Using Silica Gel Cat Litter To Readily Demonstrate the Formation of

Demonstration pubs.acs.org/jchemeduc

Using Silica Gel Cat Litter To Readily Demonstrate the Formation of Colorful Chemical Gardens Masatada Matsuoka* Komaba-toho High School, 4-5-1 Ikejiri, Setagaya-ku, Tokyo 154-0001, Japan S Supporting Information *

ABSTRACT: A chemical garden is a phenomenon by which crystals of metal salts grow in an aqueous solution of sodium silicate (also known as waterglass), visually resembling the forms of a beautiful garden. Demonstrations of this experiment are well-known, and different tube colors and shapes can form depending on the type of metal salt used. In the method described herein, a porous material soaked in a metal salt solution was used in place of the crystals. The ideal material was the easily available silica gel cat litter. The reaction was completed in ∼20 min, and since the silicate tubes are difficult to break because they are strongly bound to the cat litter, the formation of chemical gardens could be observed while holding the test tube in one’s hand. The use of cat litter reduces the preparation time for instructors and the amounts of reagents used compared to using powders and crystals of metal salts. KEYWORDS: General Public, High School/Introductory Chemistry, Demonstrations, Hands-On Learning/Manipulatives, Crystals/Crystallography

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INTRODUCTION

Sodium silicate is an indispensable material in our lives and is used in a variety of areas such as a soap additive, the bleaching of pulp, and a fireproof coating for walls. Moreover, in an era when common households did not have a refrigerator, it was possible to store eggs for several months in this aqueous solution to maintain their freshness. A chemical garden is a phenomenon by which capillary fibers are formed from crystals of metal salts placed in an aqueous solution of sodium silicate (also known as waterglass), visually resembling the forms of a beautiful garden (Figure 1). When crystals of a metal salt are placed in an aqueous solution of sodium silicate, the metal cations, under basic conditions, form a thin colloidal semipermeable membrane consisting of metal hydroxides, metal oxides, and metallosilicates. Since the osmotic pressure of the membrane is higher in the inside than on the outside, water enters the membrane, causing the membrane to rupture. Then the inside solution flows upward because of buoyancy, and the precipitation reaction continues once again, resulting in an upward growth of the insoluble tubes.1 This phenomenon was first reported in 1646 by Johann Glauber, who described the formation of what he called “philosophical trees” in a chemistry textbook entitled Furni Novi Philosophici (New Philosophical Furnaces).2 Research concerning chemical gardens has greatly progressed over the last decades, and it is expected to contribute to the future development of materials science, especially to the nanotechnology field. Chemical garden experiments can lead to the © XXXX American Chemical Society and Division of Chemical Education, Inc.

Figure 1. Chemical garden formed from a crystal powder.

formation of tubes with a variety of different colors and shapes depending on the type of metal salt used. When a chemical garden experiment is carried out, the use of chemical powders results in the formation of bulky crystals at the bottom of the beaker, which tend to consume more reagents (Figure 2). In order to create a chemical garden, a few Received: September 14, 2016 Revised: December 18, 2016

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DOI: 10.1021/acs.jchemed.6b00707 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Demonstration

millimeter-sized particles of the metal salt are chosen and dropped.2

Then the metal salt dissolves inside it, forms acidic compartments through hydrolysis, and promotes the precipitation of silicon dioxide (eq 2): Na 2O−xSiO2 (aq) + 2H+(aq) → xSiO2 (s) + H 2O + 2Na +(aq)

(2)

The aqueous solution of waterglass is basic; therefore, it contains a large number of hydroxide ions. The membrane can allow permeation of water molecules and hydroxide ions, thereby resulting in precipitation of metal hydroxide inside the tube and promoting the polymerization (eqs 3 and 4): (3)

M(OH)2 (s) → HO−[M−O−]x MOH(s) + x H 2O

(4)

