The rise and fall of the phlogiston theory: a tool to explain the use of models in science education

1 Introduction

Science is often regarded as a systematic gathering of facts and laws. High school students are not aware of the roles of science and scientists in building models and theories as tools to understand nature (Hayes & Perez, 1997). Literature review of students’ and teachers’ understanding of models and modelling in chemistry highlights the importance of incorporating the epistemological aspects of related chemical concepts (Erduran et al., 2007).

It is important that students realize that no model is entirely correct and that they understand that science is more about thinking than just describing objects (Harrison & Treagust, 2000; Treagust et al., 2002). Justi and Gilbert (2002) proposed that by using models, students’ understanding of chemistry and models might be improved, as well as their ability to produce their own chemical models. Chiu and Lin (2019), and Harrison and Treagust (2000) claim that models are more than communicative tools: they are important links in the methods and products of science.

Scientific models are used in science as learning tools, as well as representations of abstract concepts and as consensus models of scientific theories. Changes and developments in scientific models are based on facts and the discovery of new facts/phenomena, and are not easily adopted by the scientific community (Mamlok et al., 2000). Moreover, if both students and teachers understand the conceptual meaning as well as the aspects of models, it might reduce learning difficulties and misconceptions in chemistry, and help them in developing their own mental models of scientific concepts (De Jong et al., 2013; Erduran, 2001; Justi & Gilbert, 2002).

In learning science, students need to understand that science reflects its history and is an ongoing, changing enterprise. The standards for the history and nature of science recommend the use of history in school science programs to clarify different aspects of scientific inquiry, the human aspects of science, and the role that science has played in the development of various cultures (Mamlok et al., 2000).

The obvious conclusion of various studies is that the science curriculum should develop a historical approach to the teaching of science (Abd El-Khalick, 2002). Lederman et al. (2002) claimed that integrating scientific developments into science studies might also help in understanding the work of scientists. Students who are acquainted with developments in science will be more aware of scientists’ struggles during their work, and this may advance their understanding of the nature of science (Schwab, 1962).

The historical approach deals with topics regarding the role that science has played in the development of various cultures as an ongoing, changing enterprise, such as: (1) the development of scientific models in relation to economic and social aspects in different eras, (2) different aspects of scientific inquiry related to technological developments, (3) the work of scientists, and (4) the nature of science (Abd-El-Khalick, 2002; Abell & Lederman, 2007; Mamlok, 2000; National Research Council, 2011).

The rise and fall of the phlogiston theory will serve as an example, through the description of the topic ‘combustion’. Very often, the introduction of this topic is used as an excuse for teaching how to write and balance chemical equations. Research shows that many students have erroneous perceptions regarding the combustion of matter (Watson et al., 1997); thus, it was considered important to try to clarify this issue.

2 The rise and fall of the phlogiston theory (Mamlok, 2000)

The phlogiston theory was established around 1700 and lasted for about one hundred years. According to the Phlogiston Theory, phlogiston is released during heating processes, and the remaining material becomes lighter. The initial success of the phlogiston theory was its being the first consistent general theory that tried to explain chemical reactions in general and combustion in particular, as well as being a broad conceptual scheme into which could be fitted most of the chemical phenomena known in the eighteenth century.

The demise of this theory started with Lavoisier’s new insights into the phenomena of chemical reactions in general and combustion in particular, as well as about the composition of air (Wisniak, 2004). Lavoisier’s results disproved the phlogiston theory and established the applicability of the principle of mass conservation to chemical reactions, as well as composition of air. In 1783, Lavoisier criticized the phlogiston theory, but no one supported him. However, in various repeated experiments it has been found that either the remaining material after combustion becomes heavier, or its weight remains unchanged. Therefore, the scientists concluded that phlogiston is weightless! However, despite the fact that they were aware of the fact that they were working in a framework of a complicated theory that contained internal contradictions, they continued to adopt this theory (Allchin, 1997). It was Lavoisier who continued to claim that if the Phlogiston Theory could not explain so many facts in general terms, then this was a problem. His quantitative experiments, his conclusions, and the collation of his observations led to ta process in which the Phlogiston Theory and Aristotle’s four elements theory have been challenged. As a result, Lavoisier formulated the Law of conservation of Matter in Chemical Reactions, and the Oxygen theory replaced the Phlogiston Theory. Scientists reached the conclusions that the combustion process involves Oxygen. Actually, scientists had known the existence of Oxygen from the end of the 18th century, but even after its discovery they were unconvinced that it was part of every combustion process.

