Visualizing Dissolution, Ion Mobility, and Precipitation through a Low

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Visualizing Dissolution, Ion Mobility, and Precipitation through a Low-Cost, Rapid-Reaction Activity Introducing Microscale Precipitation Chemistry Bob Worley,† Eric M. Villa,‡ Jess M. Gunn,‡ and Bruce Mattson*,‡ †

CLEAPSS, Brunel University Science Park, Uxbridge UB8 3PQ, United Kingdom Department of Chemistry, Creighton University, Omaha, Nebraska 68178, United States

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S Supporting Information *

ABSTRACT: We describe here an activity that enables students to visualize the dissolution process, ion mobility, and precipitation on a microscale level, all in 30 s. A small puddle of water consisting of approximately 10 drops is placed on a plastic sheet cover. Two small crystals of water-soluble salts are transferred by toothpicks and simultaneously introduced to the puddle from opposite sides. Within 30 s, a precipitate forms where the two ionic solutions meet and the result usually looks somewhat like a cat’s eye marble. A significant feature of this activity is that it uses a minimum amount of resources.

KEYWORDS: High School/Introductory Chemistry, First-Year Undergraduate/General, Inorganic Chemistry, Laboratory Instruction, Hands-On Learning/Manipulatives, Microscale Lab, Green Chemistry, Precipitation/Solubility, Reactions

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migrate to form a precipitate, after which they perform the experiment to see how their predictions at the microscopic level and the results at the macroscopic level compare. In typical precipitation activities, students are provided with prepared and often colorless solutions, which they mix and observe as precipitation occurs instantaneously. We have expanded this already very visual demonstration of precipitation to allow students to introduce the solid salts as single crystals, transferred by toothpicks to opposite sides of a small puddle of water on a plastic sheet cover. By this process, students see the crystals dissolve and then the appearance of a precipitate shortly thereafter, illustrating the concepts of dissolution, ion mobility, and precipitation. These precipitation reactions take less than a minute in a puddle with a diameter of approximately 1 cm yet are slow enough for students to make observations regarding the above concepts. Even multiple precipitation reactions can be executed within a limited work period. The globular shape of the puddles is made possible by the hydrophobic nature of the polypropylene sheet cover on which the puddles are placed. By printing the procedure on the paper or cardstock inserted between the sheets, the student has immediate access to the procedure. Furthermore, the use of single crystals rather than solutions reduces the preparation and provides easy storage for the instructor, as well as limiting the waste produced.

recipitation reactions, historically called double-displacement or even double-decomposition reactions (terms mostly phased out at the university level),1 have a long history in the classroom and teaching laboratory.2−5 In Alex Johnstone’s 1981 textbook, he described precipitation as occurring when “two ions which make up an insoluble compound come together in solution and settle out as a solid”.6 Initially, precipitation reactions in the teaching laboratory were conducted in test tubes with a variety of ions, including many that are no longer used because of their toxicity. When well plates became available, precipitation reactions left test tubes and moved to the grid format and small scale that the well-plates offered. Reduced volumes of solution were used so that disposal would be less frequent, more cost-effective, and more environmentally friendly. More recently, plastic-covered sheets, including laminated paper and plastic sheet protectors, have allowed students to perform similar cation- and anion-precipitation analyses on the drop-sized microscale level.7 However, even in countries that score well in the Program for International Student Assessment (PISA)8 tests, students fail to grasp what is really happening at the submicroscopic level.9 It is often difficult for students to visualize how a solid dissolves into an aqueous solution and then separates into the component ions. To assist with this, students start by watching a short YouTube video showing how ions in a solid become solubilized and begin to migrate (discussed below in the Experimental Overview section).10 Through a series of three sketches, students extend what they saw in the video to predict how the aqueous ions © XXXX American Chemical Society and Division of Chemical Education, Inc.

Received: July 21, 2018 Revised: December 14, 2018

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DOI: 10.1021/acs.jchemed.8b00563 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Activity

Figure 1. Key features of the experiment. Top left: placing 10 drops of deionized water within a circle. Top right: eight solids with their toothpicks. Bottom left: adding two crystals to opposite sides of the puddle. Bottom right: formation of precipitate, usually within 30 s.

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HAZARDS There are no specific or unusual hazards associated with this student activity. If a laboratory activity involving students is designed from this procedure, prudent laboratory practices should be followed, including wearing safety glasses and washing hands with soap and water after the experiment. If metal salts such as nickel(II) or cobalt(II) are used, one should be aware of the health warnings associated with these metal ions. Provide protective equipment such as gloves if necessary.