This increases the internal pressure and causes rupture of the membrane. When the membrane ruptures, the tube continues to grow by the same process, progressing until the crystals at the bottom are exhausted. As a result, the grown tubes have a two-layer structure, in which a silica-rich layer is formed on the outside and a metal oxide-rich layer is formed in the inside.4 Balköse et al.5 have assumed the reaction mechanisms and equations to be as described above. When the created tubes are removed from the solution, they are fragile and easily prone to oxidation in air. There have been several studies of their structure and composition. For example, Cartwright et al.6 studied the formation, morphology, and composition of chemical gardens made from the chlorides of Ca(II), Mn(II), Co(II), and Ni(II). In case of Ni(II), for instance, the results of an energy-dispersive X-ray analysis indicated that the outer surface of the tube had a higher silicon content, and the composition of the compound formed on the inner surface was identified as a nickel hydrosilicate such as Ni3Si2O5(OH)4. Collins et al.7 analyzed the composition of a Cu(II) chemical garden and confirmed that the outside of the tube was formed from amorphous silica, whereas the inside was coated with a precipitate having a composition of Cu2(OH)3· NO3. In recent years, in order to enhance the reproducibility, studies have been carried out to determine how the crystals of metal salts are to be added to the sodium silicate solution. There have also been reports of experiments in which the crystals were not used directly but instead were ground, compressed, formed into pellets, and then used8 as well as experiments in which an aqueous metal salt solution was absorbed in agarose microbeads, which were used instead of the crystals.9 Moreover, on the basis of the new idea of injecting an aqueous metal solution into an aqueous sodium silicate solution at a constant flow rate rather than adding crystals into the aqueous sodium silicate solution, a study of the tube’s growth rate and composition has been reported. Steinbock and coworkers discovered that the injection of a copper(II) sulfate aqueous solution into an aqueous sodium silicate solution has growth that is different from that of the chemical garden grown from crystals; after analysis of its structure, it was concluded that the tube was not a simple bilayer structure but rather a complex structure.10 By the use of this injection method, it is possible to synthesize tubes from not only silicate but also precipitates that are nearly insoluble in water and metal hydroxides, for example.11,12

Figure 2. View of the beaker in Figure 1 from below.

Therefore, we wished to explore the use of a porous material soaked in a solution as an alternative to crystals or pellets. We investigated cat litter, a well-known commercial product. The materials used for the production of cat litter are mainly clay, paper pellets, and silica gel, and literature reports on the distinct activity of each component exist.3 In this experiment, easily obtainable silica gel cat litter (Figure 3) proved to be the ideal material.

Figure 3. Examples of silica gel cat litter: spherical (left) and flakes (right).

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BACKGROUND The chemical compositions of the tubes created in chemical garden experiments are complex. If the crystals of metal salts are added to an aqueous solution of sodium silicate (waterglass), a thin semipermeable membrane of metal silicate hydrate first covers the crystals (eq 1): Na 2O−xSiO2 (aq) + M2 +(aq) → MO−xSiO2 (aq) + 2Na +(aq)

M2 +(aq) + 2OH−(aq) → M(OH)2 (s)

(1) B



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Demonstration

MATERIALS The waterglass solution used in this experiment was prepared by diluting a commercially available sodium silicate solution (density 1.60−1.70 g/mL) consisting of 35−38% SiO2 and 17− 19% Na2O by weight. Sodium silicate solutions can be purchased not only from chemical companies but also from online retailers. For instance, 250 mL of the desired waterglass solution was obtained by placing ∼50 g of the sodium silicate solution in a 300 mL beaker. Approximately 200 mL of water was then added, and the mixture was stirred vigorously using a glass rod. The molar concentration of the waterglass solution was about 1.5 M with respect to silica. In the case of spherical silica gel litter, the mass was distributed with a range of 0.050−0.15 g and thus could be used as such. On the other hand, the mass distribution of the flake silica gel litter was 0.030−0.10 g, and in order to obtain an attractive chemical garden, 0.050 g or more of the silica gel was needed in order to minimize the risk of crushing when picking it up with tweezers. Next, 50 mL saturated chloride solutions of iron(III), cobalt(III), manganese(II), and copper(II) were prepared for use as metal salt solutions. Twenty pieces of cat litter were placed in each of these saturated solutions and left to stand for 1 h. Afterward, the cat litter was placed on a filter paper with the aid of tweezers and left to stand for 1 h to ensure complete absorption of the solution. Figure 4 shows a photograph of the

Figure 5. Chemical garden utilizing cat litter (20 min).

toxicity; therefore, the use of this substance in middle schools is not ideal. In the event of skin contact, it is advised to wash the area thoroughly with soap and water.13 Recent material safety data sheets (MSDSs) should be reviewed. To minimize the time the students are exposed to the reagents, it is recommended that the instructors prepare cat litter soaked in metal salt solutions and dispense the waterglass solution into test tubes before performing the demonstration. After the demonstration, the waterglass solution, cat litter, and chemical garden should be disposed in an inorganic waste container.

