Rethinking the introduction of particle theory: A substance-based framework

JOURNAL OF RESEARCH IN SCIENCE TEACHING VOL. 47, NO. 2, PP. 130–150 (2010)

Rethinking the Introduction of Particle Theory:A Substance-Based Framework

Philip Johnson,1 George Papageorgiou2

1School of Education, University of Durham, Leazes Road, Durham, UK2Democritus University of Thrace, Department of Primary Education,

N. Hili—Alexandroupolis, Greece

Received 2 October 2006; Accepted 21 January 2009

Abstract: In response to extensive research exposing students’ poor understanding of the particle theory of matter,

this article argues that the conceptual framework within which the theory is introduced could be a limiting factor. The

standard school particle model is characterized as operating within a ‘‘solids, liquids, and gases’’ framework. Drawing on

an analysis of scientific ideas on matter and research into students’ understanding, issues arising from the framework are

identified which could contribute towards students’ well known difficulties. The analysis leads to the proposal for a

particle model based within the framework of the concept of a substance. Results from two exploratory studies using the

substance-based particle model with children (ages 9–10) in two contrasting elementary schools in England are then

reported. After a short teaching intervention with a class in each school, individual interviews were held with a sample of

12 students from each class. Data were collected on students’ understanding of substances coexisting in different room

temperature states and phenomena involving changes of state and mixing. The results gave useful feedback on the

specification of the model and its teaching. Overall the students’ engagement with the particle ideas was encouraging and

suggests a larger scale testing of the substance-based model is merited. � 2009 Wiley Periodicals, Inc. J Res Sci Teach

47: 130–150, 2010

Keywords: particle theory; misconceptions; change of state; primary science; secondary science

The particle theory of matter is included in the early secondary curriculum (ages 11–14) of most

countries (Martin, Mullis, Gonzales, & Chrostowski, 2004). However, research indicates poor understanding

of the theory amongst secondary students. There is an argument for moving the introduction of particle theory

to later in the curriculum (Fensham, 1994; Harrison & Treagust, 2002). The cognitive demands of such an

abstract model are thought to be too great for most early secondary students. Indeed, the authors of the

National Science Education Standards (National Research Council, 1996) postponed particle ideas until high

school (ages 15–18) on these grounds (Stern, 2003). This article offers another explanation for why students

are finding the particle theory so difficult and a possible way forward. Our concerns focus on the conceptual

framework within which particle ideas are introduced. We characterize common practice as operating within

a ‘‘solids, liquids, and gases’’ framework. Instead, we propose a framework centered on the concept of ‘‘a

substance.’’

This article has three parts. The first part gives the theoretical rationale. Here the argument draws on two

strands: an examination of key ideas contributing to a scientific understanding of matter and the research into

students’ understanding of matter. We highlight the importance of the concept of a substance and show how a

‘‘solids, liquids, and gases’’ framework is scientifically inaccurate and why this could account for students’

difficulties with conventional particle model teaching. In Part 2, we present a particle model based on the

concept of a substance and discuss its implementation and development. Finally, Part 3 reports the results

Correspondence to: P. Johnson; E-mail: [email protected]

DOI 10.1002/tea.20296

Published online 14 April 2009 in Wiley InterScience (www.interscience.wiley.com).

� 2009 Wiley Periodicals, Inc.

from two small-scale studies which explored the feasibility of our substance-based model with children aged

9–10.

Part 1: The Theoretical RationaleThe Concept of a Substance and Particle Theory

The idea of a substance (pure substances) became established in the middle of the eighteenth century and

is fundamental to modern chemistry (Caldin, 2002; Toulmin & Goodfield, 1962). Figure 1 presents a concept

map which organizes typical lower secondary school content relating to matter and its behavior (Martin et al.,

2004) around the concept of a substance.

Explaining the content in Figure 1 with particle theory involves two distinct levels. A basic model

referring to the particles of a substance can deal with changes of state and mixing/separation (physical

changes). At greater resolution, identifying a substance with an atom structure (which atoms are bonded to

which) accommodates the phenomenon of chemical change. Change in which atom is bonded to which gives

new atom structures; that is, new substances. The distinction between ‘‘substance’’ particles and atoms is

crucial. Atoms are not necessarily substances. Oxygen atoms can form both ‘‘oxygen’’ (O2) and ozone (O3).

Oxygen and ozone are different substances, with different properties. A single atom of oxygen is neither

oxygen (the substance) nor ozone.

Research into Students’ Understanding of Substances and Particles

Over recent decades, many studies have addressed students’ understanding of one or more of the

phenomena covered by Figure 1, across a wide range of ages. There are several reviews of this work (e.g.,

Andersson, 1986; Barker, 2002; Driver, Squires, Rushworth, & Wood-Robinson, 1994; Gabel & Bunce,

1994; Garnett, Garnett, & Hackling, 1995; Krnel, Watson, & Glazar, 1998; Liu, 2001; Smith, Anderson,

Krajck, & Coppola, 2004; Wiser & Smith, 2008) and our dealings here will be brief.

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Figure 1. ‘‘A substance’’ as the organizing idea for matter and its behavior.

RETHINKING THE INTRODUCTION OF PARTICLE THEORY 131

Journal of Research in Science Teaching

The Concept of a Substance. As a whole, the literature suggests few secondary school students share

current scientific thinking on substance identity. For example, Stavridou and Solomonidou (1989, 1998)

found that 8- to 18-year olds did not use properties to identify substances and thus had no means of

distinguishing between physical and chemical changes. Instead of properties, students seem to use historical

criteria to assign identity (Johnson, 1996, 2000; Talanquer, 2006). Historical criteria, here, refer to where

something has come from and what has been done to it. Studies also report students (of all ages) having only

very vague perceptions of the gas state (e.g., Johnston & Driver, 1991; Lee, Eichinger, Anderson,

Berkheimer, & Blakesee, 1993; Piaget, 1929; Russell, Longden, & McGuigan, 1991; Sere, 1985; Stavy,

1988). The evidence suggests most students are far from thinking of ‘‘gases’’ as being substances.

Awell defined substance concept does not exist in everyday language (De Vos & Verdonk, 1987). As an

alternative categorization, Johnson (1996, 2002) reports students holding the idea that ‘‘solids,’’ ‘‘liquids,’’

and ‘‘gases’’ are three types of matter—literally separate species. Changes of state are then seen as puzzling

exceptions. A recent survey involving 5,500 students (ages 11–14) from 45 schools across England found a

very high prevalence of ‘‘three types of matter’’ thinking (Johnson, Tymms, & Roberts, 2008).

Particle Ideas. For our purposes, we focus on the relationship between matter and ‘‘substance’’

particles within students’ reported responses. In this regard, three distinct models can be discerned from the

research literature:

Model A: The particles are in the continuous matter, but not of it.

Model B: The particles are the matter, but they have a macroscopic character.

Model C: The particles are the matter. They do not have a macroscopic character.

For Model A, the particles are extra to the matter—they are something else (e.g., Ault, Novak, & Gowin,

1984; Lee et al., 1993). Students holding Model A may draw seemingly acceptable particle diagrams and talk

appropriately about differences in movement for the states. However, when asked to locate the matter they

shade the region between the particles (e.g., Johnson, 1998a). Accordingly, particle movement is determined

by the state of the matter in which they are embedded. It could be argued that Model A is, essentially, a

continuous view of matter.

