LEARNING
Peter W. Hewson, Section Editor
Consistency of Students’
Explanations about Combustion
J. ROD WATSON
School of Education, King’s College London, University of London, London SE1 8WA, UK
TERESA PRIETO
Facultad de Educación, Universidad de Málaga, Spain
JUSTIN S. DILLON
School of Education, King’s College London, University of London, London SE1 8WA, UK
Received 25 September 1995; revised 14 August 1996; accepted 15 January 1997
ABSTRACT: This article reports some findings of a study of 14 – 15-year-old students’ ideas about
combustion. Patterns of students’ explanation across a range of questions are described and analyzed to gain insight into both the degree of consistency of their explanations and how this may affect the processes of conceptual change in students. Data were collected by a questionnaire survey
using mainly open questions. Responses were analyzed using systemic networks. Categories from
the networks were combined to produce patterns of explanations that could be considered as theories. The general characteristics of these theories, the consistency with which they were used, and
implications for teaching and learning are discussed. © 1997 John Wiley & Sons, Inc. Sci Ed
81:425 – 444, 1997.
INTRODUCTION
This paper explores two general conclusions that appear to be implicit in much of the research literature about students’ alternative frameworks. Firstly, that students tend to be inconsistent in their explanations, and secondly, the apparently conflicting conclusion that students’
alternative frameworks are robust (i.e. difficult to change.)
Clough and Driver (1986) explored the consistency in students’ explanations across different areas of science. They found that students tended to use alternative frameworks similar to
Correspondence to: J. R. Watson
© 1997 John Wiley & Sons, Inc.
CCC 0036-8326 /97/040425-19
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one another, but that individuals were not as consistent in their use of alternative frameworks
as they were in the use of scientific explanations. Paris (1992), in her study of students’ understanding of acids and bases, also found that students were more consistent in the use of scientific explanations than alternative frameworks. Similarly, in the field of combustion,
BouJaoude (1991) claims that students’ understandings about burning “were fragmented, inconsistent and at variance with scientific knowledge.”
On the other hand, many researchers have reached the conclusion that students’ alternative
conceptions are very resistant to change (e.g., Shipstone, 1985). This appears to contradict the
inconsistent use of alternative frameworks. If students are willing to shift in their explanations
from the use of one alternative framework to another, why are they so reluctant to use scientific explanations?
In this paper, students’ responses to a questionnaire about combustion are examined to explore whether students use alternative explanations consistently. Where there is apparent inconsistency in students’ explanations, three possibilities are explored:
1.
The students have an underlying rationale for their explanations, which is not immediately apparent to the researcher. Students’ explanations may seem consistent when
viewed from the students’ perspective, but not from a scientific perspective.
2. The alternative theories give rise to explanations that are limited in scope and therefore
appear inconsistent from a scientific perspective. The students do not see the need for
an explanatory theory as general as the scientific one, and so are satisfied with less general theories that do not take into account some aspects of the phenomenon of combustion.
3. The students may be in a state of transition from acceptance of one theory to acceptance of another. Their learning may be producing conceptual change in which they are
trying to accommodate new ideas.
Black and Simon (1992) compare students’ theories and scientists’ theories to two islands,
and the teacher’s job is to build bridges between the two islands. From a researcher’s perspective the problem of inconsistency may simply be that we have failed to visit the students’ island and have viewed it from afar (previous possibilities 1 and 2), or that we are seeing the
traffic across the bridges between the islands (possibility 3).
In this paper the explanations of 150 students (aged 14 and 15 years) of some aspects of
combustion are studied using a questionnaire. The consistency of their responses is examined
in terms of three frameworks that have been developed from the literature: chemical reaction,
transmutation, and modification. The data were originally collected to characterize students’
explanations (Prieto, Watson, and Dillon, 1992), and are reanalyzed here, to explore the consistency with which students used these types of explanations. The way in which the students’
theories have been extracted from their explanations is described and the degree of consistency with which the students apply their ideas is analyzed. The results obtained are discussed
in terms of the nature of any inconsistency and the changes that must be brought about to promote more scientific explanations of combustion.
CHARACTERISTICS OF THE EXPLANATIONS OF STUDENTS DESCRIBED
IN THE LITERATURE
In this section students’ explanations are explored from the perspectives of the consistency
of their explanations and of the nature of conceptual change.
CONSISTENCY OF STUDENTS’ EXPLANATIONS ABOUT COMBUSTION
427
Some light is shed on students’ underlying rationale for their explanations by examining the
nature of their reasoning. Students are described as using linear causal reasoning and not considering all the relevant variables in the problem and relationships between them (Driver,
Guesne, and Tiberghien, 1985; Pozo, Puy Perez, Sanz, and Limon 1992). The nature of students’ linear causal reasoning is discussed by Andersson (1986a) and Gutierrez and Ogborn
(1992). In essence an agent exists (that can be a child or an instrument) whose direct or indirect action affects an object producing a change. This kind of reasoning is used by students to
make explanations and predictions about many phenomena (Andersson, 1986a). Guitierrez
and Ogborn (1992) state that an acceptable explanatory model must satisfy the constraints of
consistency, correspondence, and robustness: Consistency requires that the model must not
suffer internal contradictions; correspondence, that the model predicts what actually happens;
and robustness, that the model continues to correspond to the facts when the context is modified to another similar one. An example of such a model using linear causal reasoning is given
by Ogborn and Bliss (1990). They explain how students’ “common sense ideas” of motion are
developed, through interaction with their immediate environment, to produce logically consistent explanations of motion. The explanations take the form of causal schemes that become
successively more elaborated as a child develops. Although linear causal reasoning may lead
to explanations that appear inconsistent from a scientific perspective, work such as that of
Ogborn and Bliss (1990) shows that these explanations may be internally consistent when
viewed from the students’ perspective.
