hours) over the course of 4-6 days

• Part 1: Does the World Have Enough Water?
  • Prepare seawater solutions: 30-40 minutes
  • Assemble a solar still: 15-30 minutes
• Part 2: What’s in that Water?
  • Prepare solutions for qualitative analysis: 30-60 minutes (depends on whether solutions are bought or need to be prepared)
• Part 3: Phase Changes of Seawater
  • Prepare ice baths: 20-30 minutes (these can be prepared and stored in the freezer until needed)
• Part 4: Buffering Ability of Seawater
  • Prepare solutions: 10-30 minutes (depends on whether solutions are bought or need to be prepared)

Lesson: 150-240 minutes (2.5-4 hours) over the course of 4-6 class periods

• Note that several parts of this lesson can be completed as homework to cut down on class time needed. (See notes below and in the Teacher Notes section.)
• Part 1: Does the World Have Enough Water?
  • Complete water calculator activity: 10-15 minutes (could be assigned for homework)
  • Watch Untapped Potential video: 10 minutes (could be assigned for homework)
  • Assemble and record initial observations of solar still: 15-45 minutes (time will depend on whether observing teacher-built still or assembling their own)
• Part 2: What’s in that Water?
  • Complete Qualitative Analysis and Total Ion Content procedures: 45-60 minutes (about 10-15 minutes per station)
  • Analysis and Conclusion questions: 30-40 minutes (could be assigned for homework)
• Part 3: Phase Changes of Seawater
  • Freezing point activity (can run DI and seawater simultaneously): 15-25 minutes
  • Boiling point activity (run DI and seawater sequentially): 20-30 minutes
  • Total Dissolved Solids: 10-15 minutes (or allow to evaporate overnight and check the next day)
• Part 4: Buffering Ability of Seawater
  • Complete buffer activity procedures: 20-30 minutes
  • Analysis and Conclusion questions: 20-30 minutes (could be assigned for homework)

Materials

• Artificial seawater: Dissolve the quantities of salts listed below in 1.0 L of deionized (or distilled) water to prepare the artificial seawater. For 10-12 lab groups, approximately 2 L are sufficient to complete all of the following experiments. The exact amounts are not critical, and teachers can omit one or two salts without compromising the lab (though sodium bicarbonate is necessary if the activity on buffers is to be included).
  • 27.53 g NaCl
  • 1.23 g KCl
  • 0.27 g NaHCO₃
  • 5.79 g MgCl₂ ･ 6 H₂O
  • 14.17 g MgSO₄ ･ 7 H₂O
  • 1.82 g CaCl₂ ･ 2 H₂O
• Each part of this lesson has a student handout that accompanies it (4 student activities, plus the final reflection prompt), and the other materials listed below are per lab group unless otherwise noted.

Part 1: Does the World Have Enough Water?

• Computer or device with internet access to use the water calculator and play the video
• Materials for constructing solar stills (could do one teacher-constructed still as a demo if time/materials are limited):
  • Large container (ex: large beaker, bucket)
  • Smaller container that can fit inside the large container (ex: small beaker or dish)
  • Plastic wrap
  • Small item/weight to create a point for condensate to collect on the plastic wrap
  • Graduated cylinder for measuring the volume of water added and collected

Part 2: What’s in That Water?

Per lab group:

• Red and blue litmus (or pH) paper
• 2 watch glasses
• 10 mL graduated cylinder
• 3 small test tubes and test tube rack
• 3 50-mL beakers

One per class, set up at various stations for students to rotate through:

• Qualitative Analysis reagents, 25-50 mL each in dropper bottles
  • 1 M HNO₃
  • 0.1 M AgNO₃
  • 1 M HCl
  • 0.1 M BaCl₂
  • 0.1 M KSCN
  • 1 M NaOH
  • 1 M H₂SO₄
• Conductivity Probe and/or Salinity Sensor (for example, Vernier Conductivity Probe or Salinity Sensor)