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amount of a substance that can be used to make chemical observations. Disposal is often as simple as wiping the plastic sheet cover with a tissue and placing it in the trash. (Please consult your local rules and regulations as locales may have more stringent policies, rules, or laws regarding disposal of the chemicals used.) We have used the following combinations of salts with students, each of which forms a precipitate except for the final combination, which can be used to show that not every combination forms a precipitate: • • • • • • • •

DISCUSSION

Experimental Overview

This experiment entails three key steps: 1. A small puddle of deionized water (approximately 10 drops) is situated within a circle under a plastic sheet cover, as shown in Figure 1. The Supporting Information includes a template for the circles embedded in the instructions for students. 2. Using toothpicks with moistened tips, two salts (one crystal each or a toothpick tip of powder) are transported to opposite edges of the puddle. The two crystals are simultaneously touched to their respective edges of the puddle, and the toothpicks are immediately removed. The crystals are instantly assimilated into the puddle. Usually, it is possible to see the crystals in the puddle. 3. Watch carefully! Within 30 s a precipitate forms along a curtain-shaped line midway between the points where the crystals entered. The result often looks somewhat like a cat’s eye toy marble. We have produced two YouTube videos of the process;11,12 each is about 1 min long. Figure 1 shows these key steps, including the final precipitate.

NaCl(s) with AgNO3(s) AgNO3(s) with KI(s) AgNO3(s) with NaHCO3(s) FeSO4·7H2O(s) with NaHCO3(s) FeSO4·7H2O(s) with Ca(OH)2(s) CuSO4·5H2O(s) with NaHCO3(s) CuSO4·5H2O(s) with Na2HPO4(s) CuSO4·5H2O(s) with NaCl(s)

These precipitation combinations give consistent and reliable results and have been used several times in our generalchemistry laboratory. Both the HCO3− and HPO42− ions are used so that the pH is neutral or slightly acidic, to minimize the potential formation of a metal−hydroxide precipitate. Students are instructed that the solids follow the solubility guidelines for carbonate and phosphate, respectively, and that the products are carbonates and phosphates. Ca(OH)2(s) is a sparingly soluble powder that produces a distinctively colored precipitate with most transition-metal ions, including FeSO4·7H2O(s). As it is slow to dissolve, we instruct students to give it a 30 s head start. We have used pulverized NaOH(s) with good results. The salt dissolves quickly, resulting in a precipitation line that is frequently off-center and is sometimes murky in appearance. Crushed pellets of NaOH(s) are deliquescent in moist air, but we have stored the powder for months in an ordinary vial, as pictured in Figure 1. For use with laboratory students, we prefer Ca(OH)2, which provides clear results and is easy for students to clean up.

Reactions

Green chemistry and microscale chemistry share much in common with respect to precipitation chemistry. By selecting salts of low chemical toxicity that create little or no chemical waste, we meet the criteria expected of being green.13,14 By working with two single crystals, we are near the minimum B

DOI: 10.1021/acs.jchemed.8b00563 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Activity

After making observations on these key solution processes, students are then asked to write equations for the dissolution of salts into aqueous ions and to write balanced equations for precipitation reactions (both overall and net ionic). Balancing precipitation reactions with HCO3− and HPO42− is more challenging than with CO32− and PO43−. For students just starting their chemistry studies, an instructor may find it appropriate for students to balance the net ionic reactions with CO32− and PO43− while ignoring the pH of the solution, which is what we have adopted for General Chemistry I students. For more advanced students, balancing with HCO3− and HPO42− is not only appropriate but can lead to fruitful discussions about the speciation of a solution at various pH values. Equations for both of these approaches are presented in the Supporting Information. Students are assigned to watch one or more YouTube videos of ionic solids dissolving; a particularly useful example can be found in ref 10. As part of their preparation, students outline three sketches that demonstrate their understanding of lattice dissolution, ion migration, and precipitation. 1. The first sketch captures the early stage in the process, showing ions dissociating into solution from the lattice before they interact with ions from the other salt. This sketch is easily prepared after watching the video. The sketch should show a two-dimensional lattice as well as dissociated ions. Ions can be represented with circles of two different sizes, circles of two different colors, or formulas. Aqueous ions should be restricted to their respective halves of the puddle in this sketch. 2. The second sketch features formation of the precipitate along a vertical line similar to what students observed in the experiment. Spectator ions are mostly restricted to their respective halves of the puddle, although a few have migrated past the precipitation zone. 3. The third sketch shows the situation after ion migration is complete. In this sketch, we look for a distribution of spectator ions throughout the puddle. In all sketches, water molecules are omitted for clarity and because students are not yet familiar with ion hydrolysis.