Studies have been carried out comparing the types of explanations given by students in different ares of physics, with the genetic epistemological development of students (Monk 1990,
1991; Eckstein and Shemesh, 1993). The studies indicate a link between cognitive development and types of alternative frameworks given. Indeed, Eckstein and Shemesh have gone on
to model this link mathematically. Monk (1990) draws on the work of Vygotsky when he
claims that:
(i) there is a genetic epistemological stage related ceiling to the cognitive processing on the
part of students;
(ii) students can benefit from tuition such that their performance on test items can be increased
to the limit set by their then current stage of genetic epistemological development.
The proposal that students’ explanations are limited in scope, and that they do not see the
need for an explanatory theory as powerful as a scientific one, is supported by Driver, Guesne,
and Tiberghien (1985). Students’ explanations are reported to focus on directly observable
characteristics of the situation, while what is not perceptible is ignored. Students have a limited focus that is manifested in a tendency to interpret the phenomena in relation to the absolute properties or qualities of objects instead of considering the interaction between the
elements of a system. Students’ explanations are also often context dependent: Different ways
of explaining are often used for different examples of the same phenomenon.
A number of studies have described the difficulties of students accommodating new ideas
in changing from their alternative conceptions to scientific ones. Chi, Slotta, and deLeeuw
(1994) and Chi (1992) explain this in terms of how knowledge is categorized by students.
They propose that knowledge can be divided into distinct ontological categories. The three
major categories are matter, events, and abstractions. Entities categorized in one category,
such as matter, have certain attributes that differentiate them from entities in another ontological category, such as processes. Reiner, Chi, and Resnick (1988) give many examples from
physics where students have incorrectly categorized scientific processes as materials. This
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leads to considerable difficulties for students in developing scientific concepts. For example,
the process of “heating” may be classified incorrectly as a substantialized material “heat.”
Heating is a process that has properties such as happening during a particular time and being
caused by certain interactions, as compared with heat that can be gained or lost, can travel
along a metal bar, or can be lost during burning and account for the weight loss in many burning processes. Chi (1992) argues that conceptual change within an ontological category (nonradical conceptual change) is much easier than conceptual change across ontological
categories (radical conceptual change), and that indeed, conceptual change across ontological
categories is “nearly impossible to accomplish.”
There are no concrete operations that can transform a physical object such as a cup, into an
ontologically different entity, such as a dream. Likewise, there is no psychological learning
mechanism that can modify a concept from one ontological category into another. No mechanism of addition, deletion, generalization, discrimination, specialization, proceduralization
and so forth can change the meaning of a concept from one ontological category to another . . . (Chi, 1992).
It is suggested that the radical conceptual change needed to develop a concept in a different
ontological category may take place by developing a new concept within the correct ontological category. The new concept may then coexist with the old concept in the incorrect ontological category. Conceptual change within an ontological category (nonradical conceptual
change), although not necessarily easy to accomplish, does not present the same psychological barrier. Driver et al. (1994), for example, describe how the concept of air is developed in
students by broadening the concept of wind, through the existence of air or gas as unseen
matter, to matter with weight, and possibly eventually to the conceptualization of air or gas in
terms of particles. Such a development occurs within the ontological category of matter.
The division of conceptual change into “radical” and “nonradical” gives some insight into
the issue of consistency or inconsistency of students’ explanations. Some concepts that scientists classify as “events” may be classified by students as either “events” or “matter” depending on the circumstances. For example, in processes involving heating, students may be
influenced by circumstances to treat the phenomenon as “heating” within one set of circumstances and as “heat” in another. They could therefore be operating consistently within two
distinct ontological categories, but from a scientific perspective appear to give inconsistent
explanations.
MODELS OF STUDENTS’ UNDERSTANDING OF COMBUSTION
IN THE LITERATURE
In this section, models of students’ understanding of combustion are examined to provide a
framework for examining consistency and inconsistency in their explanations. These models
are also used in the discussion of the nature of conceptual changes required to change from
alternative explanations to more scientific ones.
A number of studies have been carried out that have attempted to synthesize findings about
students’ explanations of combustion by applying general criteria to classify the responses to
a variety of questions (Andersson, 1986b; Andersson, 1990; Meheut, Saltiel and Tiberghien,
1985; Meheut, 1982; Pfundt, 1981; Prieto, Watson, and Dillon, 1992). Andersson (1990) describes a categorization system that can be applied to students’ responses to a variety of questions concerning both chemical and physical changes. The categories are disappearance,
displacement, modification, transmutation, and chemical interaction. “Disappearance” is used
CONSISTENCY OF STUDENTS’ EXPLANATIONS ABOUT COMBUSTION
429
by some students who say that petrol is used up in the car and disappears (Andersson &
Renstrom, 1983). “Displacement” is used by some students to explain the disappearance of
water from a puddle on the floor in saying that the water had penetrated the floor, i.e. it was in
a different place. Meheut et al. (1985) give examples of “modification” in which students explain the burning of alcohol and the boiling of water in terms of modification of liquid alcohol
to alcohol vapor and liquid water to steam. “Transmutation” is used to describe changes in
terms of transmutation of substance into energy, of energy into substance, or of one substance into a new one, for example steel wool being transmuted into carbon during combustion. Ideas of “chemical interaction” are applied correctly to examples such as petrol
burning, but also incorrectly to physical changes (Osborne and Cosgrove, 1983). Some
students think that the bubbles of steam are oxygen and hydrogen gases.