In an extreme form of Model B, macroscopic properties are explained by the physical nature of the

particles, entirely. Thus, an individual particle of copper is hard, malleable, conducts electricity and copper

colored (Ben-Zvi, Eylon, & Silberstein, 1986). Melting, for example, is explained by each individual particle

melting (e.g., Griffiths & Preston, 1992). In a more developed Model B, significance is given to changes in

particle movement and arrangement for changes of state, alongside changes in individual particles (Johnson,

1998a). Students with Model B can also have difficulty in accepting empty space and may suggest ‘‘air’’ is

between the particles (Lee et al., 1993; Nussbaum, 1985). Such responses could be interpreted as retaining a

continuous view of matter. However, given students’ difficulties with the gas state, ‘‘air’’ does not necessarily

mean matter in the scientific sense. The key distinction between Models A and B is that the identity of the

matter rests with the particles in the latter but not in the former.

No study has established the frequencies of Models A and B within a representative population.

Nevertheless, the international research suggests few hold the scientific view (Model C), where the

characteristic properties of a state are explained, entirely, by the collective behavior of the particles (their

movement and spacing). Logically, with either Model A or B, it is difficult to see how ideas of atom structures

can make any sense (let alone ideas of atomic structure). As we see it, the first challenge for science education

is to develop a ‘‘substance’’ particle model.

Keeping the scientific view of matter and students’ ideas in mind, we now turn to an analysis of how

particle theory is taught in schools.

The Particle Model Used in Schools

Through the examination of research articles, De Vos and Verdonk (1996) offer the list in Figure 2 as a

description of introductory particle theory in science education. Drawing on the National Science Education

Standards (National Research Council, 1996) and Benchmarks for Science Literacy (American Association

132 JOHNSON AND PAPAGEORGIOU


for the Advancement of Science, 1993), Stern and Ahlgren (2002) have defined a set of ideas relating to the

Kinetic Molecular Theory (Figure 3). Both descriptions are consistent with long standing practice in England

and Greece (Department for Education and Employment, 1999; Department for Education and Skills, 2003;

Greek Pedagogical Institute, 2003). Together, we take Figures 2 and 3 to represent the standard particle model

promulgated in schools across many countries.

We contend that this teaching model does not give sufficient attention to the concept of a substance. The

model is set within a framework where particle ideas are applied to ‘‘solids, liquids, and gases’’ rather than the

states a substance could be in. Three issues stem directly from this which could cause difficulties for students.

Firstly, particle identities are unclear. Secondly, three types of matter are implied. Thirdly, the standard model

fails to reconcile boiling and evaporation into the air below boiling point. Each of these issues is discussed

below.

Matter and Particles. Figures 2 and 3 both begin with the generalization, ‘‘matter is made of particles.’’

What is a learner to make of this? Matter covers all kinds of familiar materials such as wood, sugar (sucrose),

water, and orange juice. If ‘‘gases’’ are considered to be matter, such things as carbon dioxide and air will be

included. ‘‘Matter is made of particles’’ makes no distinction between materials that are substances and those

that are mixtures of substances (a key distinction in Figure 1). However, as we have noted, basic particles are

particles of substances. Scientifically, there are no wood, orange juice and air particles in the same way as

1. All matter consists of entities called particles. Individual particles are too small to be

seen. They behave as hard, solid and (except in chemical reactions) immutable

objects. Their absolute dimensions and shape are usually irrelevant. In drawings the

particles may be portrayed as small circles or dots.

2. Motion is a permanent feature of all particles, because of the perfect elasticity of

collisions. There is a direct relation between the temperature of an amount of matter

and the average kinetic energy of its particles.

3. In a gas the empty space between particles is much larger than that occupied by the

particles themselves. Particles of a gas in an enclosed space are evenly distributed,

implying that gravity has a negligible effect on them.

4. There is a mutual attraction between any two particles, but its magnitude decreases

rapidly with distance. In a gas the attraction is negligible, except at high pressure and

at low temperature, when it may cause the gas to condense into a liquid or a solid.

5. In liquids and solids the particles are much closer together and subject to mutual

attraction. In solids the particles are arranged in regular patterns, with each particle

being able only to vibrate around a fixed position. In liquids the particles are

irregularly arranged and move from place to place.

6. Different substances consist of different particles, but all particles of one substance

are mutually identical. A mixture contains particles of two or more different species.

7. In a chemical reaction particles behave as if they consist of one or more sub-entities

called atoms, which are conserved in the reaction. A reaction is therefore a

rearrangement of atoms. Each of the approximately 100 chemical elements has its

own kind of atoms.

8. An atom consists of a nucleus with a positive charge surrounded by a number of

negatively charged electrons. Charged particles obey Coulomb’s law. Chemical

bond formation as well as electric currents are described in terms of the mobility of

electrons.

Figure 2. Introductory particle theory in science education (De Vos & Verdonk, 1996).



there are sugar, water and carbon dioxide particles. Moreover, strictly speaking, the idea of a state only applies

to a pure sample of a substance (Figure 1). Some mixtures do exhibit the characteristics of one of the states but

many do not (e.g., gels and pastes). Mixtures do not have precise melting points, for example, chocolate.

Explaining the more complex behavior of some mixtures is not so straightforward. Figure 3 makes no

mention of substances. Statement 6 in Figure 2 does make a link to substances, but why not start there and

avoid any ambiguity? If all of matter is understood to cover substances and mixtures of substances (Figure 1)

the universality of the theory still stands. However, the clearer cut application to pure samples can be dealt

with first. Various types of mixture can be left for later as appropriate and circumstances in which

approximations are acceptable can be appreciated (e.g., talk of ‘‘air’’ particles when distinction between the

different substances in air is not important).

As in Figure 3, the confusion is often compounded by immediate talk of atoms and molecules.

Scientifically, this terminology relates to the structures of substances. To reiterate, atoms are not necessarily

substances. However, if ‘‘atom’’ and ‘‘molecule’’ are introduced before their different meanings can be

understood, students can only treat the two terms as being synonymous. This would lead to a conflation of the

two levels of particle model noted earlier. At some point, the particles of a substance will need interpretation

in terms of atoms. However, to cover all familiar substances (e.g., copper and common salt), giant structures

as well as molecular structures will need to be recognized. A language speaking of molecules only is not

differentiated enough (and a decision when to distinguish ions from atoms is also required). Statement 7 in

Figure 2 links atoms to elements. In general terms the link is fine, but it does not necessarily differentiate

between the particles of elementary substances (which may consist of more than one atom) and individual

atoms.

Three Types of Matter? A particle model ought to be able to account for two related phenomena:

� that a substance can be in any one of the three states; and

� that different substances (under the same conditions) can coexist in different states.

In other words, one needs to understand why different substances have different melting and boiling

points. A ‘‘solids, liquids, and gases’’ framework does not afford a clear explanation. The extract below from

a. All matter is made up of particles called atoms and molecules (as opposed to being

continuous or just including particles).

b. These particles are extremely small – far too small to see directly through a

microscope.

c. Atoms and molecules are perpetually in motion.

d. Increased temperature means greater molecular motion, so most materials expand

when heated.

e. Differences in arrangement and motion of atoms/molecules in solids, liquids and

gases:

In solids, particles (i) are closely packed, (ii) are [often] regularly arranged, (iii)

vibrate in all directions, (iv) attract and “stick to” one another.

In liquids, particles (i) are closely packed, (ii) are not arranged regularly, (iii) can

slide past one another, (iv) attract and are loosely connected to one another.