Part 3: Phase Changes of Seawater

• Sodium chloride-ice bath (33 g of NaCl/100 g ice) to maintain temperature below -5 °C (this bath can reach -21 °C).
• Hotplates
• Balances
• 2 10-mL graduated cylinders
• 2 test tubes
• 2 digital temperature probes (or 2 thermometers)
• 2 metal cans (see Teacher Notes) or 100 mL beakers
• 2 100-mL graduated cylinders
• Beaker tongs
• 2 ring stands and clamps to hold temperature probes (or thermometers)

Part 4: Buffering Ability of Seawater

• 0.1 M HCl, 25 mL each in dropper bottles
• 0.1 M NaOH, 25 mL each in dropper bottles
• 6 50-mL beakers
• 2 50-mL graduated cylinders
• 1 pH probe/meter
• 2 straws

Safety

• Always wear safety goggles when handling chemicals in the lab.
• Students should wash their hands thoroughly before leaving the lab.
• When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.
• Exercise caution when using a heat source. Hot plates should be turned off and unplugged as soon as they are no longer needed.
• An operational fire extinguisher should be in the classroom.
• When working with acids and bases, if any solution gets on students’ skin, they should immediately alert you and thoroughly flush their skin with water.
• When diluting acids, always add acid to water.

Teacher Notes

• This lesson was developed as part of the AACT Chemistry & Sustainability content writing team. It connects with the following UN Sustainable Development Goals:
  • Goal 6: Clean Water and Sanitation – Ensure access to water and sanitation for all.
  • Goal 13: Climate Action – Take urgent access to combat climate change and its impacts.
  • Goal 14: Life Below Water – Conserve and sustainably use the oceans, seas, and marine resources.
• This lesson has been refined over several years of practice with Honors Chemistry students. It is typically used as a week-long summative project towards the end of the year to tie together several concepts, including solutions, acids, and chemical reactions.
• The different parts of this lesson can also be spread over several units and days to provide a recurring touchstone throughout the year.
• Either deionized or distilled water is recommended, both for preparing the seawater solution and for a comparator. In a pinch, tap water can be used, but results may vary depending on local conditions. If you want a third data point (in addition to seawater and DI water), include tap water as a sample for qualitative analysis, conductivity/salinity, and total dissolved solids.

Part 1: Does the World Have Enough Water?

• Introduce the activity by exploring a real-life problem, “Does the world have enough water?” by having students calculate their “Water Footprint” using a water footprint calculator. More high school lesson plans utilizing the water calculator are available as well, if you wanted to spend more time on this topic.
• Have students watch Untapped Potential, from Chemistry Shorts and the Dreyfus Foundation to discuss water usage and opportunities to create a circular water economy by purifying saline and wastewaters.
• Create a simple solar still to kick off the project using directions from the International Year of Chemistry 2011 Challenge and monitor its performance as students complete the seawater analyses. Ask students about the changes that they expect to see, how they could test the collected water, and/or how the collected water would compare to the seawater.
  • You can have each group build their own still or have a larger one assembled for the class to observe if time and/or materials are in short supply. The simplest design is essentially a smaller dish inside a bucket covered with plastic wrap (see diagram below). The seawater is poured in the bucket and the condensate collects in the smaller dish.

• Determine how long the still should operate; at least overnight is probably necessary to obtain measurable condensation, but that will determine on ambient temperature.
• You may wish to have students make observations at various points in time after setting up the still (for example, after 1 hour, overnight, over several days), in which case you should provide directions to record these and add space to the data table for their observations at each time interval.

Part 2: What’s in That Water?

