Investigating the influence of temperature on salt solubility in water: a STEM approach with pre-university chemistry students

1 Introduction

2 Methodology

2.2 Description of the approach

In this digital and technological era, programming is considered a fundamental skill. As stated by Koyuncu and Koyuncu (2019), “coding (programming) is one of the 21st-century skills and has great importance for future generations” (p. 70). Inspired by the positive outcomes of the experimental setup developed by Morais and Araújo (2023), we designed the didactic activity entitled Potassium nitrate bioavailability: Effect of solubility in prevention and treatment of tooth sensitivity. This activity was framed within a STEM approach and implemented with pre-university Chemistry students.

The proposed activity (described in the Supplementary Material) aimed to promote learning of chemistry concepts related to the solubility of potassium nitrate in water and its temperature dependence. Additionally, it aimed to enhance students’ proficiency in technological skills such as programming Arduino microcontrollers and utilizing Microsoft Excel software for data acquisition and analysis. As presented at the beginning of the activity exploration sheet (see Supplementary Material), the activity commenced with contextualization concerning the concept of bioavailability and its significance in the pharmaceutical industry. Bioavailability refers to the speed and extent of substance absorption by the body and its effectiveness at the desired site of action, with solubility being a crucial variable influencing it (Marques, 2015). The contextualization concluded with the specific case of potassium nitrate used in the treatment of tooth sensitivity, highlighting the need for a study to understand its solubility in water. Building upon this context, the activity consisted of 3 main stages. Stage 1 involved preparing for the activity through research on the operation of electrical and electronic equipment used in the experimental setup. This included familiarizing themselves with the Arduino Uno R3 microcontroller, breadboard, I2C LCD, LDR sensor, DS18B20 temperature sensor, a red laser suitable for Arduino, and resistors. Students also gained basic knowledge of Arduino programming and explored automatic data acquisition using the Data Streamer add-in in Microsoft Excel. This initial stage, assigned as autonomous work outside of class time, aimed to equip students with prerequisite skills, particularly in programming, to enable them to work with greater autonomy in the subsequent stages of the activity.

The first moment of active engagement with students (first 100-minute lesson) corresponded to Stage 2 of the activity. This initial lesson (and the following one) was conducted by the researchers involved. The activities were designed to promote students’ work autonomy. Therefore, researchers and chemistry teachers acted solely as guides for the student groups throughout the proposed tasks. In the chemistry laboratory, student groups had the opportunity for a guided exploration of assembling electrical circuits and controlling electrical and electronic components through Arduino programming. Initially, students were provided with step-by-step instructions to control simple electrical/electronic components, such as turning LEDs on and off, simulating a traffic light, and controlling LED combinations based on light intensity detected by an LDR sensor, simulating street lighting control in a city. Through this guided task, groups were able to apply basic Arduino programming commands, allowing them to independently solve small problems, such as controlling LEDs and/or buzzers based on temperature or light intensity values detected by the respective sensors. They also used an LCD display to visualize data. Through the provided materials, students were able to carry out the tasks of this second stage with a high level of autonomy. The students were also instructed on how to program the Arduino to perform measurements with an LDR sensor and a DS18B20 temperature sensor, in order to understand the functioning mechanism of the experimental setup. However, minor issues related to code typos or circuit assembly frequently arose. In these cases, researchers and chemistry teachers played a more active role in assisting students in identifying and overcoming these situations. These tasks and small projects were designed to enable students to understand the underlying programming/assembly of the experimental setup, which would be utilized in the proposed activity in the following lesson (Stage 3). Within the STEM approach of the activity, these initial stages aimed to foster students’ skills in the areas of Technology (T) and Engineering (E).

Stage 3 of the activity primarily focused on building and mobilizing knowledge in the areas of Science (S), such as Chemistry (and Physics), and Mathematics (M). In the second 100-minute lesson. For this stage, the students assembled the validated experimental setup by Morais and Araújo (2023) to explore the temperature dependence of potassium nitrate solubility (outlined in Figure 1). However, due to time constraints, and because programming all the components used was a bit more demanding, the complete code to be used was previously provided to them. Students worked autonomously in groups following the exploration guide of the activity “Potassium nitrate bioavailability: Effect of solubility in prevention and treatment of tooth sensitivity”.

