In the Classroom edited by
JCE DigiDemos: Tested Demonstrations
Ed Vitz Kutztown University Kutztown, PA 19530
Hard Water and Soft Soap: Dependence of Soap Performance on Water Hardness A Classroom Demonstration submitted by:
Viktoria K. L. Osorio, Wanda de Oliveira, and Omar A. El Seoud* Instituto de Química, Universidade de São Paulo, C.P. 26077, 05513-970 São Paulo, Brazil; *
[email protected] checked by:
Wyatt Cotton Science Division, Cincinnati State College, Cincinnati, OH 45223 Jerry Easdon Department of Chemistry, College of the Ozarks, Point Lookout, MO 65726
On several occasions, we have been asked why soap does not foam in seawater. Additionally, experienced travelers know that Brazilian soap bars do not work well in some parts of Europe and the United States. Soaps from foreign countries, however, usually form more lather than the local products. Water hardness and soap formulation are responsible for these differences in performance. Most Brazilian waters are soft (1), so that local soaps, unlike their foreign counterparts, are not formulated to work in relatively hard water. We have used questions regarding the dependence of soap performance on water hardness as a part of the examination for selecting high school students that will represent the State of São Paulo in the national Chemistry Olympiad. This selection process took four hours, equally divided between a demonstrations part (three demos) and a written exam. In the latter, the students were asked to explain the results of the experiments performed in terms of chemical equations or principles. Before the demonstrations, a sheet was handed out for recording observations, and the following were explained: i. The objective is to find out why soap performance depends on the water employed. ii. Commercial soap bars are mostly mixtures of sodium salts of fatty acids (2, 3). iii. Formation of rich, consistent foam is qualitatively as-
sociated with satisfactory soap performance (quantitatively, soap performance is measured by standardized detergency tests) (4). iv. Natural water usually contains salts, typically of sodium, calcium, and magnesium (5). The difference between soft and hard water is that the latter contains high concentrations of Ca2+ and Mg2+ ions (5, 6).
Experimental A list of the demonstrations and the corresponding observations is given in Table 1, and some results are shown in Figures 1 and 2. Stoppered, 50-mL graduated cylinders were used in experiments 1–3 and 6, whereas experiments 4 and 5 required a magnetic stirrer. Aqueous solutions of commercial soap bars are cloudy at room temperature, which makes it hard to see precipitate formation (Table 1). Therefore, our “soap” solution consisted of 0.04 M of sodium decanoate, prepared by neutralizing technical-grade decanoic acid (Clariant S.A., São Paulo) with NaOH. The other aqueous solutions required are: sodium ethylenediaminetetraacetate, EDTA (0.03 M), sodium carbonate (0.02 M), sodium chloride (0.02 M), and a mixture of calcium and magnesium chloride (0.01 M in each salt). The experiments consisted in mixing equal volumes, 15 or 20 mL, of each solution.
Table 1. Performance of Soap in Different Aqueous Solutions # Experiment
Observation
1 Shake soap solution with tap water. (If local water is hard, use distilled water.)
Rich foam formation.a
2 Shake soap solution with NaCl solution.
Same result as experiment 1.
3 Shake soap solution with CaCl2/MgCl2 solution.
No foam, precipitation of waxy, insoluble material.b
4 While stirring, slowly add Na2CO3 solution to the CaCl2/MgCl2 solution.
Immediate formation of a turbid suspension.c
5 While stirring, slowly add EDTA solution to the result of experiment 4.
Disappearance of turbidity.d
6 Add EDTA solution to the result of experiment 3 and shake vigorously.
Dissolution of the precipitate, formation of foam.
a
See Figure 1A.
b
See Figure 1B.
c
See Figure 2A.
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d
See Figure 2B.
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Hazards
Results and Discussion
All substances employed are safe. The solutions can be disposed of in the sink.
The demonstrations were performed for a group of 107 students. They were simultaneously projected on a large video screen and carried out in a guided style because of time limitation. Alternatively, high school students can easily carry out the experiments in a two-hour laboratory period. After performing experiments 1–3, it became clear that soap works in the presence of sodium ions, whereas its performance was adversely affected by calcium and magnesium ions. At this point, the following explanations was offered: the detergent industry solves this problem by including water “softeners” in their solid and liquid formulations (7). The objective of the next series of demonstrations was to show an example of these softeners. EDTA (structure given in ref 8 ) was chosen as an softener and sodium carbonate was employed to model formation of precipitates in soaps. Experiments 4–6 of Table 1 were then carried out. A second sheet was distributed to the students that contained the following directives and a note:
A
B Figure 1. Dependence of soap performance (as qualitatively indicated by foam formation) on water hardness: (A) Tap water (B) Water containing CaCl2 and MgCl 2 (0.01 M in each salt)
1. Write a balanced equation for the reaction of CaCl2兾MgCl2 with Na2CO3. 2. Explain the action of EDTA on the product(s) formed in the previous reaction. 3. Why does soap perform differently in the presence of NaCl or CaCl2兾MgCl2? 4. Why does soap not foam in seawater? A
5. Explain the action of EDTA on soap performance.
B
6. Explain the function of zeolites in detergent formulations. [Note: Some detergent powders contain sodium aluminosilicates as additives (builders). An important example is the class of compounds called zeolites, whose general formula is xNa2O⭈Al2O3⭈ySiO2⭈zH2O (see structure in ref 9). These are ion-exchangers; that is, the sodium ions present in their lattices are labile (10).]
