Introduction to Precipitation and Solubility Within a Computer-Enriched Module for Analytical Chemistry

Introduction

In recent years, there have been tremendous developments, both in the quantity and the quality, of computer applications for teaching chemistry. The use of Web-based materials by students in chemical education has been increasing rapidly [1]. For example, Treadway claimed that the performance levels of chemistry students were improved when they took a multimedia laboratory class [2]. In organic chemistry education, a modular approach has been used, together with self-testing, in a system termed the Web-based Enhanced Learning and Resource Network (i.e., the “WE_LEARN” system) [3]. The use of WebCT has led to an increase in learning for the students within first-year chemistry courses in chemistry [4]. Brooks actualized a software system related to Web-based classes that aims to evaluate homework and examinations automatically [5]. Robinson investigated the possible effects of using computer simulations on scientific discovery learning (6). The use of the LabVIEW Software Package for physical chemistry and analytical chemistry applications enabled students to better interface laboratory measuring devices (specifically, the potentiometer, the cyclic voltmeter, and the spectrophotometer) to computers [7–9]. In fact, Drew suggested that the workload of educators could decrease if the above-mentioned applications were included in the chemistry curriculum [10]. LUCID, which was developed for the Introduction to Chemistry and General Chemistry classes for two semesters, is a program that promotes cooperative learning [11]. All of the above-mentioned applications have been postulated to offer more advantages than the traditional teaching methods.

Educators are beginning to embrace the use of interactive multimedia packages in their lessons. The Internet accessibility of these multimedia packages makes them accessible to a wider community, as opposed to the limited and constant teaching tools available only in textbooks. Among the other advantages of these packages, university-level students could access these materials easily in libraries or their homes from anywhere in the world and learn scientific changes and developments simultaneously [12]. In a study by Carpi [13], a Web site was built that consisted of explanations of scientific concepts, explanatory animations and related links for science classes in a high school. Most of the modules included interactive activities in which the students could learn by doing. Each module contained connections to the other links. For example, in the module about the periodic table, knowledge could be acquired on the history of atomic particles through a link online from the Lawrence Berkeley National Laboratory [14]. In Carpi’s study, the Web site of the lesson and the details of its pedagogical design were explained, followed by a comparison of the examination results before and after the site was constructed. Daniel and Saat [15] in Malaysia developed a Web-based modular approach where the resources were acquired through the Internet. An examination consisting of three parts was administered to the students and they were asked to search through the Web site on the current subject. Windelspecht [16] designed a technology-based lesson consisting of modules in his study on the integration of technology into education. In a cooperative study by chemistry teachers from various high schools and universities [17], student and teacher handbooks were prepared within a project where direction had been identified as a weak point of the computer-assisted studies. Godrick and Hartman [18] created a joint laboratory class for the departments of biology and chemistry at Boston University. The subjects of the laboratory were distributed into modules called Resources Of Energy For Life, Indication Of Ascorbic Acid, Reaction Dynamics, Indication of Macromolecules and Manipulation. Ekpo used a modular system for developing and designing a chemistry laboratory safety course for secondary education [19]. Another implementation was in Gibault High School with the preparation of computer-assisted modules on subjects such as the mole concept, atomic theory, chemical links, chemical equilibrium, acid–base chemistry, and stoichiometry [20]. Banerjee and Power [21] designed modules for teachers and students at the secondary level on chemical equilibria and the application of these modules was found to facilitate the understanding of the subject. In another study concerning the effects of modular applications on the attitudes of the high school chemistry students towards chemistry, an attitude scale was administered before and after the administration of the modules and the changes in the attitudes of the students were observed. The students were observed to develop positive attitudes at the end of the sessions [22]. Bader created computer-assisted learning modules for the Introduction to Chemistry course at the undergraduate level [23, 24]. The student handbook prepared for the Solution Concentration unit involved objectives, conditions, a pretest, discussion, and 20 sets of problems while the teacher handbook consisted of application information, answers to the questions, a list of software and hardware, and 20 unit tests. In the computer-assisted learning modules developed by Manock [25; 26] for the subject of heterogeneous ionic balance within the Introduction to Chemistry course at the undergraduate level, there were teacher guidelines and student materials. Settle [27] had also chosen the subject of acid–base equilibrium for the computer-assisted learning modules that he prepared for the Introduction to Chemistry course at the undergraduate level. In the computer-assisted learning module prepared by Jameson [28] on chemical equilibrium calculations, there were 20 unit tests related to equilibrium concentration calculations. In addition, the computer-assisted learning modules prepared by Jameson for the Introduction to Chemistry course at the undergraduate level included teacher guidebooks and student materials [29]. The student guidebook on the unit related to Le Chatelier’s principle consisted of objectives, conditions, and some information for running the computer program and problems and the teacher guidebook involved application information, software, and samples of running the program using BASIC language. Apart from these, the Web-based applications of general chemistry multimedia problems within the two-term course of general chemistry, which uses video and still images from JCE Software’s “Chemistry Comes Alive (CCA)” collection, were proven to affect student performance in chemistry education by a study conducted at the University of Indiana.