Figure 1:

Schematic of the experimental setup used in the presented activity. Adapted with permission from Morais, C. and Araújo, J. L. (2023). An Alternative Experimental Procedure to Determine the Solubility of Potassium Nitrate in Water with Automatic Data Acquisition Using Arduino for Secondary School: Development and Validation with Pre-Service Chemistry Teachers. Journal of Chemical Education, 100(2), 774–781. Copyright 2024 American Chemical Society.

As suggested by Morais and Araújo (2023), after assembling the experimental apparatus and setting up automatic data acquisition,

masses of at least five samples of potassium nitrate, for example, about 13, 17, 21, 25, and 29 g, are measured into a 50 mL beaker. Subsequently, 20.0 g of distilled water is also measured and transferred to the beaker with the potassium nitrate sample. After the mixtures are prepared, each one must be heated with gentle stirring, which can be done by a magnetic stirrer (for this, a stir bar must be added to the mixture) (p. 777).

Once all the salt had dissolved, the solutions were cooled in a magnetic stirrer under gentle and continuous agitation, and the temperature at which the first potassium nitrate crystals precipitated was recorded. The experimental setup enabled the determination of the solubility of this salt in water by measuring the transmittance of the mixture and its temperature. In this activity, transmittance represents the percentage of laser beam luminosity incident on the LDR sensor. Temperature was measured using a temperature sensor placed in the mixture. As Morais and Araújo (2023) explained, “the beginning of the precipitation of potassium nitrate crystals corresponds to a decrease in transmittance, due to the dispersion of light through the mixture (Tyndall effect)” (p. 776). By utilizing the capabilities of Arduino and Microsoft Excel, automatic data acquisition of transmittance and temperature was carried out. This allowed students to simultaneously analyse and graphically process the collected data. Based on the results from multiple trials, students plotted the solubility curve of potassium nitrate in water, allowing them to formulate a hypothesis about its solubility dependence on temperature and address the set of questions provided in the exploration guide.

3 Results and discussion

In this section, we begin by presenting the results obtained by the students in Stage 3 of this activity. These results, related to the learning of chemistry content regarding the solubility of potassium nitrate in water and how the solubility of this salt varies with temperature, were qualitatively evaluated through the analysis of the questions posed to the students throughout the activity (although a scale from A to F was used to provide feedback on the students’ performance in the activity, in this section, we chose to conduct a more descriptive analysis of the students’ responses for a more fruitful discussion of these results). Based on these results, it was possible to infer the success of the implemented activity in the learning of the underlying chemistry content. The results and indicators from the implementation of Stages 1 and 2 of the project were obtained through informal notes collected by observing moments of intervention and/or conversations with the students during these moments and will be presented next. This section concludes with the presentation of the results from the overall assessment of the activity collected through the analysis of the students’ responses to the evaluation questionnaire.

Regarding Stage 3, each group of students performed only one experimental trial with each selected mass of potassium nitrate due to time constraints. The implementation of the activity was guided by an exploration worksheet (see Supplementary Material). With the experimental data obtained and shared within the class, the work groups represented the solubility curve of this salt, which was analysed and explored. As an example, Table 1 and Figure 2 illustrate the experimental results obtained.

Figure 2:

Solubility curve of potassium nitrate in water as a function of temperature.

In general, the experimental results obtained by the various groups of students are consistent with the values of potassium nitrate solubility in water described in the literature (Lide, 2006). These data support the conclusions of Morais and Araújo (2023) and validate the reliability of the experimental apparatus developed for use with secondary school students. Based on the responses to question 5 “Explain the dependence between the solubility of this salt and temperature” from the exploration worksheet, the activity, in general, allowed the students to conclude that the solubility of potassium nitrate increases considerably as the temperature of the solution also increases.