In gases, particles (i) are far apart, (ii) are randomly arranged, (iii) spread through the

spaces they occupy, (iv) move in all directions, (v) are free of one another, except

during collisions.

f. Explanation of changes of state – melting, freezing, evaporation, condensation – and

perhaps dissolving in terms of changes in arrangement, interaction, and motion of

atoms/molecules.

(Authors’ emphases)

Figure 3. Kinetic Molecular Theory (Stern & Ahlgern, 2002).



a recent National Strategy document in England (DfES, 2003), which focuses on the teaching and learning of

particles for the 11–14 curriculum, illustrates the problem:

� Particles in solids are closely packed, held by strong forces. They cannot move from a fixed point,

except to vibrate, and have very small spaces between them.

� Particles in a liquid are loosely packed in random arrangement with very small spaces between them.

The forces between particles in a liquid are weaker than in solids and the particles can move around

each other.

� Particles in a gas have, on average, larger spaces between them than in liquids or solids. The particles

in a gas move in straight lines and the forces between the particles are very weak except when they

collide. (Handout 2.9, p 43.)

In Greece and England, the classification of room temperature samples of materials into ‘‘solids,’’

‘‘liquids,’’ and ‘‘gases’’ usually precedes the introduction of particle theory (Greek Pedagogical Institute,

2003; Qualifications and Curriculum Authority, 2000). However, there are no such things as ‘‘solids,’’

‘‘liquids,’’ and ‘‘gases.’’ For substances, the state at an arbitrary temperature has no fundamental significance.

In terms of forces, the extract statements are consistent with different substances in different room

temperature states (e.g., iron, water, and oxygen). However, it is not clear whether these forces are a

determining factor for, or a consequence of, being ‘‘solid,’’ ‘‘liquid,’’ or ‘‘gas.’’ As presented, the forces are

linked to a category of matter which implies the latter: the forces are descriptive rather than predictive. The

question of why different substances have different melting and boiling points is not addressed. Furthermore,

it is not easy to reconcile these descriptions of forces with a substance changing state. On melting there is a

minimal change in average force since there is only a small change in average distance. With respect to the

relative strengths of forces, what might be true for different substances (e.g., iron and water) does not apply to

a substance in the solid and liquid states. A sophisticated understanding of the relationship between force and

distance (Figure 2, statement 4) is needed to make sense of the forces operating within the gas state. This is

surely far beyond an introductory level and as in Figure 3 and the DfES (2003) document usually not

mentioned.

Overall, we would argue that the ‘‘solids, liquids, and gases’’ framework has the danger of misdirecting

students to the idea that ‘‘solids,’’ ‘‘liquids,’’ and ‘‘gases’’ are three fundamentally separate types of matter

(Johnson, 1996). In this context, examples of category (state) change are confusing anomalies. Moreover,

three types of matter would seem to uphold Model B (three types of particle for three types of matter), thus

precluding Model C. In other words, Model B is encouraged to the detriment of developing Model C. Three

types of matter thinking is also compatible with Model A.

Reconciling Evaporation Below Boiling Point and Boiling. In the absence of the concept of a

substance, a ‘‘solids, liquids, and gases’’ framework makes no distinction between boiling and evaporation

below boiling point. Both are treated as a change of state. However, if water can change to the gas state at room

temperature, why does water need to be at 1008C to boil? The difference between a pure sample of water in the

gas state and a mixture of water and other substances (air) is ignored (see Figure 1). Furthermore, a full

reconciliation of evaporation at room temperature and boiling involves ideas of energy distribution amongst

particles. Otherwise, water particles only having enough energy to move apart from each other at 1008C sits

uneasily with the same particles escaping into the air at room temperature. Johnson (1998c) found evidence of

early secondary students struggling with this issue. Gopal, Kleinsmidt, Case, and Musonge (2004) report

second-year chemical engineering undergraduates believing that evaporation required a temperature

gradient. To avoid the problem, it is not uncommon for students to invoke the agency of the Sun in an

unspecified mechanism (e.g., Bar & Galili, 1994; Tytler, 2000). We suspect that ideas of energy distribution

are rarely introduced when evaporation at room temperature and boiling are addressed.

Ideas of energy distribution also allow one to understand why water can exist as part of a gaseous mixture

well below 1008C; that is, that the substance water can be in the air and so be a source of condensation on

cooling the mixture. Consistently, studies report droplets appearing on a cold object as one of the most

challenging events for students (Bar & Travis, 1991; Lee et al., 1993; Osborne & Cosgrove, 1983; Paik, Kim,

Cho, & Park, 2004; Tytler, 2000).



Together, we believe the above criticisms represent a serious indictment of the ‘‘solids, liquids, and

gases’’ framework for introducing particle ideas. Without linking the model to substances, the identities of the

particles are unclear, ideas of atoms are not distinguished as operating at a sub-substance level and there is no

distinction between the behavior of substances and mixtures. By failing to address the question of why

different substances have different melting and boiling points the issue of substance identity is ignored. At

worst, the ‘‘solids, liquids, and gases’’ framework might cause the students’ misconceptions noted earlier. At

best, the framework does little to remediate. Above all, it does not seem fanciful to suggest that the ‘‘solids,

liquids, and gases’’ framework entrenches the idea of three separate types of matter and hence three types of

particles (solid, liquid, and gas). From that perspective there is nothing for the model to explain. Add in the

failure to reconcile boiling and room temperature evaporation and there may be little to establish the

fruitfulness (Posner, Strike, Hewson, & Gertzog, 1982) of the standard school model in students’ minds.

We now turn to our proposal for a model based on the concept of a substance.

Part 2: A Particle Model Based on the Concept of a Substance

In addition to the foregoing analysis, our proposal is informed by an Ausubelian view of meaningful

learning (Ausubel, Novak, & Hanesian, 1978). Ausubel et al. attach central importance to the general,

inclusive ideas of a discipline which can provide anchorage for the learning of new ideas. We would argue that

the concept of a substance is a general, inclusive idea for understanding matter.

Our starting point is to establish the beginnings of the concept of a substance. Melting behavior is used to

distinguish between substances and mixtures of substances, and melting point is introduced as a means of

identifying a substance. As well as being a logical point of departure, research suggests students find melting

one of the less difficult events (e.g., Lee et al., 1993). The particle model is introduced to explain why different

substances melt at different temperatures. We should emphasize that we do not see melting, or any other

phenomenon, as evidence for particles. Rather, the model is presented as an instrument for thought, offering

unifying descriptions and successful predictions (Meheut, 1997). We will now present our specification for an

introductory substance-based particle model and discuss its application and further development to cover all

phenomena in Figure 1.

A Particle Model to Explain a Substance and Its States

The bullet points below, define a model to explain different melting points.

� A sample of a substance is a collection of extremely small particles.

� The particles of one substance are all the same and have a particular shape.

� The particles have an ability to hold on to each other—the strength is different for different

substances.

� The particles are always moving in some way—they have energy of movement.

� Heating gives particles more energy to move.

� Otherwise, the nature of individual particles is not known.

Explaining different melting points demands an account of the solid and liquid states, but these are a

possibility for each substance. At the outset, prominence is given to differences in ‘‘ability to hold.’’ The state

of a substance is seen as the outcome of an opposition between ability to hold and energy of movement (which

depends on the temperature of the sample). On melting, the particles have enough energy to overcome the

hold and move around, but still stay close together. Since ability to hold is a property of the particles it does not

change even if distances change (and the average force changes)—the potential remains. Coupled with an

emphasis on no change to the nature of the individual particles (whatever that might be), the intention is

to avoid cultivating any ideas of ‘‘solids’’ and ‘‘liquids’’ as types of matter. (NB. We prefer ‘‘hold’’ to

‘‘attraction’’ since this is more consistent with the idea of a bond as a balance between attraction and

repulsion. In this way, the model anticipates later refinements which are consistent with this simpler version.)