Qualitative Analysis

• Four tests are provided, but these can be customized as appropriate. Ideally, at least one distractor ion test should be included so that students gain experience with negative results (as written, the tests for iron and ammonium ion will give negative results). Procedures for additional tests relevant for seawater can be found in Experiments 33 and 34 from Chemistry in the Laboratory, 8^th Ed. by J. M. Postma and A. Roberts.
  • The test for sulfate is a white precipitate with barium chloride (that does not dissolve in dilute hydrochloric acid): MSO₄(aq) + BaCl₂(aq) → BaSO₄(s) + MCl₂(aq).
  • The test for chloride is a white precipitate with silver nitrate (that does not dissolve in dilute nitric acid): MCl(aq) + AgNO₃(aq) → AgCl(s) + MNO₃(aq).
  • Iron(III) ions react with SCN^- to form the red complex: Fe³⁺(aq) + SCN^-(aq) → Fe(SCN)²⁺(aq)
• Typical procedures call for 6 M HNO₃ and 6 M HCl; more dilute solutions can be used but require more volume in order to acidify the sample. Depending on resources and teacher preference, samples for chloride and sulfate ion analysis can be acidified by the teacher as part of the lab setup.
• You can set this up as a series of stations, so you only need one set of reagents for each test. Students rotate stations on your signal (about 10 minutes per station).

Total Ion Content

• The instructions provided on the student document include the use of both a conductivity probe and a salinity sensor. If you do not have both probes, you can just include the instructions for the probe you have and remove the other information from the procedures and data table before giving the handout to students. The procedures also include a tap water sample. You can remove tap water if time is short, although these tests don’t take very long to complete.
• If there is an upper limit to the conductivity meter, it is necessary to dilute the seawater sample to obtain a measurement (and multiply the measurement by the dilution factor in order to get the value of the actual sample). Suggestions for this are provided in the student procedures, but you may need to adjust them depending on the range of your conductivity meter.

Part 3: Phase Changes of Seawater

• For the most efficient use of lab time, you may choose to pair up lab groups and assign one lab group to measure boiling points and the other freezing points, then have the groups swap data.
  • Measuring the boiling point is more difficult for students than measuring the freezing points. It may be best for the teacher to provide students with experimental data for the boiling point of deionized water (the values for both seawater and deionized water tend to be lower than the ideal value of 100 °C). Sample graphs can be found in the answer key available for download.
• If using digital temperature probes that allow continuous readings, data can be downloaded directly and used to make the graphs.
• While multiple samples can be run in the same ice bath, the number of hot plates available will limit how many concurrent boiling points can be measured.
• Temperatures below -5 °C can be reached using a sodium chloride-ice bath (33 g of NaCl/100 g ice). During data acquisition, it is recommended that you keep the bath in an insulated ice bucket (as shown below) to help maintain a constant temperature.

• The boiling points and freezing points can be estimated by averaging a series of data points that are within ±0.5 °C. Alternatively, a spreadsheet program can be used to estimate the y-intercept of a straight line as shown in this explanation of how to obtain a freezing or boiling point in Google Sheets.
• Beakers can be used to measure boiling points and TDS, but I have found using small metal cans (such as those from the American Scientific Calorimeter Set Item 3235-00 american-scientific.com/product/calorimeter-set-2-2/, see picture below) works well for constant heat transfer.

• The amounts of inorganic salts dissolved in water can be measured in multiple ways: in Part 2 of this lesson, students measure conductivity (µS/cm) and/or salinity (parts per thousand, ppt), and Part 3 has students measure TDS (in parts per million, ppm). As an extension, students could convert each of their measurements to the same unit (ppm) to understand that that while the values from different measurements may differ, they are the same order of magnitude (sample data in the table below).
  • It is reasonable to assume that differences between TDS and salinity measurements are due to the presence of MgSO₄ and NaHCO₃. The Vernier Salinity Sensor claims to measure the concentration of all non-carbonate salts in solution. It appears that it primarily measures the concentration of the chloride salts in solution.
  • To convert salinity from ppt to ppm, just multiply by a factor of 1000. The conductivity can be used to estimate the total dissolved salt concentration of a water sample using the following equation: TDS (in mg/L or ppm) = 0.67 x electrical conductivity (in µS/cm). When multiple salts are present in solution, differences in molar ionic conductivities between ions complicate a direct correlation and may provide salinity values that are lower than the TDS they measure by boiling off the water. This resource from Lake Superior Streams provides more information about the relationship between conductivity and TDS, as well as other information about water quality and pollution.

