ijse -- 1998 -- children's understanding of changes of state involving the gas state

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Children's understanding of changes of state involving the gas state, Part 1:Boiling water and the particle theoryPhilip Johnson a

a School of Education, University of Durham, UK

Online Publication Date: 01 June 1998

To cite this Article Johnson, Philip(1998)'Children's understanding of changes of state involving the gas state, Part 1: Boiling water andthe particle theory',International Journal of Science Education,20:5,567 — 583

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INT. J. Sci. EDUC., 1998, VOL. 20, NO. 5, 567-583

Children's understanding of changes of stateinvolving the gas state, Part 1: Boiling water and theparticle theory

Philip Johnson, School of Education, University of Durham, UK

As part of a three-year longitudinal study which explored the development of children's concept of asubstance (ages 11 to 14), this paper reports the findings in relation to children's understanding ofboiling water and particle ideas. Evidence is presented which suggests that, for most of the pupils,particle ideas provided the means for them to begin to accept that the bubbles in boiling water were thewater changed to the gas state. The importance of this, in terms of children's understanding of a sampleof gas as a sample of a subtance, is discussed. It is argued that boiling water must be seen to have acurriculum significance which goes far beyond its association with a defined temperature.

Introduction

The scientific idea of a substance is a high-order conceptualization, contingentupon a number of other ideas (Johnson 1996). Crucially, one must appreciatethat the identity of a substance is independent of state and so a substance couldbe in any of the three physical states. Along with conceptions of the states them-selves, change of state has been an area which has received attention within therecent research into children's understandings (Pfundt and Duit 1994). In keepingwith children's understanding in other areas of science the research has high-lighted no little confusion. Within the literature we have been given 'snapshots'of the situation but this does not tell us how an individual pupil's thinking mightdevelop. This paper reports findings from a three-year longitudinal study whichhas given insights into the development of children's understanding of changes ofstate; insights which suggest a way forward for science education. The study, as awhole, explored the development of children's concept of a substance and sochange of state was but one of a number of interrelated ideas that were examined.Part one of this paper is restricted to pupils' understanding of boiling water, parttwo (Johnson 1998) will concern evaporation at room temperature and condensa-tion of atmospheric water vapour. Melting and freezing were also addressed withinthe study, but the changes involving the gas state were far more problematic forthe pupils.

Existing research

By their nature, the states and change of state feature, to varying degrees, in manystudies. Reviews covering this area can be found in Driver et al. (1985), Andersson

0950-0693/98 S12·00 © 1998 Taylor & Francis Ltd.

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568 P.JOHNSON

(1990), Driver et al. (1994), Wandersee et al. (1994), Garnett et al. (1995). Whenboiling, water is changing to the gas state. What does the literature tell us aboutpupils' understanding of this end-point? Although it seems that most children atage 11 do seem aware that there is 'something' called air all around us (Sere 1985,Russell et al. 1991), and many would call it a gas, children also regard sprays, mists(steam), flames and smoke as gases too (Russell et al. 1991). Gases can be seen ornot seen (Stavy 1988, Johnston and Driver 1991, Russell et al. 1991) and, indeed,the main qualifying characteristic seems to be an ability to 'get into the air andspread out' and, usually, 'up'. Some pupils also seem to use 'gas' more specificallyas a name for the familiar fossil fuel, and others use it more generally as somethingharmful and intangible (Sere 1985, Russell et al. 1991). Andersson (1984) com-ments:

. . . pupils tend to consider gas and air and air and oxygen as equivalent to each other.In other words, they have not understood that gas is a superordinate concept and thatthere are different gases with different properties. They say, for example, that 'air isoxygen and gas' and that 'oxygen is something you breathe, that is, air.' (p. 45)

For a task where a drop of clear liquid was vaporized in a sealed container (of fixedvolume), both Mas et al. (1987) and Stavy (1990) report most 11- to 13-year-oldsnot conserving mass. Piaget (1929) found that 'air' and 'gases' tend to be associatedwith what seems immaterial and unexplained. The overall picture, then, is verymuch one of children having only rather vague perceptions of what a gas is and anassociated looseness of terminology.