All of these computer-based methods contrast strongly with traditional teaching methods. In traditional teaching methodology, a lecturer presents a lesson through an oral presentation. This method is specific with “interpretative,” “informative,” and “illustrative” characteristics. The advantages of this method include the fact that it is useful for the presentation of the beginning of a lesson and for transferring knowledge to large groups. To date, a single lecturer has been easier and more economical, accounting for the usage of traditional methods from the very beginning of one’s chemistry education and then throughout one’s entire chemistry career. Teachers, however, need to know the characteristics of the method clearly to obtain maximum effectiveness. Long and boring presentations can cause incomplete communication where the students enter a passive learning mode in which they do not ask questions [30].

Despite the extensive list of computer techniques/technologies that have been employed in the teaching of chemistry and were listed above, the fundamental issue of the quality and quantity of student learning remains. Within the concept of knowledge management and how knowledge is acquired, Dubinskas differentiates between knowledge building and access to knowledge [31]. Within this theoretical framework, knowledge building is equated with teaching and learning. Questions of how well did they learn or how fast did they learn are appropriate. Because electronic technologies are playing an ever more central role in the teaching/learning experience, a comparison of one type of Web-based technique to the standard traditional teaching methods to determine if the knowledge building process (i.e., the way that students learn to think and navigate through a building knowledge base) is enhanced with electronic technology.

We report here our results concerning our efforts to test the level of knowledge gained by a group of students using Web-based modules as compared to the level of knowledge gained using traditional teaching methods. The level of knowledge gained was measured using standard pre- and posttest methodology, as described below. In this study, the web-based modules were found to be a more effective teaching device than were traditional teaching methods.

Experimental Methodology

Participants and Division into Groups. This study involved 84 third-year university students (54 females and 30 males), who were attending the Internet Class of Chemistry Education at Hacettepe University, Faculty of Education, Department of Chemistry Education and who were attending courses in chemistry education and attending Chemistry Education Seminar. All of the students had previously taken and passed the course “Introduction to General Chemistry.” These students were randomly divided into an experiment group and a control group of 42 students each. Overall, the students were allowed to choose their own group without knowing in advance which group would be the control group and which group would be the experimental group. The directors of the project ensured that the groups were relatively equal in length of time within the university, previous courses taken, and scholastic aptitude.

Pre- and Posttesting. The data to be evaluated were gathered by applying a pre- and a posttest concerning the material to be learned in these modules. We have termed this test a Chemistry Achievement Test (CAT). The test consisted of ten open-ended questions, which are listed in Appendix 1. These questions were based on concepts in the subject area of precipitation and solubility, which is the same area as the Web-based modules that were used in this study. The Chemistry Achievement Test was prepared from the question bank belonging to the Department of Chemistry Education. The question bank consists of questions related to precipitation and solubility that had been honed and refined over the past 20 years and were presented many times to different student groups. The inner validity of the Chemistry Achievement Test was achieved by consulting with outside experts in the field of knowledge, with a reliability of 0.89. Both the experimental group and the control group took the CAT as a pretest prior to any instruction on the subject matter. The Chemistry Achievement Test was administered as a post-test one week

Table 1. Content of the Web-Based Modules Teaching Precipitation and Dissolution

Concepts taught	Demonstration experiment
1. Solubility constant	Precipitation of Alkaline Earth Salts (Figures 1 and 2)
2. Precipitate formation	Precipitation of Phosphates as a Function of pH value (Figure 3)
3. Precipitating reactant	Precipitation and Dissolution of Sulfides of the H₂S Group (Figure 4)
4. Dissolution	Precipitation and Dissolution of Silver Salts (Figure 5)
5. Solvent reactant	Precipitation and Dissolution of Pb(II) Salts (Figure 6)

M²⁺ + CO₃^2- ® MCO₃

M²⁺ + C₂O₄^2- ® MC₂O₄

M²⁺ + SO₄^2- ® MSO₄

M²⁺ + CrO₄^2- ® MCrO₄

M²⁺ = Ca²⁺, Sr²⁺, Ba²⁺

Figure 1. Precipitation of alkaline earth salts.