Figure 2. Demonstrating the softening effect of EDTA. Part (A) shows the result of addition of sodium carbonate to a mixture of calcium- and magnesium chloride (precipitation of calcium and magnesium carbonates). Part (B) shows the dissolution of the precipitate formed on addition of EDTA.
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Most students, 92%, correctly wrote the reaction of CaCl2兾MgCl2 with Na2CO3, whereas only 35% concluded that EDTA acted on the CaCO3 or MgCO3 that had precipitated. Twenty percent of the students attributed this “action” to the formation of soluble complexes of EDTA with Ca2+ and Mg2+ ions. This modest percentage reflects the fact that coordination chemistry is an optional part of the Brazilian high school chemistry curriculum. Thirty-four percent attributed the difference of soap performance in NaCl solution and in seawater to the presence of Ca2+ or Mg2+ ions in the latter (questions 3 and 4). The softening effect of EDTA (experiment 6) was explained on the same base as experiment 5. That is, the additive forms soluble complexes with Ca2+ and Mg2+ ions of the precipitated fatty acid salts, transforming them into the corresponding (soluble) sodium salt. The softening action of zeolites was correctly explained by 32% of the students; namely, it is due to the removal of the Ca2+ and Mg2+ present via ion-exchange with (builder-based) sodium ions. The choice of Na2CO3 as a model for soap proved to be good. Most students were able to deduce the parallelism; that is, if calcium or magnesium carbonates are water insoluble,
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the corresponding salts of fatty acids might behave likewise. This leads to soap precipitation and inhibition of foam, for example, in seawater, a typical example of hard water. Conclusions The demonstrations have the following attractive features: (i) use safe, inexpensive reagents and simple glassware and equipment, (ii) introduce important, everyday topics, for example, types of water and soaps, detergents, and additives employed therein (11–14), (iii) underline the importance of topics (complex formation and ion-exchange) that are optional in the Brazilian high school chemistry curriculum, and (iv) stimulate the students to consider the wider consequences of water hardness; namely, drinking water should be made softer, for example, by treatment with ion-exchange resins, and water softeners should be included in detergent formulations for efficient detergency (4, 15–18). Literature Cited 1. ANEEL—Monitoramento da qualidade das águas no Brasil (Monitoring the quality of Brazilian waters), http:// www.mma.gov.br/port/srh/acervo/publica/doc/oestado/texto/175184.html (accessed Oct 2004). 2. Hill, G. C.; Holman, J. S. Chemistry in Context, 4th ed.; Nelson: London, 1995; pp 553, 562–563. 3. Seager, S. L.; Slabaugh, M. R. Chemistry for Today: General, Organic, and Biochemistry, 2nd ed.; West: New York, 1994; p 511. 4. (a) Spangler, W. G. In Detergency: Theory and Test Methods, Part I; Cutler, W. G., Davis, R. C., Eds.; Marcel Dekker, New York, 1972; pp 414–449. (b) Sosis, P. In Detergency: Theory
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and Test Methods, Part II; Cutler, W. G., Davis, R. C., Eds.; Marcel Dekker: New York, 1975; pp 625–658. 5. Hard Water—To Soften or Not to Soften. http:// www.ca.uky.edu/agc/pubs/ip/ip7/ip7.htm (accessed Oct 2004). 6. Hill, J. W.; Petrucci, R. H. General Chemistry; Prentice Hall: New Jersey, 1996; p 299. 7. Snyder, C. H. The Extraordinary Chemistry of Ordinary Things, 3rd ed.; J. Wiley: New York, 1998; p 337. 8. Yappert, M. C.; DuPre, D. B. J. Chem. Educ. 1997, 74, 1422– 1423. 9. Coker, E. N.; Davis, P. J.; Kerkstra, A.; van Bekkurn, H. J. Chem. Educ. 1999, 76, 1417–1419. 10. Kotz, J. C.; Treichel, P., Jr. Chemistry and Chemical Reactivity, 4th ed.; Saunders: New York, 1999; pp 1026–1027. 11. Hill, J. W.; Kolb, D. K. Chemistry for Changing Times, 7th ed.; Prentice Hall: New Jersey, 1995; pp 560–570. 12. Selinger, B. Chemistry in the Marketplace, 5th ed.; Harcourt Brace: London, 1998; pp 43–51. 13. Freemantle, M. Chemistry in Action, 2nd ed.; Macmillan: London, 1995; pp 253–255, 668–670. 14. Snyder, C. H. The Extraordinary Chemistry of Ordinary Things, 3rd ed.; J. Wiley: New York, 1998; pp 323–339. 15. Donovan, T. R.; Poole M. C.; Yack, D. J. Chemicals in Action, 2nd ed.; Holt, Rinehart and Winston: Toronto, 1995; pp 350– 351. 16. Stoker, H. S. Chemistry, A Science for Today; Macmillan: London, 1989; pp 384, 669–671. 17. Botkin, D. B.; Keller, E. A. Environmental Science: Earth as a Living Planet, 3rd ed.; J. Wiley: New York, 2000; pp 420– 422. 18. Shakhashiri, B. Z. Laboratory Manual; General, Organic, and Biological Chemistry; Structures of Life; Benjamin/Cummings: Redwood City, CA, 2002; Vol. 3, Section 9.38, pp 345–347.
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