Meheut et al. (1985) developed a way of categorizing the responses of students (aged
11 – 12) based on their ideas of conservation. There are some similarities with
Andersson’s categorization scheme, but Meheut et al. also incorporate the nature of the
combustible material in the categorization system. Students’ responses can be divided
into two groups according to the nature of the combustible substance. The first group includes responses about metals, wax, and water, which are said to melt or evaporate, rather
than burn, or using Andersson’s terminology, are modified. The second group includes responses about wood, cardboard, paper, and air, which are seen to burn and be changed
into another substance or nothing. Using Andersson’s categories, these substances disappear or are transmuted. An important feature in the categorization of Meheut et al. is that
during transmutation each substance is transmuted separately. Students often fail to realize that matter is interacting.
The role of oxygen and air is not dealt with adequately in either Andersson’s or
Meheut’s model. One difficulty in interpreting responses to questions designed to elicit
students’ understandings of the role of oxygen and air in combustion is that not all students have a good understanding of the nature of air (Russell et al., 1991; Brooks &
Driver, 1989). Brooks & Driver reported that, even at age 16, about three quarters of students thought that air had zero or negative weight. Similar findings are reported by Stavy
(1990). Students may, therefore, believe that matter continues to exist when it is involved
in a chemical change, but does not necessarily conserve weight.
In an earlier paper (Prieto, Watson, and Dillon, 1992) the general descriptions of the
categories of Andersson and of Meheut were refined into operational criteria through a
preliminary qualitative analysis of the questionnaire. Three categories of students’ explanations of combustion were produced to fit our data. These categories are frameworks
that link different elements of students’ explanations to form a coherent whole. The categories were modification (M), transmutation (T), and chemical reaction (C). The disappearance category of Andersson is subsumed under transmutation as a limiting form of
transmutation, where matter is transmuted to nothing (Pfundt, 1981). Examples of displacement were not found in our data. The three categories are defined in Table 1.
There is a small amount of overlap between the categories T and M. In both T6 and M3
(Table 1) the flame/fire causes a change. There is no reason, however, why the explanatory frameworks should be different in every respect. Initially some responses were given
a dual code (T6/M3), and the final code has been allocated in light of the rest of the response to that question.
It can be seen that the alternative frameworks “transmutation” and “modification” share
many of the characteristics of students’ alternative explanations described in the literature
reviewed earlier: These frameworks use a particular kind of reasoning, are limited in their
scope, and incorporate concepts that are placed in incorrect ontological categories.
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TABLE 1
Operational Definitions of Students’ Exlanations of Combustion
1. Role of oxygen/interaction
Chemical reaction
C1: Students recognize that the combustible substance and oxygen/air interact.
C2: The reaction is irreversible.
Transmutation
T1: There is no interaction between the combustible substance and the oxygen/air.
T2: Oxygen/air may or may not be recognized as necessary for combustion to take place.
T3: Burning is a destructive process.
T4: The destructive process may release or liberate substances from the combustible substance.
T5: Burning is irreversible.
Modification
M1: Oxygen/air is not involved in the change.
M2: The change is reversible.
2. Flame/fire
Chemical Reaction
C3: Energy changes may be observed but are not explained.
C4: The flame/fire is evidence of a chemical reaction.
C5: The flame contains both the combustible substance and oxygen/air reacting.
Transmutation
T6: The flame/fire is an active agent of change.
T7: Air/oxygen may be needed to “feed the flame” or “keep it alive.”
T8: Air/oxygen is transmuted by the flame/fire or is consumed by it.
T9: Matter may be transmuted into heat and vice versa.
T10: Flames contain only the combustible substance or oxygen/air or possibly both but with no
interaction.
Modification
M3: The flame/fire is a source of heat to make the modification occur.
3. Products and reactants
Chemical Reaction
C6: The products contain the reactants in a different chemical combination.
C7: Mass is conserved provided that students think that gases weigh something or gas is not
“lost” to the atmosphere.
C8: Properties are not conserved.
Transmutation
T11: Substance is changed from one substance to another or into nothing during combustion.
T12: Oxygen/air may be transmuted separately into a product.
T13: Oxygen may be needed but does not interact in a chemical sense.
T14: Mass may increase, decrease, or stay the same (because the transmuted products are different from the reactants).
T15: Properties are not conserved.
T16: Properties may be substantialized and therefore be involved in the transmutation.
Modification
M4: One substance changes to a different form of the same substance.
M5: Substance is conserved.
M6: Mass may be conserved, but this depends on whether different forms of the same substance
are considered to weigh the same.
M7: Some properties are conserved.