Figure 2.Solubility constants of alkaline earth salts.

after each group began their study methodology. The students were notified one day in advance that the CAT post-test exam was to be given. Both groups were able to revisit the materials that they had used for study. The experimental group was able to revisit the Web-based modules. The control group was allowed to revisit their textbooks and class notes. A rubric, in the form of an answer key, was used to evaluate the answers to the questions, in order to provide consistency of grading among the two data sets.

Subject Matter That Was Taught

Whether using the conventional methods or the Web-based modules, the same concepts were presented to the students. These concepts were taught in a total of 6 hours of instruction, with three sessions each of two hours of instruction. The same length of time of instruction was utilized for both groups. A rough outline of the concepts, and how these concepts were explained is shown in Table 1.

The first topic taught was the solubility product constant. Included within this topic was the subconcept of the multiplication of solubility in which slightly soluble salts are in an equilibrium reaction with their ions in solution because the precipitation and dissolution speeds of the salts are becoming equal to each other. This equilibrium is called the solubility equilibrium. As examples of this concept,reactions and information related to the precipitation and the dissolution of alkaline earth salts, particularly as the carbonate, oxalate, sulfate, and chromate salts, were presented to students.Precipitation was defined for both the experimental group and the control group as the separation of a product in the solid state from a solution that has reached its saturated concentration as a result of a reaction of a substance with a reactant. Precipitate formation is demonstrated by the observation of reactions and color changes that occur upon the addition of calcium-, barium-, silver-, magnesium- and aluminum-molybdate-containing solutions into phosphate-containing solutions. The importance of pH in these changes has also been explained. A related subconcept that is introduced at this time is the precipitating reagent. The precipitating reagent is one that converts a completely soluble ion into a slightly soluble compound after its addition to a sample solution. Thus, reactions, and their consequent precipitates, formed by the addition of the precipitating reagent H₂S to solutions containing the ions of the elements from the H₂S group have been given as examples. Silver halides allow the demonstration of the precipitation of an ion that can be followed by its own dissolution. The additions of the appropriate halides to an Ag⁺-containing solution cause the precipitation of the silver halide. The precipitated silver halide can be dissolved again by the addition of concentrated ammonia, dilute ammonia, cyanide, or thiosulfate. Finally, the concept that a reagent can cause the dissolution of a precipitate is further exemplified by the dissolution of Pb(II) salts, which can be dissolved by the addition of H₂O, I^–, (NH₄)₂C₄H₄O₆, NaOH or diluted HNO₃.

Traditional Methods Instructional Methodology. The control group was taught about the subject of precipitation and solubility by traditional methods of instruction. To the extent possible, all methodology for teaching was the same, except for the introduction of the concepts. The content of the subject matter is exactly the same as is taught by the Web-based modules; however, the chemical equations were discussed using the medium of the blackboard and chalk. The only educational materials were the textbook, blackboard, chalk, and an eraser. Identical homework problems were assigned to the two groups (i.e., the traditional methods instruction group and the Web-based modules group). Discussion sections were held to help the students learn the materials.

Na₂HPO₄+	Ca²⁺	®	CaHPO₄	(white) ¯
	Ba²⁺	®	BaHPO₄	(white) ¯
	Mg²⁺ / NH₃	®	MgNH₄PO₄	(white) ¯
	Ag⁺	®	Ag₃PO₄	(yellow) ¯
	(NH₄)₂MoO₄	®	(NH₄)₃[P(Mo₁₂O₄₀)]	(yellow) ¯

Figure 3. Precipitation of phosphates as a function of the pH value.