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RESULTS AND DISCUSSION This experiment resulted in the growth of chemical gardens with different colors and shapes, as summarized in Table 1. Table 1. Chemical Garden Colors and Tube Shapes Metal Cation 3+

Fe Co2+ Mn2+ Cu2+

Color

Tube Shape

Brown Purple White Blue

Thick tubes break and bend while growing Widespread growth of thin tubes Widespread growth of thin tubes Widespread growth of thick tubes

Dynamic growth of chemical gardens can be viewed in the video (with a 300× playback rate) in the Supporting Information. In addition, Figure 6 shows the relationship between the reaction time and the length of the chemical garden.8,12 Figure 7 shows microscope photographs of the chemical gardens. These precipitation structures were equivalent to the ones obtained when tubes were grown from metal

Figure 4. Cat litter after absorption of the different metal salt solutions.

cat litter after absorption of the different metal salt solutions. We were able to repeatedly use the saturated aqueous solutions and keep the cat litter immersed in the aqueous solution for a sufficiently long period of time.

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PROCEDURE First, 15 mL of the waterglass solution was added to each of the four test tubes (18 mm in diameter). A single piece of cat litter that had been immersed in a chloride solution of iron(III), cobalt(II), manganese(II), or copper(II) was then placed in each test tube. After ∼5 min, the silicate tubes began to grow from the silicate metal salts. As shown in Figure 5, after ∼20 min, they reached the water surface.

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HAZARDS Students and instructors should wear lab coats, safety goggles, and suitable gloves when handling the waterglass solution and metal chloride. Sodium silicate (waterglass) and metal chlorides may cause irritation of the eyes and skin. Cobalt chloride is suspected to cause adverse health effects and environmental

Figure 6. Relationship between the reaction time and the length of the chemical garden. C



Demonstration

experiment, we used a 20% waterglass solution (1.5 M with respect to silica) and examined differences in the growth for different dilutions of the solution. Figure 9 shows the results

Figure 9. Differences in the shapes of chemical gardens due to different concentrations of waterglass solution. Figure 7. Microscope photographs of chemical gardens: Fe3+ (1:50), Co2+ (1:150), Mn2+ (1:150), and Cu2+ (1:150).

obtained using cat litter containing copper(II) after standing in the waterglass solution for 30 min. When the concentration of the waterglass solution was low, a sufficient number of silicate crystals was produced to cover the cat litter, but tube growth did not take place. The composition and density of commercial sodium silicate solutions differ according to the manufacturer. The rate of tube growth may be improved by increasing the concentration of the waterglass solution. As reported in the literature, a waterglass solution density of 1.05−1.10 g/mL and a silica molar concentration of 0.6−1.6 M in aqueous solution may be used as standard conditions to achieve chemical garden growth.4 In this experiment, a waterglass solution with a concentration of 20% (1.5 M with respect to silica) was used, which is indicated as the standard concentration.

salt crystals (e.g., ref 2 in the case of iron(III), ref 5 in the case of cobalt(II), ref 6 in the case of manganese(II), and ref 8 in the case of copper(II)). The silica gel litter used in this experiment is a porous material. The metal salt solution could flow between the gaps on the surface of the silica gel (Figure 8); therefore, the grown

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CONCLUSION Recrystallization of metal salts and the metal displacement reaction (silver tree) also form products with beautiful patterns; therefore, these experiments are popular among students of middle and high schools. As the reaction mechanism in the chemical garden experiment is complex, the aim is not for students to understand the mechanism per se but rather to promote curiosity about science in students through visible tube growth using this experiment. Middle school students enjoy observing the tube growth while becoming familiar with laboratory etiquette and basic experimental techniques. Furthermore, high school students may learn the sort of development content that may lead to the pursuit of scientific principles such as the differences in the solubility of various salts, the meaning of semipermeable membranes, and osmotic pressure. The time needed for the demonstration of this experiment is about 20 min. In addition to reducing the preparation time for instructors, the use of cat litter in place of metal salt crystals is superior in that it reduces the volume of reagent consumed.