There are two main papers which specifically address the composition of thebubbles in boiling water. The first of these is Osborne and Cosgrove (1983),which, indeed, stands out as the seminal paper in the area of change of state.The events and key questions, reproduced from the paper, are given in figure 1.Only the first two of these events need concern us here.

Four commonly held views on the bubbles in boiling water are reported byOsborne and Cosgrove (1983) (numbers of students out of the sample of 43, ages 8to 18, are given in parentheses):

• the bubbles are made of heat (3);• the bubbles are made of air (18);• the bubbles are oxygen or hydrogen (6);• the bubbles consist of steam (5).

In relation to the second event most of the students identified the visible mistcoming away as 'steam', and, it was also noted that 'many of these pupils consid-ered that when steam was no longer visible it had changed into air; i.e. it hadbecome air' (p. 829). Although using different expressions, all seemed to appreci-ate the steam was turning back into water on the saucer. At this point someconcern must be registered, particularly in the light of children's perceptions ofthe gas state itself noted earlier. The authors suggest that 'steam' is the acceptablescientific view for the composition of the bubbles. However, this hardly seems fairon the students who might be noting the contrast between the clarity of the bub-bles and the mistiness of what they called 'steam' (and were asked about as steam).Given that the students talked of steam changing into air it seems a pity that thestudents' meaning for 'air', in reference to the bubbles, was not explored (Johnsonand Gott 1996). Similarly, concern must be raised in relation to a multiple-choice


CHANGES OF STATE INVOLVING THE GAS STATE, PART 1 569

Events and key questions

1. An electric jug with water boiling in it.

The question: What do you see in the jug?then: What are the bubbles made of ?then: What is boiling?

2. Steam from a jug condensing on a saucer placed above the jug.

The question: What is steam made of?then: What is on the saucer?

3. Water evaporating from the surface of the saucer.

The question: What has happened to the water on thesaucer?

4. Blocks of ice placed in a household jar and the lid screwedon.After a time moisture from the air condensed on the cold partof the jar.

The question: From where has the water on theoutside of the jar come?

5. A block of ice melting on a teaspoon.

The question: What is happening to the ice on thespoon?

Figure 1. Events and key questions.Source: Osborne and Cosgrove (1983: 826).

survey of a larger sample of students where 'air', 'steam', 'heat', and 'oxygen orhydrogen' were given as the only options; each could be interpreted differently bypupils and none seems particularly satisfactory as an answer.

The study by Bar and Travis (1991) was closely modelled on the work ofOsborne and Cosgrove. They report most children at age 11 in a category of'when water is boiled vapour is seen; this vapour is coming from the vessel' (p.368). On the basis of this they claim that 'many children know that water changesinto gas by the process of boiling' (p. 371), and later that, 'Children from a youngage have an almost correct view about boiling; they only confuse steam withvapour. Otherwise they understand that during boiling liquid is changed intogas' (p. 378). These researchers have conflated a mist with the gas state, as ifthe distinction was of no importance. This conflation is unexpected, especiallysince they also note the paradox of most of these pupils saying the bubbles were'air' in other responses. Unless the pupils were using 'air' as a general term for thegas state, and, therefore, meant that the bubbles were 'water turned into an air' itseems difficult to see how one can say they appreciated this change of state forwhat it is — the water itself becoming a body of gas.

From this work, while it seems one can say that secondary pupils are awarethat water somehow leaves boiling water as a mist (which then disappears), anyconnection with the bubbles is in doubt. This suggests that the change from theliquid to gas state is not well understood. Furthermore, the meanings of 'steam'and 'air' would seem to be problematic.


570 P. JOHNSON

Methodology

For a full description of the methodology the reader is referred to Johnson (1995)and Johnson and Gott (1996). Only a brief account giving those aspects mostrelevant to the content of this paper can be given here.