As an example, some responses from student groups to this question are shown:

With the experimental activity, we can observe that the solubility of the salt increases with increasing temperature. Thus, the dissolution reaction of potassium nitrate is favoured by the temperature increase, making it an endothermic reaction in the forward direction (Group A).

As the temperature increases, the solubility of potassium nitrate also increases. This means that the reaction is endothermic in the forward direction, i.e., favoured by the temperature increase (Group B).

Although all groups concluded that the solubility of potassium nitrate increases with increasing temperature, some student groups presented small mistakes in the analysis of the experimental results, particularly regarding the explanation of the mathematical relationship between the two variables under study: “the solubility of the salt and the temperature are directly proportional” (Group C). For example, some groups performed a linear fit of the experimental points, others used a second-degree polynomial function, and others used an exponential function (question 4). Although the differences in these fits were not significant for the range of data presented, they influenced the responses to question 6, Estimate the solubility of potassium nitrate at a body temperature of 36 °C and determine the maximum amount of potassium nitrate that can be dissolved in 150 mL of water at that temperature.

To determine whether the students also understood how the experimental apparatus worked and how it allowed for the acquisition of data on the solubility of potassium nitrate, they were asked to explain how the use of this experimental equipment allowed the determination of the solubility of potassium nitrate (question 2). The majority of student groups answered this question correctly:

Solubility is the value of the concentration of the saturated solution. Thus, using the laser that passes through the solution together with the LDR on which the laser beam strikes, we can detect the moment when the salt begins to precipitate. The studied solution was previously heated and then placed in a magnetic stirrer. Finally, Arduino and Excel software made it possible to record the moment when the transmittance decreased (the solution became turbid), that is, the moment when the salt precipitation began (Group D).

Firstly, we heated the potassium nitrate solution until it completely dissolved. The laser passes through the solution and strikes the LDR, which allows for measuring the transmittance of the solution. As the temperature decreases, we reach the point at which potassium nitrate starts to crystallize. From that moment on, the formed crystals make the mixture increasingly opaque, resulting in a more pronounced drop in the measured transmittance value (Group E).

Finally, the exploration worksheet ends with a reflective question about a real-life situation in the context of the activity (question 8). This question asked the students to analyse the labels of two different toothpaste brands (one for daily use, with a concentration of 50 mg/g of KNO₃, and another for treating tooth sensitivity that contained 5 % of KNO₃) and critically reflect and discuss the differences between the two kinds of toothpaste. In this question, almost all groups concluded that both toothpaste brands had the same mass concentration of potassium nitrate. However, only about half of the students engaged in critical discussion of these data. Some examples of responses from these discussions include:

We can conclude that both toothpastes have the same amount of potassium nitrate. Thus, through brushing (1 g of toothpaste), the maximum mass of potassium nitrate that can be absorbed is 0.05 g in both situations. Therefore, there are no differences in terms of KNO ₃ concentration between the daily use toothpaste and the one recommended for tooth sensitivity (Group F).

The analysis of the students’ responses to the evaluation questionnaire of this activity (Figure 3), along with the collected informal records, allows for a better understanding of some of the aspects related to the students’ results in the exploration worksheet.

Figure 3:

Students’ responses to the evaluation questionnaire of the implemented activity. (Note that item 4 refers to the difficulty of performing the activity, so the response scale should be read inversely to the others).

Regarding the learning objective proposed in the Chemistry curriculum for this level of education (to experimentally investigate the effect of temperature on the solubility of a solid solute), it is concluded that this was a successful teaching approach. Firstly, as mentioned earlier, the students provided scientifically correct answers in the exploration sheet, allowing them to conclude how the solubility of potassium nitrate varies with temperature. Secondly, more than 70 % of the students (with no negative responses) stated in question 11 of the questionnaire that they learned more about chemistry content related to salt solubility, and only 19 % of the students had difficulties understanding the relevance of the explored context for the activity. Additionally, from observations and conversations with the students, emerged several indicators that the proposed activity promoted the development of different literacies (e.g., scientific, chemical, technological, and digital), which are considered essential for a 21st-century education, as defined in the Portuguese Chemistry curriculum. Furthermore, as mentioned by Morais and Araújo (2023) and Stehle and Peters-Burton (2019), the involvement of students in various stages of this STEM activity – introduction to programming the experimental apparatus to its assembly, data acquisition, analysis, and discussion – promotes the development of critical and creative thinking and other higher-order thinking skills.