CONSISTENCY OF STUDENTS’ EXPLANATIONS ABOUT COMBUSTION
431
Linear causal reasoning is used in both “transmutation” and “modification” (c.f. Driver,
Guesne, and Tiberghien, 1985; Andersson, 1986a). In transmutation, the flame or fire is
an active agent of change, which acts on the combustible material, and perhaps the oxygen separately, in a destructive process to produce a product that is completely different
from the reactant(s) (T3, T5, T6, T11, T12, T14, T15). In modification, the heat or fire
acts on the combustible substance (M4) to change it into a different version of the same
substance (M4, M5, M6, M7). Piaget’s stage theory can be applied to categorize the type
of thinking required for the different framework. Transmutation or modification require
only concrete operational thinking as compared with “chemical reaction,” which requires
formal operational thinking (c.f. Monk 1990, 1991), and this may explain the low numbers of students who give “chemical reaction” explanations. The thinking needed for
modification involves the reversible transformation of a substance with certain properties
(e.g. solid properties) into the same substance with some different properties (e.g. liquid
properties) and is characteristic of early concrete operational thinking. Combustion as
transmutation involves an increase of the product at the expense of the combustible substance; i.e. the system contains only two components, which are polar opposites: less
combustible substance, more product. This is characteristic of late concrete operational
thinking. “Disappearance,” which has been subsumed under the category of transmutation, could be considered to make lower cognitive demands as an indicator of early concrete or even preoperational thinking. In thinking about combustion as “chemical
reaction,” a student must have a model that contains at least three components (i.e. combustible substance, oxygen, and product), and the relative ratios of these co-vary in a direct and indirect fashion. This could be interpreted as requiring the compensatory
thinking that is one of the characteristics of formal operational thought.
The limited scope of the alternative frameworks is seen in their heavy dependence on
immediately observable characteristics of combustion (c.f. Driver, Guesne, and
Tiberghien, 1985). Oxygen or air, which cannot be seen, plays no role in interacting with
the combustible substance in transmutation (T1, T2, T7, T8, T12 and T13) and plays no
role in modification at all (M1). Gaseous products and weight changes are unimportant in
transmutation (T14) and modification (M3) explanations and receive little attention from
the students. The alternative explanations are also mainly descriptive in nature (c.f.
Driver, Guesne, and Tiberghien, 1985). The explanations have predictive validity, in that
they allow students to predict changes that will occur in what they perceive as similar circumstances, but they fail to go beyond the immediately perceptible. As predictive explanations, transmutation functions well on the types of materials that students normally
give as examples of burning, i.e. carbon/hydrogen-based materials such as wood, paper,
plastic, cloth, and living things. Modification functions well on examples of burning, that
students incorrectly perceive as similar to changes of state, such as a candle or a metal
burning (melting) and alcohol burning (evaporation): Under not too close scrutiny the
original substance reforms after burning as a solid product (freezing) or as a liquid (drops
smelling of alcohol condensing on a cold surface from an alcohol flame).
The limited scope of these frameworks is also seen in the imprecise use of concepts
and language (c.f. Driver, Guesne and Tiberghien, 1985) that is very evident in the students’ responses. For example, the confusion between the concepts of burning and heating are obvious in the modification explanation.
Both transmutation and modification can be categorized in the same ontological
category as chemical reaction, i.e. the category “constraint based event” category (Chi,
1992), but the concept of heat/heating that is used within the transmutation explanation is
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classified incorrectly by some students. Some students substantialize heat (T9), i.e. categorize it as “matter” instead of as a “constraint-based event.”
RESEARCH METHODS
A questionnaire containing nine open questions, including one that also contained a
fixed response item, was developed through a series of four pilot studies that involved
about 150 students in Spain and England. The questionnaire was designed to elicit responses that could be used both to develop further the aforementioned explanatory frameworks and to allocate students to the frameworks. Different questions focused on different
aspects of the frameworks: the requirements; the process; the products, including aspects
of conservation; and the examples of combustion that students give. The questions that are
the focus of this paper are given in the appendix. The data relating to examples of combustion that students give are discussed elsewhere (Watson, Prieto, and Dillon, 1995). The
questionnaire was administered to 150 students aged 14 and 15, who were in their third
year of their secondary school education in mixed-ability co-educational state schools in
Spain. The data were collected and analyzed in Spanish, so all the quotations of student responses given in the results section of this paper are translations. Ages 14 to 15 were chosen because the students were familiar with burning outside the school context and had
studied some elementary examples of combustion such as burning wood and candles, and
the oxidation of some metals. The approach taken in school was mainly nonpractical, with
about one fifth of the lessons involving practical work. There was a small emphasis on
quantitative work involving weight and volume changes.
This study builds on a study reported earlier (Prieto et al., 1992), by reanalyzing the
same data from the perspective of consistency. The first stages in the analysis are reported
in the earlier study and are repeated here for clarity. The responses to the questionnaire
were analyzed, firstly by developing systemic networks (Bliss, Monk, and Ogborn, 1983)
to capture the essence of the responses to open questions and to code them in a systematic way. Some of the codes are more useful than others in gaining insight into the students’ thinking. For example “It is a chemical reaction that happens when a substance
gains such a high temperature that it cannot stand it” (student 32) could be taken to indicate an understanding that one chemical is seen to be reacting with another. This meaning
is not, however, clear from the latter part of the response. Similarly, “It is a combustion
process” (student 70) could be taken to mean the reaction of a combustible substance
with oxygen. The rest of the response shows clearly that this is not the case with a change
of state being given as an example of combustion.
Next the networks were analyzed to identify indicators of different categories of thinking, and these indicators were used to modify Andersson’s (1990) categories and to generate the detailed descriptions of different categories of thinking, given in Table 1 (Prieto
et al., 1992). The detailed descriptions were then used to categorize students’ responses
question by question.
The new analysis reported in this paper concerns the consistency with which students’
explanations fitted different explanatory frameworks. The students were divided into different groups according to the consistency of their responses across questions. A detailed
qualitative analysis of the responses of students in the different groups was then carried
out to examine the nature of the consistency or inconsistency of each group.