After establishing the model for melting (and freezing), students can be invited to speculate about the

continued heating of a liquid sample. At some point will the particles have enough energy to move apart? If so,

what might the sample look like? These questions can lead to the demonstration of a drop of water turning to a



large clear volume when injected into a closed hot gas syringe (well above 1008C). Thus the model is used to

predict the gas state for a sample of a substance. In keeping with the research evidence, there is no assumption

that students have any real understanding of what ‘‘a gas’’ is. Furthermore, a longitudinal study has suggested

that particle ideas can open up the possibility of the gas state in students’ thinking (Johnson, 1998b). The

argument that ‘‘gases’’ at room temperature are substances where the ability to hold is very weak follows.

Although a full understanding of room temperature states requires the relationship between

temperature, entropy and energy (Johnson, 1990), we believe seeing ‘‘ability to hold’’ as the determining

factor is a helpful first stage. Assuming a rough equivalence in the energy of movement for different

substances at room temperature seems preferable to leaving students with the non-scientific idea of three

types of matter. Particles of a substance in the gas state at room temperature have more freedom than a

substance in the solid state, but they do not necessarily have more energy. Given students’ difficulties with the

idea of empty space between particles, we recommend the initial focus is on the particles being the substance

and their closeness for both the solid and liquid states. In time, and especially when a sample of gas is

understood as being a substance where the particles are far apart, the logic of empty space can be pursued.

The foremost reason for introducing particle ideas is to explain why different substances coexist in

different states, thereby countering any ideas of ‘‘solids,’’ ‘‘liquids,’’ and ‘‘gases’’ as three separate types of

matter. In this respect, the idea of ‘‘substance’’ particles should apply to all substances. Clearly, this is a

compromise for ionic substances, but from a student’s perspective it makes no sense to exclude them. For

example, surely, it is far better for students to appreciate that substances such as common salt (sodium

chloride) melt and boil, too. We will return to this issue later. Without further development, the model can also

account for dissolving. Here, it must be noted that the specific holding ability only extends to particles of the

same substance. The model cannot predict hold strength between particles of different substances. A full

understanding of solubility requires entropy, again, which is rarely considered below degree level.

Nevertheless, a general notion of compatibility could be introduced to explain very different solubilities in

different solvents (e.g., common salt in water and hexane respectively). Mixtures that are not solutions (e.g.,

gel, paste, emulsion, mist, and foam) could be tackled, if desired.

Developing the Model to Explain Evaporation into the Air below Boiling Point

As noted earlier, reconciling boiling and evaporation below boiling point involves the idea of an energy

distribution. Much can be done with a simplified picture entailing high, medium, and low energy particles and

changes in their proportions as temperature changes. Evaporation of water at room temperature is explained

by the escape of high energy particles (which can overcome the hold) and the ejection of others by

bombardment with high energy air particles. If desired, the latter can be interpreted as an overall transfer of

energy from the room as a result of the cooling effect of the former. Thus, water particles leave individually,

one by one. Boiling is contrasted as a collective phenomenon. It takes more than one particle to form a pocket

of gas inside the liquid (a bubble). This only happens when there are enough high energy particles at any one

time; that is, at a certain temperature.

Once evaporated, the continued existence of water as part of the air demands further elaboration. A

distribution of energies will re-establish amongst the water particles and the chances of two competing

processes need to be considered:

(a) medium and low energy water particles meeting up and joining—which largely depends on their

overall concentration (humidity); and

(b) a nascent droplet being broken up by a high energy particle (most likely air)—which depends on

temperature.

If b > a, the water particles will remain separated. If a > b, droplets of condensation will form. Thus,

the appearance of condensation on cooling a mixture of water and ‘‘air’’ and variations in the required

temperature can be explained.

Developing the Model to Explain Chemical Change

To accommodate chemical change, the next stage is to introduce atoms. A ‘‘substance’’ particle is now

defined in terms of bonded atoms, where a bond is a strong hold. Here we advocate immediate differentiation



between molecular and giant structures. For molecular structures the ‘‘substance’’ particles are the molecules

and in giant structures they are the repeating unit (both represented by the formula of a substance). Looking

ahead, the ground is prepared for atomic structure and types of bonding, but at this stage it is important for

students to realize that they cannot be expected to understand why different structures arise. The two types of

structure account for the wide range in melting and boiling points. Introducing the two structures on an equal

footing will hopefully counteract any tendency to think in terms of molecules when students meet ionic

structures later (Taber & Coll, 2002). Of course, the major advance is the ability to explain chemical change.

In parallel with the gas state, as noted earlier, we believe a macroscopic understanding of substances changing

into other substances cannot be assumed and we suggest the model should be used to predict the possibility of

such a phenomenon (Johnson, 2002). A detailed exemplification of the whole approach is given in Johnson

and Roberts (2006).

Part 3: Two Exploratory Studies

We have explored the viability of our substance-based teaching model with two Year 5 classes (ages 9–

10). The two classes were in different elementary schools in North East England. Particle theory had not been

taught to these students before and any informal learning outside the classroom was unlikely (Maskill,

Cachapuz, & Koulaidis, 1997). In this way, we hoped to gain insight into how students engaged with the ideas

on first exposure. Given their age and the available time, chemical change was not addressed. Otherwise, we

attempted to teach the ideas as discussed above.

Methodology

Participants. The first study (n¼ 30) was at the start of the academic year. The second involved the Year

5 half (n¼ 15) of a mixed year class midway through the academic year. The geographical locations and

relative performances in national assessments of the two schools suggest the participants spanned a wide

range of socioeconomic backgrounds and expected academic achievement.

Instruction. The first study comprised 4, weekly, 80-minute morning sessions (split by a break) and the

second comprised 12, weekly, 30-minute afternoon sessions. All teaching was by the first author. Appendix A

outlines the content of the studies. Both contained the same core relating to substances and their states,

evaporation below boiling point and condensation of water from air. However, a broader consideration of

properties and materials was dropped in favor of attention to dissolving and separation of mixtures for the

second study. The second study also had more extensive supporting written materials. The teaching methods

involved an unexceptional mix of small group practical work, demonstration, videos, exposition, whole class

discussion and written work.

Data Collection. Twelve students from each class were interviewed, individually, by the first author.

Based on the class teacher’s overall judgment, six boys and six girls were selected to represent the range of

expected achievement for the first class. For the second study, the practicality of reaching a total of 12 out of

the 15 drove the process. In both studies, the interviews took place a few weeks after the last lesson, over a 2-

week period. With this delay, we sought to probe more enduring understandings rather than temporarily

remembered forms of words.

The interviews were of the clinical type (Posner & Gertzog, 1982) and were essentially the same for both

studies (see Appendix B). Drawing on earlier work (Johnson, 1998a,b,c), the interviews had two distinct

phases. Using contexts met in the instruction, the first phase gave students the opportunity to apply particle

ideas to phenomena (melting, boiling, room temperature evaporation, and condensation on cooling air). In the

second phase, there was direct questioning on the particle model itself for different substances in different

room temperature states. Students were also asked to explain dissolving. For the first study, dissolving had not

been included in the teaching and this was a chance see whether the students could apply particles ideas to a

new situation. Dissolving was taught in the second study and in that respect was no different to the other

phenomena. The interviews were audio-taped and lasted around 40 minutes.