Item 1, regarding the enjoyment of the activity due to the involvement of Arduino programming, received the highest percentage of positive responses (98 %). Similarly, items 6 and 12, which are related to the interest in this differentiated activity compared to the usually performed laboratory activities and the desire to engage in more STEM activities at school, gathered over 70 % positive responses, demonstrating that this could be a future path of interest for Chemistry Education. In this regard, the development of collaboration and active cooperation between and within groups was also a highly positive aspect observed throughout the implementation sessions of the activity. This aspect can be considered highly successful, as evidenced by the students’ responses to item 7. This factor may help explain why the overall responses of the work groups to the questions in the exploration sheet were so positive since there was clear engagement from all group members in executing the activity and discussing the answers to the presented questions.

Regarding the experimental apparatus developed for collecting data on the solubility of potassium nitrate, items 3, 4, and 10, concerning the understanding of the functioning of the experimental apparatus for data acquisition, mobilizing content from other areas beyond Chemistry (i.e. Physics), received very positive evaluations from students (less than 25 % of students responded negatively to these items), aligning with the students’ responses to question 2 in the exploration sheet.

Additionally, 97 % of the students considered automatic data acquisition to be a significant advantage for the activity (item 2). Informal records also highlight the importance of this feature of the experimental apparatus since, as also mentioned by Morais and Araújo (2023), it allowed students to perform tasks simultaneously (e.g., answering questions on the exploration sheet) while waiting for the solution to cool and precipitate the potassium nitrate crystals.

One of the questions in the exploration sheet that resulted in the most incorrect answers was related to the use of Excel to plot the curve fitting the experimental data. These results are corroborated by item 5 of the questionnaire and the collected records, as students mentioned having little experience and interest in using this software for data processing. In Portugal, the acquisition and processing of data from Physics and Chemistry laboratory activities are predominantly done using the potentialities of graphic calculators since they are a mandatory tool in Chemistry, Physics and Mathematics subjects in Portuguese Secondary Education.

Items 9 and 13 require a more detailed reflection as the notes from observations and conversations with students contradict the trend of their responses to these items. For example, 86 % of students stated that they did not have difficulty assembling the electrical circuits for the proper functioning of the experimental apparatus (item 9), and no student reported difficulties in performing the various tasks requested throughout the activity (item 13). However, during the exploration of the second part of the activity (guided exploration of assembling electrical circuits and controlling electrical and electronic components through Arduino programming), students faced many difficulties. They mentioned having never worked with electronic components or programming, which they considered the main reason for the difficulties. This is also why the activity was designed in distinct stages to contribute to the acquisition of foundational knowledge and skills, enabling students to carry out Stage 3 (conducting the experimental activity) with greater autonomy. However, some indicators of self-efficacy do not correspond to the observed reality, perhaps because students perceived the activity as solely focusing on Stage 3, centred on the experimental activity related to the solubility of potassium nitrate.

Overall, the majority of students highlight the acquisition of new knowledge and skills related to computational thinking and “introduction to programming in real scenarios” as positive aspects of the activity (Student P) and as something that “could be useful in the future” (Student Q). A smaller number of students also emphasize their involvement in assembling electrical circuits, active learning of chemistry content through experimental activities, and recognizing the importance of Chemistry for the pharmaceutical industry as a positive aspect. In this line, many students mentioned not finding any negative aspects to mention, or they mentioned negative aspects such as having “only one activity in the school year” (Student R) or having “only 2 sessions” (Student S). However, a considerable number of students referred to “difficulty in using Excel” (Student T) since, despite its potential, as mentioned, the software is unfamiliar to the students. Some students also stated that the fact of “not knowing how to program in Arduino beforehand” (Student U) was a fact that made it challenging to carry out the proposed activity since the introduction to Arduino programming, covered in one class, is clearly insufficient for students to master those basic skills.