The analysis of the data revealed a dilemma inherent in the design of this study. On
the one hand, it was necessary to consider responses from many students to a range of
CONSISTENCY OF STUDENTS’ EXPLANATIONS ABOUT COMBUSTION
433
questions. On the other hand, it was necessary to be able to achieve a depth of response and richness of data, to be able to interpret the reasons why students gave the
responses that they did. In view of the limited resources available for this study, it was
decided to sacrifice some of the quality of the data to achieve quantitative results. This
did lead to some difficulties in allocating responses to particular categories of explanation. For example, one difficulty was that it was often difficult to decide whether a response should be categorized as “transmutation” (T) or “chemical reaction” (C). The
aforementioned response given by student 32 is an example of this. The use of the
words “chemical reaction” may be an indication of “C,” but this is ambiguous. Similarly, there are indications in the response of a destructive process, “T,” but this is not
clear. There is no mention of oxygen or air. This response has, therefore, been categorized as intermediate between “C” and “T ” as “X.” Other responses are not ambiguous but contain aspects of both “C” and the “T ” explanations. These have also been
classified as “X.”
Some questions, such as question 1 and question 6, allowed students to give, in different parts of their response, both modification explanations and transmutation/chemical reaction explanations, e.g. student 70 mentioned previously. Such responses received a
double classification, in this “X” and “M.”
In spite of these difficulties in interpretation, the operational definitions have allowed a
high degree of reliability to be achieved in allocating responses to particular categories of
explanation. A 10% sample was coded by two independent coders and 94% reliability
was achieved.
Results. Classification of responses to each question. The responses to each question were
categorized using the operational definitions in Table 1. Each response was examined to
identify whether it contained phrases that could be classified according to the code descriptions in Table 1. Different parts of the same responses to questions 1 and 6a could be
classified using different categories. For example, two students referred to the wick of the
candle being changed to black ash (T) and the wax melting (M). Some responses did not
contain enough information to be categorized and are given as “not coded” in the table.
The allocation of responses to categories is shown in Table 2.
TABLE 2
Categories of Responses for Different Questions
Question
Category
1
5
6a
6b&c
7
9
Chemical reaction (C)
Intermediate (X)
Transmutation (T)
Trans.(T) 1 Mod.(M)
Modification (M)
Not coded
Blank
2
10
106
13
3
16
0
1
2
140
0
0
5
2
1
17
20
2
65
3
42
1
24
95
0
10
11
9
6
14
73
0
1
9
47
43
3
40
1
21
28
14
Total
150
150
150
150
150
150
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WATSON ET AL.
TABLE 3
Combinations of Different Types of Explanations Used by Students (%), N 5 150
Chemical reaction (C)
and/or intermediate (X)
1 transmutation (T)
Transmutation (T) only
Total
Modification (M)
No Modification
Total
28
22
50
34
62
16
38
50
100
The patterns that can be seen in the table are that transmutation was the most commonly used type of explanation in questions 1, 5, 6b and c, and 7. Modification is only
important in the case of the candle (Q6) and to some extent the magnesium (Q9). The scientific theory chemical reaction is little used but some students use some elements of it
(X). Many more students are classified as having some understanding of chemical reaction (C) in question 9, than in other questions, presumably because the forced choices in
this question made it easier for them to identify a chemical reaction explanation.
The extent to which students used one or more types of explanation is indicated in
Table 3. This table represents students’ explanations in two dimensions. The chemical reaction (C), intermediate (X), and transmutation (T) explanations form one dimension. In
this dimension transmutation is viewed as a framework in which changes occur without
interaction between substances, as compared with chemical reactions where there is interaction. The second dimension depends on whether students used modification explanations or not. Frameworks in which modification responses are not used are seen to reflect
a view in which features of burning have been generalized over several contexts, whereas
students who use frameworks in which modification responses are used, have not
achieved this generalized view of burning (see Prieto et al., 1992, for further discussion).
The columns in Table 3 show that about two thirds (62%) of the students used modification explanations at least once, whereas about one third (38%) never used modification
explanations. The predominance of transmutation explanations is shown in the rows. The
students in the first row (50%) used a mixture of transmutation explanations with chemical reaction and/or intermediate explanations. The students in the second row (50%) used
transmutation explanations.
In the next section, the response of students are considered in more detail to shed light
on whether the students were responding in a consistent or inconsistent way.
The degree of consistency in students’ responses. In this section the consistency or inconsistency of students’ responses is examined from the perspectives introduced in the introduction, i.e. students may be consistent, students may have an underlying rationale for
their explanations that is not immediately apparent to the researcher, students may be using explanations that are limited in scope, or students may be in a state of transition from
one theory to another.
Use of transmutation explanations only. The simplest group to consider is the group of
students in Table 3 (16%) who are completely consistent in their responses, using only
one type of explanation, i.e. transmutation with no modification. An example of a student
who consistently used transmutation explanations follows:
Student 62
Q1(a) What I understand by burning is when a material is able to produce heat with another
material which helps it to produce flames. (b) It consumed.
CONSISTENCY OF STUDENTS’ EXPLANATIONS ABOUT COMBUSTION
435
Q5 You get ashes and residue.
Q6(a) Wax has been consumed and the flame has been extinguished because of lack
of oxygen. (b) The air has been consumed and there is no supply of oxygen. (c) (No
response)
Q7 It is made of different parts of heat with different intensities.
Q9(e) because as a material burns it disappears and all that is left is a powder that weights less
than in its original state.
Use or non-use of modification explanations. No students used only modification types
of explanations. Modification is always used in combination with a different kind of explanation. From a scientist’s perspective, students are being inconsistent in their explanations of combustion: Sometimes they give one kind of explanation, sometimes another.