Other salt combinations are certainly possible, but they may require special waste handling, depending on local regulations. We have had excellent results with salts of nickel(II), chromium(III), and cobalt(II), which are predominantly in the form of easy-to-manage crystals: NiCl2·6H2O(s), Mg(NO3)2·6H2O(s), CrCl3·6H2O(s), and CoCl2·6H2O(s). Finally, a pairing such as CuSO4·5H2O(s) and NaCl(s) can be used to illustrate that not every combination will produce a precipitate. This can provide a very useful disruption to the pattern of precipitations being observed for students to discuss the processes encountered and the solubility rules. Experiment

The eight solids are provided in small vials (Figure 1), each containing approximately 100 mg of salt, which can be utilized by several groups. Bottles and caps are numbered and toothpicks are used until the end of the experiment and then discarded. If a toothpick is not immediately returned to its respective vial it is replaced with a new one. After the experiment is over, the vial caps are replaced and all eight vials are stored in the 9 oz (250 mL) plastic cup. The sets of cups and vials together with the plastic sheet covers, instructions, and boxes of toothpicks take up very little space and can be stored in a single drawer. Setting up the activity takes only minutes. This highly visual precipitation activity can be conducted as a classroom demonstration with the assistance of a document camera; conducted as a small-group, in-class activity; or developed into a laboratory experiment for students. For our purposes, we use the full set of reactions presented above as a complete laboratory experiment early in the first semester of general chemistry, when classes of chemical reactions are being introduced. We have found that this activity quickly and easily allows student to visualize complicated solution processes. After the students have deposited the crystals into the puddle (Figure 2, left), they watch the crystals slowly dissolve. In the case of the

Student Responses to the Activity

Students were asked before performing this activity to rate their level of comfort with skills such as writing chemical reactions (both overall and net ionic) and visualizing the steps of a precipitation reaction. The 126 students were again polled after the experiment was completed and showed consistently higher scores, illustrating the worth of the experience. A summary of the questions and average responses is provided in the Supporting Information. Throughout the series of experiments, students properly balanced equations, understood why the precipitation occurred, addressed the question of what happened to the waters of hydration, and explored and hypothesized why the CuSO4·5H2O(s) + NaCl(s) reaction did not show a precipitate. Most completed the experiment, decided on the products, and wrote all of the reactions with minimal assistance from the instructor or TA.

Figure 2. Left: Crystals of CuSO4·5H2O(s) and NaHCO3 on the left and right sides of the puddle. As the crystals slowly dissolve, students will see the relative amounts slowly decrease. Here, it is essential for the students to be reminded about how ions are able to migrate through solution. Right: Precipitate between the two starting points. This precipitation continues to grow as the reaction continues. Note that the light blue color from the Cu2+ ions is not observed on the other side of the precipitation interface until all of the HCO3− is consumed.

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CONCLUSION The precipitation activity presented here is rich in solution chemistry and provides ample opportunities for observations regarding the solution process, ion migration, and the formation of precipitates. Students and instructors alike will appreciate the implicit elegance of the activity. Perhaps the most surprising

colored solution of Cu2+, they can monitor the progress of ion migration. Soon after the dissolution process begins, the appearance of a faint precipitate is observed at the interface between the two opposing ion migration fronts (Figure 2, right). Students then make a sketch in their laboratory notebook of the precipitation observed in the puddle. C