Regarding aspects to improve in the activity, the majority of students stated that there was nothing to improve (“Nothing, I liked everything” (Student V) or “nothing, the activity was quite interesting and different from other activities previously performed” (Student W)). Nonetheless, many students believed that the introduction to programming should initially start with “block programming” (Student X) and that some time should be dedicated to allowing students to “better explore Excel for data acquisition and processing” (Student Y). Lastly, another aspect mentioned by students pertains to the adequacy of the activity to the available class time (“Only more time would be necessary” (Student Z)).

Despite the difficulties observed in assembling the experimental setup, including the mentioned electrical circuit assembly, and the lack of prior knowledge in Arduino programming, the interest, motivation, and autonomy of the students in carrying out the various stages of this differentiated experimental activity were clearly noticeable to the teachers and the researchers during intervention moments. Thus, combining these observations with the students’ responses to the exploration worksheet and evaluation questionnaire, it is possible to conclude that through these innovative and differentiated pedagogical dynamics, the proposed learning objectives were achieved, expanding and mobilizing students’ knowledge from various scientific and technological areas while equipping them with essential skills and abilities for their education (Taber, 2016).

4 Conclusions

From the analysis of the results from this differentiated and innovative pedagogical approach, a set of positive indicators of the success of this intervention with secondary school students emerged, allowing us to infer that STEM activities could be a fruitful path in the future of Chemistry Education (Avargil et al., 2020; Aydin-Gunbatar et al., 2018). These reveal that, overall, the activity Potassium nitrate bioavailability: Effect of solubility in prevention and treatment of tooth sensitivity, successfully allowed students to investigate the effect of temperature on the solubility of potassium nitrate salt in water, aligning with one of the proposed learning objectives in the Portuguese Chemistry curriculum, while also learning and mobilizing knowledge from other scientific areas such as Physics or Computing (in addition to Chemistry) and technology.

It is an almost unanimous opinion among all participants that Arduino programming was a differentiating factor with a very positive impact, especially on their motivation and interest in carrying out the proposed activity. In this regard, several students also mentioned that the development of these knowledge and skills would be very important for their future academic and professional paths. However, some student responses, as well as the observations made, revealed some difficulties experienced by students in completing the proposed tasks. Firstly, the lack of prior programming knowledge among almost all participants limited the Arduino introduction activity to the most basic tasks. In this sense, some students suggested that in a future activity, block programming could be introduced as it is more intuitive and easy to learn. Nonetheless, when students were asked to program something new to respond to a given challenge, despite the difficulties, they showed commitment and motivation in researching and testing possible solutions. Difficulties were also noted in the assembly of electrical and electronic circuits as many of the components used (such as the breadboard) were completely unfamiliar to the students. Similarly, the use of Excel for automatic data acquisition and processing proved to be one of the tasks that posed the most difficulties for students due to a lack of knowledge of the capabilities of this software.

It is evident that this STEM activity centred on Chemistry but with programming also playing a central role, proves to be an enriching pedagogical strategy, not only because it promotes students’ interest, motivation, and meaningful learning of the addressed content, but also because it fosters the mastery of important skills and abilities for their future (Akrami, 2022).

As presented, the use of Arduino in the Chemistry laboratory offers numerous new possibilities for didactic innovation, as it is a way to deepen the use of educational robotics and stimulate the development of new skills, such as programming or computational thinking, essential for 21st-century employability. Additionally, due to its low cost, it enables the automatic acquisition of data that would not otherwise be possible, due to the high cost of traditional laboratory equipment. However, mastering programming may be a limitation for Chemistry teachers, so the development of collaborative approaches with ICT teachers may be an interesting and highly potential path for updating science teaching practices in general, and Chemistry in particular.