From a student’s perspective there may, in fact, be no inconsistency: Their lack of precision in the use of language, their lack of development of certain concepts, and their lack
of discrimination between concepts, allows them to use both kinds of explanations. For
example students sometimes fail to distinguish between heating and burning, and consequently between change of state and combustion:
Student 3
Q1(a) It is when a substance burns. (b) It becomes ashes or changes to another material. For
example, if it is wood it becomes ashes, and if it is plastic it melts.
In many ways, burning and change of state have similarities: Burning alcohol and boiling water over a Bunsen burner are similar. A flame is seen, heat can be felt, the volume
of liquid gets less, and a vapor is formed. There are also similarities between burning wax
in a candle and melting a block of ice, both melt on heating/burning and solidify again on
cooling (or at least most of the wax resolidifies, if the candle does not burn too long). The
concepts of change of state and burning are sometimes not well developed in students
and become confused, because the students do not use adequate criteria to distinguish between them.
A factor that appears to control which type of explanation is used is the nature of the
combustible material. The present study confirms the results of Meheut et al. (1985) in
showing that students’ have a greater tendency to use modification explanations with wax
and metal. There are insufficient data in the present study to show whether students discriminate between different combustible substances in a consistent way: There are too
few different types of combustible material included in the study. There are, however, indications from the literature that this type of explanation, as well as being dependent on
the nature of the material being burnt, is also age dependent, with its use decreasing with
age (Russell, Longden, and McGuigan, 1991; Sanmartí, 1989; Driver, Child, Gott, Head,
Johnson, Worsley, and Wiley, 1994; BouJaoude, 1991).
Use of chemical reaction, intermediate and transmutation explanations. Fifty percent
of the students gave responses that included transmutation explanations as well as explanations that indicated some aspects of chemical reaction or intermediate explanations (first row, in Table 3). Most of these students seem to be using transmutation
explanations most of the time, but at the same time are beginning to incorporate aspects of chemical reaction explanations in their responses. They could be characterized as students in a state of transition from one theory to another. An example is
given with student 64:
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Student 64
Q1(a) Something burns when it is on fire. (b) Depending on the substance, a grey or black
powdered substance remains.
Q5 You destroy it.
Q6(a) If the candle is small it is consumed. If the candle is big, the oxygen is consumed before. (b) The oxygen is being consumed and the container is full of carbon dioxide.
Q7 of gunpowder that as it is burning releases nitrogen.
Q9(c) (No explanation).
This student chooses the correct option for question 9, which explains an increase in
weight of magnesium by its reaction with oxygen. The idea of interaction is not so clear
in response to question 6: The candle is consumed and the oxygen is consumed and replaced by carbon dioxide, but it is not clear whether these are two independent processes
of transmutation or whether there is chemical interaction between the two substances.
The responses to questions 1 and 5 are transmutation explanations with the combustible
substance being transmuted to a destruction product. When asked about the role of oxygen in combustion, this student, therefore, can choose the correct option in a multiple
choice question (Q9), has difficulty in articulating the role of oxygen in an open format
question asking about oxygen (Q6), and fails to consider oxygen in the other questions
where he is not specifically asked about it. It appears that this student is struggling to incorporate the role of oxygen into a transmutation framework that did not previously take
oxygen into account.
Another example shows similar efforts to incorporate oxygen into a transmutation
framework.
Student 102
Q1(a) is a form of combustion which requires the presence of oxygen to be able to react with
it and produce fire. (b) It decomposes and is converted to ashes.
Student 102 expresses the idea of interaction more clearly, but again views the products as
destruction products. There is no mention in this student’s response of oxides as products and
certainly not of any gaseous products.
Student 138 uses ideas of interaction but this time the interaction is between air and the
flame.
Student 138
Q6(a) (No response). (b) The air is absorbed by the flame and when the air has finished the
flame goes out.
Q7 Oxygen and materials from the match.
The student has incorporated ideas of interaction into a transmutation framework by
viewing oxygen/air as necessary to interact with the active agent of change, the flame, to
allow burning to take place.
In some students there seems to be some progression toward the chemical reaction explanation in terms of the language used. Some students incorporate “scientific” language
CONSISTENCY OF STUDENTS’ EXPLANATIONS ABOUT COMBUSTION
437
such as “combustible,” “chemical reaction,” “combustion” into what are essentially transmutation responses:
Student 95
Q1(a) a chemical reaction is where there is a combustible substance, air and something that
consumes the combustible substance, fire in this case, and ashes is what is left from this reaction. It is what remains. (b) become ashes.
Q5 You get ashes which is the result of the chemical reaction from wood and fire.
Q6a (No response)
Q6b As it is enclosed, there is a combustion with a small bit of the combustible air and as it is
finished the fire goes out.
Q7 Fire.
Q9(e) because the magnesium has been burnt and the rest of the gases that are produced by the
fire are taking off weight.
In summary, it appears that the students in the first row in Table 3 are inconsistent in
their use of explanatory frameworks. This may be a result, not of illogical or ad hoc
thinking, but of a struggle to come to terms with two mutually exclusive explanatory
frameworks. It can be seen that some of the students’ explanations may be attempts to introduce new ideas from the chemical reaction framework into a transmutation framework.
DISCUSSION
The evidence presented in this paper shows that the students can be divided into three
overlapping groups with regard to consistency. If the students’ responses are categorized
using coherent alternative frameworks, derived from students’ explanations, it is possible
to say that one group of students (T only) is totally consistent in the use of their alternative frameworks (column 2, row 2 in Table 3); a second group appears to be consistent
from their own perspective (column 1, row 2 in Table 3) in the way that they use or do
not use modification explanations, in that whether they use them depends on the nature of
the combustible substance; and a third group (row 1 in Table 3) is inconsistent in the use
of different alternative frameworks in that they use aspects of both transmutation and
chemical reaction explanations.