DOI: 10.1021/acs.jchemed.8b00563 J. Chem. Educ. XXXX, XXX, XXX−XXX

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(7) Sattsangi, P. D. Microscale Procedure for Inorganic Qualitative Analysis with Emphasis on Writing Equations: Chemical Fingerprinting Applied to the n-Bottle Problem of Matching Samples with Their Formulas. J. Chem. Educ. 2014, 91, 1393. (8) Science Performance (PISA), 2018. OECD Data. https://data. oecd.org/pisa/science-performance-pisa.htm (accessed Dec 2018). (9) Tan, K. C. D.; Goh, N. K.; Chia, L. S.; Treagust, D. F. Multiple Representations in Chemical Education; Gilbert, J. K., Treagust, G., Eds.; Springer: Dordrecht, 2009. (10) Manton, B. Dissolving NaCl, 2011. YouTube. https://youtu.be/ 9aYLonML69w (accessed Dec 2018). (11) Mattson, B. Puddle Precipitation, 2015. YouTube. https://youtu. be/f2FA1p5KHCE (accessed Dec 2018). (12) Mattson, B. Microscale Puddle Precipitation, 2018. YouTube. https://youtu.be/Nsfwr9-rWno (accessed Dec 2018). (13) Anastas, P. T.; Warner, J. C. Green Chemistry: Theory and Practice; Oxford University Press: New York, 1998. (14) 12 Design Principles of Green Chemistry. American Chemical Society. https://www.acs.org/content/acs/en/greenchemistry/ principles/12-principles-of-green-chemistry.html (accessed Dec 2018). Specifically, the first 4 of the 12 principles are relevant to the activity described in this paper: prevent waste, atom economy, less hazardous synthesis, and design benign chemicals. (15) Instructors have benefitted from experience with this activity through CLEAPSS, a nonprofit-making advisory service based at Brunel University Science Park, Kingston Lane, Uxbridge UB8 3PQ, U.K., that provides support in science and technology for a consortium of local authorities and their schools, including establishments for pupils with special needs in England, Wales, and Northern Ireland. Independent schools, post-16 colleges, teacher-training establishments, curriculum developers, and others can apply for associate membership. It has close connections with the U.K. Government Department for Education and the U.K. Health and Safety Executive. The CLEAPSS homepage is available at http://www.cleapss.org.uk/ (accessed Dec 2018).

feature of the process, even to well-seasoned chemistry instructors, is how quickly ions dissolve in water and begin to migrate. The precipitate forms in the shape of a ribbon somewhere between and perpendicular to the spots where the crystals were introduced. Students learn as much about dissolution, solvation, and ion migration as about precipitation. Instructors, too, can learn these concepts more deeply through experience with the activity.15 Another interesting phenomenon is that the precipitating ions do not migrate past the precipitation line; this can be seen by the color of the metal ion exclusively remaining on one side of the precipitate line. For example, when crystals of CrCl3(s) and NaHCO3(s) are used, the intensely green color of Cr3+(aq) migrates toward the middle but does not cross the precipitation line, where the concentration of bicarbonate prevents further migration. This sharp demarcation holds for 15 min or more. The reactions here offer an accessible way to quickly and visually illustrate difficult-to-understand topics of dissolution, ion migration, and precipitation. This simple set of microscale precipitations yield visually appealing results illustrating the intricacies of these seemingly simple aqueous precipitation reactions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.8b00563.

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Microscale-precipitation-chemistry student instructions (including a template with the circles for the seven precipitation reactions), discussion of lab results related to the solubility rules (the expected work product from the students for this experiment), balanced equations, inquiry-based enrichment activities, and assessment of effectiveness (PDF, DOCX)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Eric M. Villa: 0000-0001-9883-1993 Bruce Mattson: 0000-0002-7625-8513 Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Clarke, J. B.; Hastie, J. W.; Kihlborg, L. H. E.; Metselaar, R.; Thackeray, M. M. Definitions of Terms Relating to Phase Transitions of the Solid State (IUPAC Recommendations 1994). Pure Appl. Chem. 1994, 66, 577−594. (2) Solomon, S.; Fulep-Poszmik, A.; Lee, A. Qualitative Analysis of Eleven Household Compounds. J. Chem. Educ. 1991, 68, 328. (3) Oliver-Hoyo, M. Problem Analysis: Lesson Scripts and Their Potential Applications. J. Chem. Educ. 2001, 78, 1425. (4) Furlong, W. R.; Quackenbush, M. R.; Indralingam, R. What Is That Colorless Solution? A Qualitative Analysis Laboratory for General Chemistry. J. Chem. Educ. 2009, 86, 953. (5) Treagust, D. F.; Tan, K. C. D.; Goh, N. K.; Chia, L. S. Major Sources of Difficulty in Students’ Understanding of Basic Inorganic Qualitative Analysis. J. Chem. Educ. 2004, 81, 725. (6) Johnstone, A. H.; Morrison, T. I.; Reid, N. Chemistry about Us; Heinemann Educational Books: London, 1981. D

DOI: 10.1021/acs.jchemed.8b00563 J. Chem. Educ. XXXX, XXX, XXX−XXX