Data Analysis. All audio-tapes were transcribed by the first author, using the bullet points in Appendix

B to organize responses. Subsequent analysis identified the propositions within the responses. This led to the

construction of categories which picked out those characteristics which seemed to represent qualitatively

distinct responses for each of the events.

Results

Firstly, we report the students’ responses relating to a melting point, at the macroscopic level, since this

is the basis for introducing the particle model. Next, are their explanations for the phenomena covered in

Phase 1 of the interview plus dissolving from Phase 2. Then, we give the students’ responses to the

questioning on the particle model itself in Phase 2 of the interview. Finally, we examine the relationship

between students’ particle models and their explanations across the phenomena. Numbers 1–12 and 13–24

identify students in Studies 1 and 2 respectively.

Melting Point. Table 1 gives response categories and their frequencies to the questioning on melting and

solidifying wax, ordered by their congruence to the intended view.

The first category represents a sound understanding. These students had abstracted the idea of a melting

point as a divide between the solid and liquid states. Thus, with a melting point of 648C, a sample of wax

would not show signs of melting (i.e., any liquid) until its temperature reached 648C. On cooling from 808C, it

will start to solidify as soon as the temperature drops below 648C. (The teaching had not addressed latent heat

of fusion and solidifying at just below rather than at 648C was deemed acceptable.)

Students in the second category saw the melting point as marking the start or end of a narrow zone over

which the change of state took place. Thus, the wax might start to melt at 628C and be fully melted at 648C, or

start at 648C and be fully melted at 658C, and vice versa on solidifying. All students in both of the first two

categories appreciated that a bigger lump would take longer but still melt at/over the same temperature/s

(if maintained against any cooling effect).

The seven students in the third category thought melting started just before the melting point but did not

see the change from liquid to solid as the exact reverse. Even though the term ‘‘freezing’’ was not used by the

interviewer (to avoid influence of its everyday meaning), they said the temperature would need to drop well

below the previously discussed melting temperatures for wax to solidify. Two of these students also thought

that a small lump of wax would melt at a lower temperature than a bigger lump—their grasp on state and

temperature seems weaker than the others in this category.

The two students in the fourth category said a lump of wax would change to a runny liquid on heating, but

the idea of a melting point meant little to them. Finally, the pair in the last category did not seem to appreciate

that wax would go runny on heating. Instead, they talked about ‘‘going sticky’’ and ‘‘drying up.’’ One of these

students had been absent for the experiment to find wax’s melting point. Later, both could talk about ice and

lead changing to runny liquids, so this seemed to be a specific problem with wax.

Table 1

Descriptions of melting and solidifying wax

Melting wax response category

Number of students

Study 1 Study 2

1 Clear concept of change between solid and liquid states. Melting point/solidifyingpoint as a sharp divide

4 4

2 Clear concept of change between solid and liquid states. Melting/solidifying takesplace over a narrow temperature zone. Melting point/solidifying point marks thestart or end of the zone

2 3

3 Clear concept of change between solid and liquid states. Melting point marks theend of a narrow temperature zone over which the change takes place. Solidifyingtakes place at a much lower temperature or not sure

5 2

4 Clear concept of change between solid and liquid states. No concept of melting point 0 25 No concept of change between solid and liquid states 1 1a

aAbsent when the experiment was taught.



In relation to other kinds of materials, all 20 students in the first three categories were happy with the idea

of lead melting at a higher temperature than wax. The contrast between wax and chocolate was more of a

challenge. Sixteen noted a difference in behavior but only six (three in each study) were able to offer an

explanation in terms of purity. The rapid introduction of this distinction seems to have been an overload for

most students.

On the whole, notwithstanding the specific example of wax and the issues of solidifying temperature and

purity, all were aware of melting as a phenomenon and nearly all appreciated differences in melting points.

Explanations of Phenomena. Table 2 presents results for the five phenomena. Importantly, Table 2

shows that students did draw upon particle ideas. Each first category represents a sound application of the

model and second categories suggest a good first step along the way. Only one student (Study 2) made no

mention of particles at all. This student had considerable learning difficulties and the interview was for

reasons of inclusion rather than expectation. Omitting the bottom categories (‘‘difficult to interpret/no use of

particles’’), we will now say a little more about the responses for each phenomenon.

Table 2

Explanations for the five phenomena

Code Phenomenon and explanation category

Number of students

Study 1 Study 2

MeltingM1 A change in the movement of the particles. Idea of a hold between the particles

weakening/being overcome. Higher melting point of lead explained bystronger hold

6 4

M2 A change in the movement of the particles explains the change from solidto liquid. Differences in melting point explained by macroscopiccharacteristics

3 3

M3 Particles move apart on melting—there is no discussion of movement 0 1M4 Difficult to interpret/no use of particle ideas 3 4

Dissolvinga

D1 Solute particles separate and mix with water particles. Particle diagram showsboth types of particle

7 6

D2 Solute particles separate and mix with water. Particle diagram only shows soluteparticles

1 2

D3 Difficult to interpret/no use of particle ideas 3 4Big bubbles in boiling water (including revisit after syringe—see Appendix B)

B1 The bubbles are the substance water in the gas state. Particles move apart tocreate the gas state

3 3

B2 The bubbles are water changed to gas. Particles are mentioned but there isuncertainty about the identity of the gas—it is not simply the substance water

3 4

B3 Difficult to interpret/no use of particle ideas 6 5Evaporation below boiling point

E1 Some water particles can leave on their own (have enough energy/are strongenough), others are knocked out/lifted by air particles. Air and water particlesshown in diagrams

3 3

E2 Air particles knock out/pick up the water particles. Air and water particles shownin diagrams

3 1

E3 Water particles leave and go into the air. Only water particles shown in anydiagram

3 4

E4 Difficult to interpret/no use of particle ideas 3 4Condensation of water from the air on a cold object

C1 Water particles in the air, on cooling, cluster together to form droplets of liquidwater

1 2

C2 From water particles in the air, but it is not clear how these to form droplets ofliquid water

4 2

C3 Tentative link to evaporated water. No mention of water particles 2 2C4 Difficult to interpret/no use of particle ideas 5 6

aDue to a truncated interview, this event was not covered with one student in Study 1.



Melting. Nine of the ten students in the first category (M1) offered particle ideas spontaneously. All

used changes in particle movement (from fixed places to moving around) to explain the change of state, and

strength of hold (grip, joints, or cling) to explain different melting points. However, most saw a weakening of

the hold as the cause of melting instead of a changed balance between hold and energy due to an increase in

temperature. Four of the students kept the particles almost as close for the liquid state, but the rest seemed to

view a reasonably marked separation as a condition for translational movement. All but one displayed a sense

of intrinsic motion for the molten state, perhaps encouraged by the heating as an obvious agent.

Students in M2 used changes in movement to explain the change from solid to liquid (although five of the

six also required a significant separation). However, none used ideas of interactions between particles to

explain differences in melting point. Even though half associated the change in movement with a weakening

of the hold, all invoked macroscopic properties to explain why lead had a higher melting point than wax (e.g.,

lead is harder/stronger than wax). Four had a good sense of intrinsic motion and three used the model

spontaneously.

The one student in M3 talked of particles moving apart ‘‘because it gets too hot and it like . . . can’t take it

anymore’’—there was no mention of a change in movement to explain the runniness of the liquid state.

Dissolving. Students in the first category (D1) accounted for the disappearance of the solute (sugar/salt)

in terms of a separation of particles previously drawn together for a grain/lump. Their particle diagrams

showed a mixture of sugar/salt and water particles. Additionally, many talked about the two types of particle

clinging on to each other, which is moving towards an explanation of why in terms of interactions.