A theoretical difficulty arises here. Table 3 is a summary of inferences drawn from data
by the researchers about the consistency or otherwise of students’ explanations. The researchers have tried to understand the alternative frameworks used by the students, and
the extent to which they have succeeded in doing this will be reflected in the level of consistency reported. It does appear, however, that many students do have an underlying rationale for their explanations and that some of these underlying rationales are
unscientific, in that they are limited in scope, yet coherent in their own terms. There is,
however, a large group of students whose explanations appear to be inconsistent. Deeper
research may reveal consistencies that this research has not discovered, but a possible explanation for the data is that students are in a state of transition from one theory to another, and the following discussion is intended to elucidate some of the difficulties that
students may have in making a transition from a modification framework to a chemical
reaction framework and from a transmutation framework to a chemical reaction framework.
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An examination of students’ explanations (see Table 1) reveals that certain aspects that
are normally present in scientists’ explanations of chemical change are completely or almost completely absent. Students fail to mention imperceptible products such as gases.
Even students who write about the combustible substance interacting with oxygen usually write about an ash as the product and fail to mention gaseous products. Oxygen is
rarely mentioned spontaneously. Students usually only mention oxygen in response to direct questions about the role of oxygen.
The weight of reactants and products, although playing an important role in scientists’
explanations, was never mentioned spontaneously by students. This is not surprising as
many 16-year-old students fail to realize that gases have weight, and some even think that
gases have negative weight (Brooks and Driver, 1989; Stavy and Stachel, 1985; Stavy,
1990). This precludes even students who understand combustion as a combination of the
combustible material with oxygen from drawing conclusions from experiments involving
measurement of weights. For students using transmutation explanations there is no necessity for a product to weigh more, the same, or less than the reactant, because it simply becomes a different substance.
Students make no mention of atomic or molecular particles. This could explain why attempts to change students’ conceptions by teaching about particles have failed (e.g.
Meheut et al., 1985). The students may see no relation between their own explanations
and acceptable scientific explanations in terms of particles.
A comparison of the operational definitions of modification and the chemical reaction
explanation (Table 1) reveals the following differences: In modification there is no role
for oxygen (M1), the process is reversible (M2), the flame/fire is the source of heat to
make the change occur (M3), and the product is the same substance as the reactant (M4,
M5, M7). In chemical reaction, oxygen interacts with the combustible substance (C1), the
process is irreversible (C2), the flame/fire is evidence of a chemical reaction (C4), and the
product or products are different substances from the reactants (C6, C8).
Of the students’ frameworks presented in this paper, modification is the simplest. It requires the lowest level of cognitive processing. It also depends on selective or inaccurate
observation: For example modification can only be maintained for candle wax if the
candle is observed burning for a limited time, so that the disappearance of the wax is not
apparent. Finally, it explains the most limited range of observations: For example it does
not explain the role of oxygen, whereas transmutation can explain oxygen as being
transmuted separately, or as being needed to “feed” the active agent of change, the
flame.
The fact that modification explanations coexist with transmutation or chemical reaction explanations indicates that students are not limited to this framework because of
limitations in the cognitive processing power of the students (c.f. Monk 1990, 1991).
Nevertheless, students have to shift from a framework that involves linear causal reasoning to one that involves the interaction of two chemicals to form at least one product, often more. The modification explanation, like chemical reaction, falls into the
ontological category of explaining a “constraint-based event” (Reiner et al., 1988) and
so nonradical conceptual change is required to bring it closer to the chemical reaction
explanation.
To bring about the conceptual change required, students have to learn to distinguish between heating and burning and to distinguish between burning and changes of state. The
comparison of the types of students’ explanations discussed previously suggests that
practical exercises need to be constructed in which students carefully compare heating, as
CONSISTENCY OF STUDENTS’ EXPLANATIONS ABOUT COMBUSTION
439
a process in which energy is transferred from an external source to the reactants, with
burning, in which energy is transferred to the surroundings during the process of reaction.
Similarly, change of state from solid to liquid, or from liquid to gas, can be compared
with combustion in which solids or liquids form gaseous products. In these changes of
state, energy is transferred to the system, whereas in combustion, energy is transferred
from the system to the surroundings. Related to this is the fact that during a change of
state, flames are external to the system, whereas in combustion, flames are generated as a
result of the process of combustion. The comparison of the two frameworks also suggests
that more attention needs to be paid to comparing the products of the reaction with the reactants to show whether the process is reversible. It appears that relying on visually obvious attributes of reactants and products is insufficient for many students to realize
whether the process is reversible on cooling.
A similar comparison of transmutation and chemical reaction explanations reveals that
the differences between these two frameworks are much more subtle and that the conceptual changes needed to move from transmutation explanations to a chemical reaction explanation are more difficult to achieve. In the transmutation framework, burning is a
destructive process (T4) in which the combustible substance, and oxygen if it is mentioned, are transmuted separately (T1, T2) into separate products (T11, T12). The role of
oxygen is unclear (T2) but is sometimes seen as interacting with or feeding the flame (T7,
T8, T10). The flame/fire is the active agent of change (T6), and heat may be substantialized so that matter can be transmuted into heat and vice versa (T9). The product of combustion of the combustible substance is a substance different from the reactant and so has
different properties (T11). Also it may weigh less, the same, or more than the reactant. In
the chemical reaction framework, oxygen interacts with the combustible substance to
produce products that contain the combustible substance and oxygen in a different chemical combination (C1, C6), and therefore the products weigh more than the combustible
substance. Students need to understand that gases weigh something and includes gases
“lost” to the atmosphere (C7). The role of energy is unclear (C3, C4). The products have
different properties from the reactants (C8).