Figure 8. Microscope photograph (1:500) of cat litter (left side of Figure 3).

tubes were thinner and more vivid compared with the chemical gardens grown from metal salt crystals. Furthermore, the tube roots were strongly fixed and solid, so that when the test tubes were held by hand they would not break, even when tilted. Generally, tubes grown in chemical gardens are affected by the concentration of the waterglass solution. Therefore, in this D



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Demonstration

(12) Stone, D. A.; Lewellyn, B.; Baygents, J. C.; Goldstein, R. E. Precipitative Growth Templated by a Fluid Jet. Langmuir 2005, 21 (24), 10916−10919. (13) Young, J. A. Sodium Silicate (aqueous solution). J. Chem. Educ. 2009, 86 (8), 918.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.6b00707. Movie of the demonstration shown in Figure 5, in which 30 min changes have been compressed to 6 s through time-lapse photography (MPG)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Masatada Matsuoka: 0000-0002-9555-5062 Notes

The author declares no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Japan Science and Technology Agency (JST) under the Program for Promotion of PreUniversity Research Activities in Science. The author thanks Dr. Masayuki Inoue in the Graduate School of Mathematics and Science Education at The Tokyo University of Science for his generous advice and acknowledges the contribution of Daiki Matsuno and Yusuke Miyata in determining the appropriate reaction conditions.

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REFERENCES

(1) Barge, L. M.; Cardoso, S. S. S.; Cartwright, J. H. E.; Cooper, G. J. T.; Cronin, L.; De Wit, A.; Doloboff, I. J.; Escribano, B.; Goldstein, R. E.; Haudin, F.; Jones, D. E. H.; Mackay, A. L.; Maselko, J.; Pagano, J. J.; Pantaleone, J.; Russell, M. J.; Sainz-Díaz, C. I.; Steinbock, O.; Stone, D. A.; Tanimoto, Y.; Thomas, N. L. From Chemical Gardens to Chemobrionics. Chem. Rev. 2015, 115, 8652−8703. (2) Steinbock, O.; Cartwright, J. H. E.; Barge, L. M. The fertile physics of chemical gardens. Phys. Today 2016, 69 (3), 44−51. (3) Celestino, T.; Marchetti, T. The Chemistry of Cat Litter: Activities for High School Students To Evaluate a Commercial Product’s Properties and Claims Using the Tools of Chemistry. J. Chem. Educ. 2015, 92 (8), 1359−1363. (4) Cartwright, J. H. E.; García-Ruiz, J. M.; Novella, M. L.; Otálora, F. Formation of chemical gardens. J. Colloid Interface Sci. 2002, 256 (8), 351−359. (5) Balköse, D.; Ö zkan, F.; Köktürk, U.; Ulutan, S.; Ü lkü, S.; Nişli, G. Characterization of Hollow Chemical Garden Fibers from Metal Salts and Water Glass. J. Sol-Gel Sci. Technol. 2002, 23 (3), 253−263. (6) Cartwright, J. H. E.; Escribano, B.; Sainz-Díaz, C. I. ChemicalGarden Formation, Morphology, and Composition. I. Effect of the Nature of the Cations. Langmuir 2011, 27 (87), 3286−3293. (7) Collins, C.; Zhou, W.; Klinowski, J. A unique structure of Cu2(OH)3·NO3 crystals in the s̀ ilica garden’ and their degradation under electron beam irradiation. Chem. Phys. Lett. 1999, 306 (3), 145− 148. (8) Kaminker, V.; Maselko, J.; Pantaleone, J. The dynamics of open precipitation tubes. J. Chem. Phys. 2014, 140 (24), 244901. (9) Makki, R.; Al-Humiari, M.; Dutta, S.; Steinbock, O. Hollow Microtubes and Shells from Reactant-Loaded Polymer Beads. Angew. Chem., Int. Ed. 2009, 48 (46), 8752−8756. (10) Makki, R.; Roszol, L.; Pagano, J. J.; Steinbock, O. Tubular precipitation structures: materials synthesis under non-equilibrium conditions. Philos. Trans. R. Soc., A 2012, 370 (1969), 2848−2865. (11) Batista, B. C.; Steinbock, O. Chemical gardens without silica: the formation of pure metal hydroxide tubes. Chem. Commun. 2015, 51 (65), 12962−12965. E


Using Silica Gel Cat Litter To Readily Demonstrate the Formation of

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