During this period, the scientific community was divided into three groups: (1) scientists who assumed that the two theories were complementary, (2) scientists who supported the Phlogiston Theory, and (3) scientists who supported the Oxygen theory. The rise and fall of the Phlogiston theory is a good example to the process of the replacement of one theory by another, due to new facts and new discoveries. In addition, it stresses the advantages and limitations of scientific models and theories, as well as the nature of science (Lederman & Lederman, 2019). Therefore, a brief program, planned for two lessons, was developed around the Phlogiston Theory, in the framework of teaching and learning the “Science: An Ever-Developing Entity” program (Blonder & Mamlok-Naaman, 2020). Its objective was to present the students with the ancient theory of combustion, and explain the confrontation between the Phlogiston and the Oxygen theories.

Two examples of lesson plans (90 min each) aimed at high school students (10th grade), are presented in this paper. The goal of these lessons is to introduce the students with the rationale of the Phlogiston theory versus the Oxygen theory, and clarify the difference between heating and combustion. One lesson plan deals with concepts referring to the Phlogiston and the Oxygen theories, while the other one is an experiment, aiming at explaining the “Conservation of Matter” law according to both theories, as well as the concepts of combustion versus, heating:

1. The first example refers to student understanding the concepts of Oxygen and Phlogiston.

2. The second example deals with heating the CuSO₄·5H₂O.

Both lessons were carried out during teaching and learning the subject of Energy. The teacher may start teaching the topic with the one of the activities, or enact it at the completion of the topic. In addition, the phlogiston theory maybe integrated at each stage of discussing energy issues. Students can learn that the phlogiston theory preceded the oxygen theory. It is recommended to demonstrate the internal logic of the phlogiston theory despite the difficulty involved, and conduct discussions about theories prevalent in various periods in contrast to theories accepted today. It should clarify the importance of realizing that previous opinions were always based on logical arguments, relevant to the periods in which they existed.

It would be interesting to check students’ attitudes towards science before and after an activity like the one proposed. Does the historical approach of how theories are replaced by other theories increase the value of science in students’ views, or decrease it? How do studnets understand the role of a scientist in light of the stories? Do they understand that scientific terms are “invented” by scientists and not “discovered”? Do they understand the connection of today’s science to the previous theories, and the contribution of the previous theories to today’s science?

The elaborated examples of two lesson plans aimed at high school students (10th grade), regarding the Phlogiston Theory are included in the Supplementary Materials.

3 Data collection and analysis

Data was collected from the teachers as well as the students.

4 Findings

5 Conclusions

We may conclude, that the brief program, planned for three lessons, and developed around the Phlogiston Theory (in the framework of teaching and learning the “Science: An Ever-Developing Entity”), reached its goals. The students, who studied the program, learned that the scientists:

1. Were active during a period in which people dared to propose bold scientific hypotheses, while at the same time searched for facts that would support their hypotheses (Erduran et al., 2007);

2. Were curious, and their curiosity made them search for answers to a variety of phenomena;

3. Laid the foundations of the scientific research, that combines observations, hypotheses, asking questions, conducting experiments, argumentation, and building new models and theories (Katchevich et al., 2013; Lederman & Lederman, 2019).

The teachers assumed that as a result of such or similar activities, it will be possible to develop critical thinking in chemistry students, and a broader worldview of the world of science in all its many contexts. The teachers hope that as a result of such or similar activities, it will be possible to develop critical thinking in chemistry students, and a broader worldview of the world of science in all its many contexts.