It is evident that it is difficult to establish clear criteria to differentiate between the two
frameworks through experimentation. Both frameworks can explain what happens to the
oxygen, although the transmutation framework’s explanation of why it is needed is
weaker, i.e. it is needed by the flame. Both frameworks can explain the role of heat or
heating, although the transmutation framework fails to explain why energy is transferred
from the reactants to the surroundings in the process, as an input of heat is needed as the
active agent of change. It is often difficult to distinguish in class experiments, however,
whether energy is being transferred to or from the system during the process of combustion. Normally burning is started by heating the combustible substance using a flame.
Weight change are often used to reinforce the scientists’ view of combustion. Copper can
be heated in a tube connected to gas syringes containing air, and the increase in mass of
the copper can be explained in terms of the decrease in volume of the air. This experiment, which seems so persuasive to a chemistry teacher, must seem rather strange to a
student who believes that air has no or negative weight. The transmutation explanation is
relatively simple, i.e. the copper has been transmuted to carbon, which being a different
substance has a different weight, or possibly because it contains heat, has an increased
weight; the oxygen has been transmuted to nothing.
The transmutation framework can be used to explain many everyday observations of
combustion. One reason for its persistence may be that it is a simple framework that
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requires a lower level of cognitive processing than the chemical reaction framework.
Many students have yet to reach the stage of cognitive development that will allow them
to use a more sophisticated framework (Monk 1990, 1991), and change from using linear
causal reasoning to using a framework that involves the interaction of two chemicals to
form at least one product, often more. Another explanation of the persistence of the transmutation framework could be the fact that although, like the chemical reaction framework, it falls into the ontological category of explaining a “constraint-based event”
(Reiner et al., 1988), certain concepts, i.e. heat/heating, embodied in the framework, are
incorrectly classified as materials instead of constraint-based events. For this concept to
be placed in the correct ontological category, radical conceptual change (Chi, 1992) must
take place.
The previous analysis suggests why the transmutation framework may be so persistent. The explanatory power of the transmutation framework and the chemical reaction
framework is similar for explaining the phenomena included in the frameworks. It is
only when aspects that are not included in the frameworks are emphasized that the
transmutation framework starts to fail. It ignores the role of gases in combustion and is
not capable of being used to predict quantitative aspects of combustion, including
weight changes and volume changes. The transmutation framework, as an explanation
of combustion as a destructive process, works well with carbon- or hydrogen-based
substances, but is less satisfactory in explaining the combustion of metals, where there
is more solid substance left after burning and where a greater variety of products is
formed. It therefore appears that the process of conceptual change from a transmutation
framework to a chemical reaction framework will be aided by providing experience of
burning with a wide range of substances including more non-carbon/hydrogen based
substances, by identifying the products of combustion, by emphasizing the role of
gases in combustion, and by emphasizing quantitative aspects of combustion, both
weight changes and volume changes.
The chemical reaction explanation is capable of being developed to include these aspects, whereas for the transmutation explanation, this is more difficult. Whether such
conceptual change is within the cognitive capabilities of the students is not clear in the
light of Monk’s work, and further research is needed to explore this.
It appears from the current research, that many students were changing from the transmutation framework to the chemical reaction framework by adapting the former to include aspects of the later and that the extent to which they did this varied according to the
question. White (1991) describes the development of concepts as a continuous multidimensional process, that in theory never ends. In changing from one framework of
chemical transformation to another, students develop the precision with which they use
language, replace aspects of the old framework with aspects of the new, incorporate new
concepts, and sometimes retain aspects of both frameworks simultaneously. In view of
the complexity of the change between the frameworks, the process of change may take
place over a substantial period of time. Inconsistency in the students’ explanations may,
therefore, be an indicator of conceptual change.
ACKNOWLEDGEMENTS
We would like to thank Dr. Martin Monk for his help and advice in classifying the categories of students’ explanations according to the level of cognitive processing required. We are also grateful for an
Accion Integrada grant that has facilitated the collaboration between the University of Malaga and
King’s College London, University of London.
CONSISTENCY OF STUDENTS’ EXPLANATIONS ABOUT COMBUSTION
441
APPENDIX
Compressed Version of Selected Questions from Questionnaire
1. a. Please explain, in your own words, what you understand by the word “burning.”
b. What happens to a substance when it burns?
5. Is anything produced when wood burns?
Yes
No
Don’t know
Please explain your answer.
6. The candle in the gas jar goes out after a few seconds.
Figure 1. A candle burning.
a. What do you think has happened to the wax of the candle?
b. What has happened to the air in the gas jar?
c. Is anything formed that you cannot see? Please explain your answer.
7. When a match burns, you see a flame. What is the flame made of?
9. A student heated 6g of magnesium ribbon in a crucible and it burnt to form a white powder. At
the end of the experiment the student weighed the burnt magnesium and found that it now
weighed 10g. Why did the weight increase?
a. The oxygen of the air mixed with the magnesium, and because of this the weight increased.
b. When the magnesium was heated, it expanded and so its weight increased.
c. The magnesium reacted with the oxygen of the air, and because of this the weight increased.
d. The magnesium gained heat from the flame, and because of this the weight increased.
e. The result is impossible. The weight cannot have increased.
Please explain your choice:
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