Those in D2 invoked the separation of sugar/salt particles but their particle diagrams showed water as a

continuous background. Although not essential to explaining the disappearing grain/lump, this may be a sign

of less confidence in particle ideas.

Bubbles in Boiling Water. Understanding the bubbles in boiling water rests on two crucial ideas: that a

sample of water can change into a body of gas and that the gas is still the substance water. Those in the first

category (B1) seemed to have accepted both. Furthermore, all used particle ideas spontaneously and

explained the change of state in terms of the particles being further apart inside a bubble compared to the

surrounding liquid. Four of the six students reasoned there was literally nothing between the particles. The

other two did not go this far—their later responses on oxygen (Phase 2) suggested uncertainties about the gas

state.

With varying degrees of confidence, students in B2 saw the bubbles as water changing to a gas but were

not sure about its identity. The idea of water (the substance) being in the gas state was a step too far. Everyday

use of the word ‘‘water’’ is tied to the liquid state, but there seemed to be something more fundamental than a

simple issue of terminology. At the first asking, Student 8 gave an apparently sound account of particles

moving apart (as the grip fades), but to him the change was irreversible:

S8 it started at water it probably still wouldn’t be able to go back

I why do you think that

S8 um because it has already turned into gas and it would probably be able to get back to from water

to ice but it wouldn’t be able to get back from er gas to water and go back to ice

The questioning on a drop of water injected into a hot gas syringe elicited similar difficulties. For

example, Student 7 who also applied macroscopic thinking to the particles:

S7 do the particles escape because it is so hot . . . and then start pushing out . . . seeing as there’s

loads of particles in gas . . . and they expand . . . and when they expand . . . it kind of makes

it . . .makes it a bigger area

I when you say it expands . . . do you mean the particles get bigger

S7 . . . do they get filled up with something

For her, too, it was irreversible:

I when it changes to a gas . . . is it still water

S7 . . . um . . . no . . .!



I why do you say that . . .!

S7 well say you have um a tap which . . . been really hot . . . and the steam comes up and it sticks to

the ceiling . . . and it turns back to water droplets . . .well it can’t really do that with gas I don’t

think

Other comments were: ‘‘it’s not just nothing at all, there is something in there but I don’t know what it

is’’ (Student 9) and ‘‘it would make air and . . . like different sorts of gases which pushed it out’’ (Student

21—she did not think it was the same as normal air).

Evaporation below Boiling Point. Students in the first category had begun to grasp the concept of a

distribution of energy. They distinguished between water particles that had enough energy (or were ‘‘strong’’

enough) to leave on their own and those removed by the air. None suggested why some water particles had

more energy: there was no sense of an energy exchange within continual motion. None talked of air particles

of different energies. Eviction was either by collision or a less specific sense of lifting. Some examples are

given below.

Student 7: all the particles around the outside leave because they have got more energy . . . and air

particles crash into these ones without . . .which will send some off

Student 20: when the strong particles escape and the air particles carry the other ones away

Student 17: the strong ones will come up into the air . . . say that’s some air . . . [the triangles are

what] . . . the air pieces . . . and that bit’ll come in and might knock that [knock that

one . . . that square one there] and there might be some particles already lifted . . . and

they’ll be trying to separate them . . . stop them getting back together to form a liquid.

Later, Student 17 noted the parallel between dissolving and evaporation:

the water’s going to get in between the salt . . . and like separate the particles [so what will we notice] it

will start to disappear . . . it’s a bit like when the water’s in the air.

Although not asked directly about the persistence of water particles in air, the extract from Student 17

implies a sense of intrinsic motion keeping them apart. Student 20 superimposed a circular air particle upon a

triangular water particle ‘‘to show that the air particle has caught it.’’

Those in E2 only focused on the role of air particles which, nevertheless, was providing a mechanism to

explain why evaporation could happen at room temperature without apparent heating. Student 4 made the

point as follows:

the water would have evaporated but it wouldn’t be caused by heat it would be [why would that happen

then] because air particles would be crashing into it and knocking a few water particles out [so it

wouldn’t be caused by heat] it wouldn’t . . . it could be caused a bit by heat if it was on the

radiator . . . but not where it is now

Again, some seemed to address the continued existence of water in the air. Student 18 surrounded each

water particle with air particles that had ‘‘taken it prisoner.’’ Student 8 paired them up, and Student 6 said

‘‘they grip to the air particles and turn into gas.’’ All in the first two categories drew diagrams showing both

water and air particles.

Student 21 said ‘‘air particles would come down and like grip them or something,’’ but her diagram only

showed water particles. Revealingly, she drew air as lines and showed an airborne water particle transforming

to lines ‘‘because it’s been swept away . . . and it’s turning into a gas.’’ For this reason she was placed in the

third category. E3 contains responses where the disappearance of liquid water was explained in terms of water

particles going into the air, but their fate and the role of the air was more of a mystery. None of the students in

E3 showed air particles in any diagrams.

Finally, it should be noted that over the Study 1 interviews, a number of students made reference to a

subsequent lesson with their class teacher on evaporation, which seemed to have involved radiators and a



particle role play. The extent to which this was aligned with Appendix A is unknown, but it was noticeable that

all used particle ideas spontaneously for this phenomenon. For Study 2, half needed a gentle nudge.

Condensation of Water from the Air on a Cold Can. Qualification for the top category required a clear

sense of separated water particles joining together to form the droplets on the can. For example, Student 4:

S4 the water particles what’s gone to gas hit that and cool down . . . and become water

I can you draw a picture of water particles when in the air and then what it is like on the can

S4 this is for when in the air like zoom around freely [he draws well separated particles]

I so those are water particles . . .would there be any air particles there as well . . .!

S4 yes [he surrounds each water particle with air particles]

I so green ones are the air particles

S4 yes there’ll be a lot more air . . .!

I and then what would it be like on the side of the can so you are showing me now the

S4 they’ve gone back together [he draws a tight cluster of water particles]

I and the reason they’ve gone back together is

S4 because the . . . they don’t have enough heat to give them energy any more

However, as above, the focus was on the energy of the water particles, rather than that of the air

particles which had been keeping the water particles apart.

Those in C2 identified water particles in the surrounding air as the source, but their explanation for the

appearance of the liquid state was not clear. A response such as ‘‘the water particles in the air when they hit

that they made condensation’’ (Student 8) leaves too much uncertainty. There could be thoughts of

clustering but it could also mean a change to the individual particles (gas to liquid). Student 7 used the idea

of joining together but only after a change to the particles:

could water particles cling onto the can . . .well they’re not really water particles at the moment . . . but

they are in a different form and when they touch something cold . . . like this . . . they turn back into

water particles and then into water droplets [earlier she had spoken at length about water particles

joining together to make droplets in clouds]

This is not necessarily a contradiction to her stance on the syringe experiment (above) where she did

not think the gas could go back to water: water changed to a gas by strong heating could be different to

water evaporating gently into the air at room temperature.

Students in the third category (C3) had a vague sense of a connection to evaporated water. They did, at

least, show some appreciation that water could exist as part of the air. However, they made no mention of

water particles. For example, Student 10, ‘‘does it come from the air somehow?’’.