Sb₂S₃+ (NH₄)₂S_x	→	2[SbS₄]^3-

PbS + HNO₃ (concentrated)	→	Pb²⁺ + HS^-+ NO₃^-

HgS + Na₂S	→	2Na⁺ + [HgS₂]²

Figure 4. Precipitation and solubility of sulfides from the H₂S group.

Ag⁺ + I^- ® AgI (yellow) ¯

Ag⁺ + Br^- ® AgBr (light yellow) ¯

Ag⁺ + Cl^- ® AgCl (white) ¯

Ag⁺ + 2NH₃ ® [Ag(NH₃)₂]⁺

Ag⁺ + 2S₂O₃^2- ® [Ag(S₂O₃)₂]^3-

Ag⁺ + 2CN^- ® [Ag(CN)₂]^-

Figure 5. Precipitation and dissolution of silver salts.

Pb²⁺ + 2Cl^- ® PbCl₂(s) (white) ¯

Pb²⁺ + 2I^-® PbI₂(s) (yellow) ¯

Pb²⁺ + SO₄^2- ® PbSO₄(s) (white) ¯

Pb²⁺ + CrO₄^2- ® PbCrO₄(s) (yellow) ¯

Pb²⁺ + S^2- ® PbS(s) (black) ¯

PbI₂	I^- ®	[PbI₃]^-	I^- ®	[PbI₄]^2-
PbSO₄		(NH₄)₂C₄H₄O₆ ®		[Pb(C4H₃O₆)₂]^4-
PbCrO₄		OH^- ®		CrO₄.PbO(s) chrome red
		OH^- ® (excess)		[Pb(OH)₃]^- + CrO₄^2-
PbS		HNO₃ ® (concentrated)		Pb²⁺
PbCl₂		®		dissolves in hot water

Figure 6. Precipitation and dissolution of Pb(II) salts.

Table 2. Statistical Evaluation of the Results Obtained by tThe Different Study Methodologies

		PreTest	PostTest
	N^a	x^b	X^c	S^d	T^e	P^f
Experimental Group	42	15.8	67.2	12.693	-0.982		0.000
Control Group	42	16.6	58.8	12.693	-0.982		0.000

^aNumber of students. ^baverage. ^cStandard deviation. ^dt-Test coefficient. ^eSignificance.

Web-Based Modules Instructional Methodology. Web-based modules were used as the primary teaching mechanism. These modules have been published on the Internet as a portion of Creative Chemistry on the Internet [32] and were prepared by the Chemistry Contact Network [33], which is a project of Professor Nesper of the Department of Inorganic Chemistry of the ETH Zurich. Experiments about the subjects of precipitation and solubility within the CCI on the ETH Web site can be watched by using Real Player. During the display of experiments, instructions are shown on the screen. In addition to this, students could watch detailed information about the experiments and the reactions at the time of viewing the module. For each of the experiments, students of the experimental group were provided with color pictures, graphics, diagrams, tables, animations and short educational films in order to enable them to see and learn about the chemicals that were in the module, the results of the experiments, obligatory safety measures, and color changes. We emphasize that all of these materials, especially, the color pictures, animations and short films, were presented via Internet methodologies that the students could use and reuse at their own leisure. Significant data related to each experiment and reactions were presented to the students in subtitles. While working on the modules that involve experiments such as the precipitation of alkaline earth salts, the pH-dependent precipitation of phosphates, the precipitation and dissolution of sulfides from the H₂S group, and the precipitation and dissolution of silver (Ag⁺) and Pb(II) salts, students were given the opportunity to view images related to experiments through step-by-step film slides on a full screen. Examples of these materials are given in Appendix 2. By presenting important theoretical information and the chemical reactions just below the images, the correlation between the practical and the theoretical aspects of the subjects is easily illustrated. In order to minimize defective and false learning of students during the use of computer-assisted learning modules, students were allowed the opportunity to repeat experiments. More details of these modules are given in Appendix 2.

Data analysis

The results of these post-test evaluations are shown in Table 2. When analyzing this data, it is immediately obvious that the group that used Internet-assisted learning modules showed a significantly different average on the posttest compared to the control group, which was taught using the traditional methodology.