• Evaporation of brine and seawater is one way to harvest salt. A supplemental reading that may interest students is Traditional Ways of Knowing: Salt Harvesting.
• Differentiation: For advanced students who are comfortable with calculations, students can determine percent recovery of salts. Using the volume of seawater and the molarity values calculated in the pre-lab, students determine how much of each salt, both individually and in total, should be present, and compare this value to the value of solids isolated. Students could calculate the total theoretical ionic molality of the seawater sample and determine the percent error of their molality values (see van’t Hoff factors).

Part 4: Buffering Ability of Seawater

• Students tend to be surprised to discover that the pH of deionized water isn’t 7 (and that it may differ between groups and trials). This variation is due to the absorbance of carbon dioxide from the air. “Fresh” DI water tends to have a pH of about 6.5 to 6.8, but if it's sitting in an open beaker, it rapidly absorbs carbon dioxide, and the pH can drop to about 5.5. In addition, the low conductivity of deionized water impacts the electrode response of pH meters.
• The effect of carbon dioxide absorption on the pH of seawater provides an illustration of buffer capacity, as the pH remains constant until the buffer is exhausted and pH declines. This can be readily seen if the pH is monitored continuously while blowing into the water, rather than simply measuring initial and final pH values. (The buffer region is evident in the right-hand graph below, between 0 and ~5 seconds.) If you have pH meters that will record data continuously (ex: a reading per second) and can export it, you could have students use those and see it for themselves, or you could show them the graphs below:

Final Reflection

• Students are asked to write a short essay (3-5 paragraphs) summarizing what they learned about water throughout the course of this lesson. As this should be an individual reflection, it could be assigned for homework and students could discuss big-picture takeaways in class if time permits. Students essays will likely address a wide range of topics – some sample excerpts written by students are included in the answer key, but all students will respond differently and there is no single “correct answer” to this prompt!
• The prompt is intentionally very broad to allow for students to write about whatever parts of the lesson were most impactful for them. This lack of an explicit question to answer may be intimidating or confusing to some students. If they need more guidance, encourage them to think about some or all of the following questions:
  • What did you learn that surprised you?
  • What can you do to be more environmentally friendly in your water use?
  • In what ways does seawater differ from DI water? Why do these differences matter?
  • How did what you learned influence how you think about your relationship with water?

References

This lesson was derived from several related experiments and has been refined over several years’ practice with Honors Chemistry students. Some useful references are summarized below:

“Desalination.” U.S. Geological Survey, 11 Sept. 2019, https://www.usgs.gov/special-topics/water-science-school/science/desalination.

Postma, James M., and Anne Roberts. Chemistry in the Laboratory. 8th ed., W.H. Freeman, Macmillan Learning, 2017.

“The Chemistry of Seawater” Module developed by Joseph Baron, Marvin Blevins, and Barbara Sawrey, ChemSource Sourcebook, Version 3.0, 2010, Principal Investigator M. V. Orna.

Selco, Jodye I., et al. “The Analysis of Seawater: A Laboratory-Centered Learning Project in General Chemistry.” Journal of Chemical Education, vol. 80, no. 1, 2003, p. 54, https://doi.org/10.1021/ed080p54.

“What Are the Dissolved Solids in Seawater?” The Royal Society of Chemistry and the Nuffield Foundation, 17 Aug. 2015, https://edu.rsc.org/experiments/what-are-the-dissolved-solids-in-seawater/1785.article.

“Electrical Conductivity (EC25) and TDS.” Lake Superior Streams, accessed 22 June 2022, https://www.lakesuperiorstreams.org/understanding/param_ec.html.

Hale, George. “Acidic Seas. How Carbon Dioxide Is Changing the Oceans.” ChemMatters, 2018, pp. 10–12.