Students’ Particle Models. Based on the particle models discussed in Part 1 of this article, Table 3

categorizes responses in the second phase of the interview. Here, students were asked directly about the model

for different substances in different room temperature states. Earlier applications to the phenomena were

highly consistent with these later responses and in a few cases helped to decide the classification. Model C

Table 3

Particle models

Particle model

Number of students

Study 1 Study 2

C: Particles are the substance and properties of state are explained by their collective behavior 1 1B: The particles are the substance, but they have macroscopic character (they are literally

small bits of it).7 7

U: A substance is made of a particular combination of particles 1 0A: The particles are in the continuous substance but not of it (they are something else) 3 3X: Continuous substance—particle ideas have no meaning 0 1



students drew particles close together for the solid and liquid states, apart for the gas state and reasoned the

intervening space was nothing. Different room temperature states for sugar/salt, water, and oxygen were

explained by different strengths of hold. When asked to imagine touching single particles of these three

substances, they did not think there would be any differences (apart from shape or size). Students (11) who

said single particles would feel like the macroscopic room temperature sample were placed in Model B. Three

further students (two in Study 1 and one in Study 2) were also assigned to B. These students had indicated

that the particles were the substance but their responses on single particles were insufficient to

distinguish between B and C. Their answers in Phase 1 were far more in line with B than C. For those

in Model A, the nature of the particles was unknown (as with model C) since the particles are extra

to the continuous substance. That said, two of the six students did show Model B thinking for a substance

in the solid state. Model X speaks for itself—the one entry is the very weak student noted earlier.

Finally, the student categorized as Model U was an unusual case. For her, no one particle was the substance but

together the particles were the substance. This view is akin to thinking at the atom level, but there was no

suggestion that she knew about atoms: she said it was her own invention (perhaps thinking in terms of

ingredients).

Overall Profiles across the Interview. The six students with Model A and the one student with Model X

are in the bottom category for each of the five phenomena in Table 2. Given the nature of Model A, any

explanations mentioning particles were difficult to interpret. Using a scoring system, Table 4 presents profiles

for the remaining 17 students. Each first category in Table 2 is awarded three points, second categories two

points and, if judged to represent at least some progress towards the intended understanding, third categories

are scored one. This gives an equal weighting to each phenomenon which allows crude ranking of both

students and phenomena.

It is no surprise to see one of the two Model C students heading Table 4 with a full score. Unfortunately,

an unguarded remark deprived the other of the chance to match this. Caught up by the overall quality of the

response, the interviewer interjected with the idea of particles gathering for condensation. Almost certainly, if

allowed it would have been volunteered by the interviewee. Overall, these two students, one from each study,

demonstrated very impressive achievement.

Table 4

Student profiles

StudentID Study

Particlemodel

Phenomenon score

Melting Dissolving Boiling Evaporation Condensation Total

17 2 C 3 3 3 3 3 155 1 C 3 3 3 3 2 14

19 2 B 3 3 3 3 2 1418 2 B 3 3 3 2 3 144 1 B 3 3 3 2 3 146 1 B 3 3 3 2 2 137 1 B 3 3 2 3 2 138 1 B 2 3a 2 2 2 119 1 U 3 3 2 3 0 11

21 2 B 3 3 2 1 1 1022 2 B 2 3 2 1 2 1023 2 B 2 2 2 1 1 820 2 B 2 3 0 3 0 810 1 B 3 2 0 1 1 711 1 B 2 3 0 1 1 712 1 B 2 3 0 1 0 624 2 B 1 2 2 1 0 6

43 48 32 33 25

aThis is an estimated response. The event was omitted due to insufficient time to complete the interview.



Table 4 shows that those categorized as Model B spanned a wide range of progress in applying particle

ideas to the phenomena. In fact, apart from saying that individual particles of different substances in different

room temperature states would be different, Student 19 met the criteria for Model C in all other respects.

Another (Student 18) was also close to C, but used differences in energy rather than strengths of hold to

explain room temperature states. In their explanations of the phenomena, both of these students only used the

arrangement and movement of particles (the collective dimension) to account for the macroscopic

observations.

Moving a little further away from Model C, Students 4 and 6 used hold to explain different melting and

boiling points and hence room temperature states, but drew particles of oxygen close together. Interestingly,

earlier on, both of these students had talked about water particles moving apart to form the bubbles in boiling

water (B1). When this was pointed out during the interview, their replies suggested the heat made the

difference:

Student 6: because that’s been heated up and air particles are um different to water particles and

um . . . they like . . .well when you move they come apart a bit . . . but when they are staying

still I think that they’re clinged together.

Student 4: if there was nothing else in. it would have to have. be very close together because there

would be nothing to hold it up [and for the bubble] that was trying to get away from each

other

Placing the particles close together for room temperature oxygen was common amongst the other

students in B. It is an intrusion of macroscopic level thinking and probably signals a corresponding difficulty

with the related ideas of empty space between the particles and intrinsic motion. Otherwise, Students 4 and 6

displayed much collective thinking in their explanations. If not quite there yet, their responses to boiling,

evaporation, and condensing suggest good progress towards understanding the gas state and mixtures of

water and air.

Further down the Bs, the collective dimension is strong enough to deal with phenomena involving the

solid and liquid states, but those involving the gas state are more of a mystery. That said, the idea of water

particles separating in the hot syringe allowed some to start to think of the bubbles in boiling water as resulting

from a change of the water as opposed to something else coming out of the water.

Finally, Student 9 with her unusual model (U) showed a high degree of collective thinking in her

explanations. However, she saw water’s change to the gas state (and mixing with air) as an irreversible

separation and, therefore, could not account for condensation.

Discussion

Given the differences between the two studies, direct comparison with each other must be resisted. It

cannot be argued that one was more effective than the other and for this or that reason. Furthermore, the data

do not allow a comparison between our substance-based model and a three types of matter-based model. It

must also be acknowledged that the same person carried out the instruction, interviewing and analysis. The

purpose of the two studies was to gauge the viability of the substance-based particle model and the two classes

were selected with the aim of achieving an exposure to a wide range of complete beginners.

After very short and intensive interventions, a good proportion of students in both groups made

encouraging progress. Indeed, for some it was quite remarkable. Across the phenomena, Table 4 shows that

17 of the 24 students populate at least two of the top two categories of each phenomenon (nine and eight in

Studies 1 and 2 respectively). All of these students seemed to be viewing the particles as being the substance.

Although most were categorized as Model B it must be emphasized that this was a developed Model B which

entailed a good degree of collective thinking. All of these students used changes in arrangement and/or

movement to explain melting and dissolving. None said that individual particles would melt or dissolve as

often reported elsewhere. For half (of the Model B upwards students), their model was strong enough to tackle

phenomena involving the gas state with success. The ‘‘one particle’’ question which decided between B and C

is a stern test and its fairness could be challenged. When asked so directly about individual particles of

different substances in different room temperature states, relinquishing any thoughts about the particles’

physical nature probably requires a very secure, confident grasp of the model.



As well as the viability, the exploratory studies have also provided feedback on the teaching model itself

and its implementation. The exactness and equivalence of melting and freezing points presented more of a

challenge than expected. While this did not derail the interventions, more attention here would likely be of

benefit. Furthermore, the contrasting behavior of mixtures should perhaps be left until later. One must resist

trying to explain too much at once. The concept of a substance can be introduced for what it is, first.

As anticipated, the students enjoyed more success with the solid and liquid states than the gas state. If

dissolving (solute starting in solid state) is the easiest phenomenon (Table 4), and given the issues with

melting point, there could be an argument for using dissolving as the introductory event rather than melting.