Discussion And Conclusions

The effects of Web-based learning modules on the achievement and learning levels of students were examined in this study on the subjects of precipitation and solubility. After the students were distributed into control and experiment groups, the students in the control group were taught the subject of precipitation and solubility together with basic concepts and important features using the traditional methods of lecture and blackboard with an emphasis on related reactions and solvent and precipitate reactivity. No student-centered teaching methodologies or technologies were used within this group. In contrast, students in the experiment group were provided learning modules that were prepared on the general subjects of precipitation and solubility as they are generally taught within the area of general chemistry. These modules consisted of the basic concepts and Internet links to demonstration experiments in order to facilitate the understanding of the subject matter. The students were provided with photos relating to the subject matter, short movies, tables, graphics and animations of experiments, essential concepts, color changes, and reactions. The possible misconceptions and misunderstandings were overcome through providing replays when necessary. The modules, with their Internet links, enabled not only the students to learn at their own pace, but also allowed for the educational processes to be extended beyond the physical boundaries of the school environment. A significant difference in the performance on the achievement test, favoring the experiment group, was observed when the posttest results were evaluated. The students with high levels of comprehension and memorization skills in both control and treatment groups were successful in the pretest. The traditional method used in the teaching of the control group was less effective in the achievement levels when compared to that of the treatment group. Similar studies have also shown that the web-based materials acquired via the Internet were effective in increasing the achievement levels in chemistry education [34, 35].

In summary, the application of computer-assisted learning modules in chemistry education was found to have a positive effect on the students’ achievement and learning levels.

References and Notes

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Appendıx 1: The Chemıstry Achıevement Test (CAT)

The Chemistry Achievement Test consists of the following open-ended questions.

1. What is the meaning of K_sp?

2. What are the factors affecting the solubility of precipitates?

3. While calculating the solubility of CaCO₃ in water, which reactions do you use (Take the hydrolysis of CaCO₃into consideration)?

4. The solubility product for aluminum hydroxide precipitation is extremely low. If fluoride ions were added to an aqueous solution of aluminum hydroxide, how would the solubility of aluminum hydroxide be affected?

5. In a solution that is saturated with H₂S, how do you show the relationship betweensulfide ion concentration and hydroxonium ion concentration?

6. The total ionization constant of H₂S is shown below:

a. [H₃O⁺]²[S^2–] = K₁K₂

i. [H₂S]

7. While a system is reaching equilibrium, it changes

i. Towards the state with the least energy, and

ii. Towards the state with the greatest randomness.

8. Explain the dissolution of NaCl in water, describing how both of these factors are achieved in the dissolution process.

9. What can you say about a solid substance that is dissolved in water and that finally comes to an equilibrium state?

10. What are the precipitating reagents used for the alkaline earth metals (Take the hydrolysis of alkaline earth metals into consideration)?

11. What has to be done to cause the dissolution of HgS salt, which is exceedingly hard to dissolve?

12. 10. Comment on the solubility of Ag₂S(s) → 2Ag⁺ + S^2–?

13. K_sp(Ag₂S) = 8 ´ 10^–51

Appendıx 2. Overvıew Of The Web-Based Materıals

The following is a brief overview of the materials used during the Web-based portion of this study.

Solubility Constant: Precipitation of Alkaline Earth Salts

The first concept to be taught in the subject area of “Precipitation and Solubility” is the solubility constant (see Figure 1). The precipitation of alkaline earth salts was used as the case in point. Experiments, in which Ca²⁺_,Sr²⁺, and Ba²⁺ were each individually treated with carbonate, oxalate, sulfate, and chromate solutions, respectively, were presented as a slideshow on the Web to the students. Of specific importance is that the anion and the metal ion concentrations are each approximately 10^–3 mol/L before the additions. This indicates that the formation of the precipitate occurs at a concentration of about 10^–6 mol²/L² and that the solubility of precipitated salts is low. These data may be compared to the known K_sp data, which is shown in Figure 2. As shown in both the pictures of the experiments and in Figure 2, solutions of calcium chromate, calcium sulfate, and strontium sulfate are soluble under these conditions, corresponding to their relatively large K_sp values (i.e., pK_L < 6, K_L > 10^–6 mol²/L²). With an extremely careful eye, one can see turbidity in the solution of strontium chromate, which has been attributed to a subtle difference between precipitation and dissolution speeds. Further, ions of calcium, strontium and barium have weakly bound hydrate structures in dilute solution. These hydrate ions do not produce any reaction as acids practically, but they do produce slightly soluble salts having a high ionic charge.