Given the danger of cultivating ideas of ‘‘solids’’ and ‘‘liquids,’’ our instinct is to caution against. Starting

with different melting points, so necessitating the idea of an ability to hold, is designed to avoid ideas of types

of matter. However, dissolving soon followed by melting and different melting points might represent a

sequence of more manageable steps. Certainly, it should be investigated as an alternative lead in. With regard

to ‘‘ability to hold,’’ most of the students explained melting in terms of the hold weakening as energy

increased rather than the changed balance between hold and energy. This subtle distinction is probably not of

great concern at an early stage. Differences in ability to hold for different substances still explain different

melting points. The students also seemed to exaggerate the increase in the spacing between particles on

changing from the solid to liquid states. So long as the accompanying change in particle movement is seen as

the only factor needed to explain the change of state, this could be allowed to begin with, though not

encouraged. Follow up work, using the example of water where the spacing actually decreases on melting

would be appropriate later on.

Using the particle model to predict the gas state appears to have promise. Extending the ‘‘drop of water in

a hot syringe’’ experiment to show the reversibility of the change on cooling may help with the idea of the

substance water being in the gas state, but ultimately this view probably depends on accepting the logic of

invariant particles and a collective account of the gas state. Compared to the solid and liquid states, a workable

model of the gas state is more dependent on ideas of empty space and intrinsic motion. In common with other

studies, the students found these to be most challenging. Ideally, more time would be spent consolidating the

model for the solid and liquids states before moving on to the gas state.

For evaporation of water at room temperature, an active role for the air in itself seemed to have more

appeal than ideas of energy distribution at a temperature. Vapor pressure aside, from the substance-based

perspective the event is a mixing phenomenon rather than the change of state as viewed from a three types of

matter framework. As noted by Student 17, the parallel is with dissolving. The idea that salt melts in water is

widely accepted as a misconception. Is the idea that water changes to the gas state when it evaporates into the

air at room temperature not also a misconception? Giving an active role to the air can explain why the event

takes place at room temperature without an obvious source of heat (as opposed to boiling at 1008C), and

perhaps should be used as a first level explanation. The fuller story involving the distribution of energies could

wait—introductory level explanations of dissolving do not draw on the concept of energy distribution.

Condensation on cooling a clear mixture of water and air can be explained, largely, by the air particles no

longer being able to keep the water particles apart rather than just the water particles losing energy (the change

of state view).

Conclusion

There seems to be a widely held consensus that students find particle theory difficult because it is

inherently difficult. We agree that the standard school model does present difficulties. However, we have

argued that the standard school model is set within a three types of matter framework and that this may be

causing unnecessary difficulties. Instead, we have proposed a model set within a framework based on the

concept of a substance. The key focus of our model is to explain why a substance can be in any of the three

states rather than ‘‘solids, liquids, and gases.’’

In our small scale exploratory studies, the 9–10 years old students’ responses to the substance-based

model are encouraging. At least, we believe there are grounds for carrying out further work on a larger scale,

incorporating the refinements to the model and its implementation discussed above. One can only speculate

upon what could be achieved by a more measured pace, reiteration and consolidation over a number of years,

but plausibly it would be much more. Older students meeting the ideas for the first time would be expected to



achieve higher levels of success. Whichever framework is used at whatever age, it seems likely that many

students will adopt Model B type thinking to some degree. However, we would argue that the substance

framework is more conducive to further development to Model C. The three types of matter framework could,

unintentionally, restrict many students to a primitive Model B. Research which compares the two frameworks

in terms of the effect on students’ learning would be instructive. However, even if the substance-based particle

model proves equally difficult, we contend it is scientifically more accurate and preferable for that reason.

Finally, the young age of our student sample deserves some comment. The appropriate grade placement

for curriculum content is an empirical question. At younger ages the proportion of students engaging

profitably with the ideas is likely to be lower and damage that compromises later learning could be done. On

the other hand, early introduction as part of a carefully structured long term plan gives more time for

understanding to develop. If the results reported here have wider application, they would support Metz’s

contention that the elementary school curriculum underestimates students’ capacity for grasping abstract

ideas (Metz, 1995). Using a substance-based framework, there could be a case for an earlier rather than a later

introduction to the particle theory of matter.

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Appendix A

An outline of the two trials

Study 1School: Above average attainment in national testsClass: Year 5, n¼ 30Timing: Near the start of the academic yearDuration: Four, weekly, 80-minute morning sessions (split by a break)Content: Distinction between material and object in relation to properties

� properties that depend on the material only. Malleability, solubility, melting behavior andfloating/sinking behavior of lumps of wax, copper and salt (small group practical/demonstration)

� properties that depend on the material, amount and design. Flexibility of lengths of wood and plasticof various cross-sections (small group practical)

Using melting behavior to define a pure sample of a substance� contrasting the melting behaviors of candle wax and chocolate (small group practical)� melting point and melting samples of lead and salt (video)Using a substance particle model to explain different melting points.Predicting the possibility of the gas state� A drop of water in a hot (1508C) gas syringe (video)� Bubbles in boiling water (video)� Boiling point and its use to identify a substance (boiling wax—video)� ‘‘Gases’’ as substances with a very weak hold between their particlesA sample of a substance could be in one of three statesUsing a ‘‘developed’’ model to explain evaporation below boiling pointCondensation of water from the air (on a cold drinks can)

Study 2School: Below average attainment in national testsClass: Year 5, n¼ 15 (half of a mixed age class)Timing: Midway through the academic yearDuration: Twelve, weekly, 30-minute afternoon sessions (end of the day)Content: As above, omitting distinction between material and object in relation to properties, plus:

Dissolving in water and explanation with ‘‘basic’’ particle model� solutes in solid and liquid states (small group practical work) and gas state



Appendix B

An outline of the interviews for both studies

Phase 1

Thinking was probed in the areas below. A gentle prompt for particle ideas was given if not invoked

spontaneously by the interviewee.

(a) Melting� What happens when lumps of wax and chocolate are heated and the reason for different melting

behavior (if necessary, a prompt for the different behavior was given).

� How to find the melting point of wax and whether this can be done for chocolate.

� What happens if wax at 808C is left to cool.

� How the melting point of a small lump of wax compares to a large lump.

� An explanation for why water, lead, salt, and wax can melt.

� Why wax and lead have different melting points.

(b) Boiling water

� What happens if a container of water is heated on a cooker (if necessary, a prompt for the big bubbles

was given).

� What the big bubbles are and why they form.

� What happens to the level of water in the container.

� An explanation for the drop of water in hot gas syringe experiment. Revisiting a beaker of boiling

water.

� What are oxygen, carbon dioxide and air.

(c) Evaporation

� What happens if a saucer of water is left in a room and why.

(d) Condensation

� Why droplets appear on the surface of a cold drinks can, previously wiped dry.

Phase 2

The questioning here probed the interviewee’s understanding of the model for substances in different

states at room temperature. Dissolving as a phenomenon was also covered.

� The interviewee’s drawing of some of the particles for a grain of sugar (if they could be seen).

� The interviewee’s drawing of some of the particles for a pool of water (if they could be seen).

� An explanation for sugar dissolving in water.

� The interviewee’s drawing of some of the particles for a jar of oxygen (if they could be seen).

� Why samples of sugar, water and oxygen are in different states.

� Any differences if single particles of sugar, water and oxygen could be touched.

NB. For the interview in the second study, the melting behavior of chocolate compared to wax was

moved to Phase 2. Large lumps of salt were used instead of granular sugar.

Separating a mixture� filtration (small group practical work)� distillation (video)Water cycle in terms of particles (cut and stick matching exercise)



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Rethinking the introduction of particle theory: A substance-based framework