Data were collected over a three-year period (1990—93), in an English non-selective (comprehensive) secondary school, from a cohort of pupils (JV = 147) as itmoved from year 7 to year 9 (ages 11—14). The principal means of data collectionwas the periodic interviewing of a sample of pupils from the cohort (N = 33).Interviews were of the 'clinical type', using objects and events as a stimulus forquestioning (Posner and Gertzog 1982, Bell et al. 1985). Four teaching units,directly concerned with the development of the concept of a substance, formedthe background to the study. Figure 2 shows the relationship between the teaching

Academicyear

6

7

7

7

7

8

8

8

9

9

9

Dates

June-July 1990

Sept. - Nov.1990

Nov. - Dec.1990

Feb.- April1991

June - July1991

Nov. - Dec.1991

Feb. - March1992

June-July1992

Nov. - Dec.1992

Jan. - Feb1993

June - July1993

'Activity7

PRELIMINARYINTERVIEW

UNIT1Properties of

substances

INTERVIEW 1

UNIT 2Pure substances

INTERVIEW 2

INTERVIEW 3

UNIT 3The earth as a

resource

INTERVIEW 4

UNIT 4Substances and change

INTERVIEW 5

TEACHING'INTERVIEW

Figure 2. The timing of the interviews and teaching units.



units and the interviews. Figure 3 outlines the content of the teaching units inrelation to all changes of state and figure 4 gives an outline of the tasks andquestioning on boiling water only. It should be noted that, in addition to theinterviews, pupils also answered a questionnaire as part of the first teachingunit. Squaring evaporation below boiling point with boiling (a special case ofevaporation) is not an easy matter and it was for this reason that the two phenom-ena were separated within this study - for both the teaching units and the inter-views.

The findings

The findings are an unfolding story and so some discussion is necessary as part oftheir presentation. For the big bubbles in boiling water, the pupils' responsescould be placed in any of four categories. These categories are defined belowand the frequencies at each data collection point are given in table 1.

UnitlPupils had experiences of changes of state; boiling salt water to dryness, simple distillation(water from salt water), boiling water in a glass beaker, and melting ice. The followingmacroscopic explanation for the bubbles in boiling water was 'given';

About 30 "C Small bubbles start forming on the bottom of the beaker. These arebubbles of air. Cold water has air dissolved in it. Hot water can'tdissolve air very well. So, as the water gets hotter the air can't staydissolved and so bubbles out.

About 90 °C Big bubbles try to form but collapse.At 100 °C Big bubbles form, rise to the top and burst. The water is boiling. The

temperature stays at 100 "C. This is the boiling point of water. The bigbubbles are water as a gas - sometimes called steam.

The teacher's guide also suggested it be emphasised that proper steam can't be seen and that thevisible mist is condensed steam.

Unit 2

There was a heavy emphasis on exact change of state points in connection with ideas of purityand identity. Boiling points of ethanol, ethanol/water, and water samples were tested. Samplesof wax, chocolate, lead, tin and zinc were melted.Particle ideas were introduced as an explanation for the three states and change of state. For a'demonstration' of the change from liquid to gas, a flat balloon containing a little water wasplaced in a glass fronted oven (internal temperature of 140 °C). It was emphasised that theclear big bubbles in boiling water were water in the gas state and that visible steam iscondensation.

Unit 3

This considered the evaporation of water below its boiling point and condensation of atmosphericwater on cooling. A particle explanation was covered.

Unit 4

This re-examined the idea of exact change of state points, now using temperature-time graphs,and gave some prominence to the energy changes involved. Boiling water was a particularfocus. As a demonstration, an empty glass gas syringe, heated in the glass fronted oven, wasinjected with a drop of water. Again, particle ideas were revisited.

Figure 3. Outline of the content of teaching units with respect to changeof state.


572 P. JOHNSON

QuestionnaireAs a class experiment, pupils heated a beaker of water to boiling point, and maintained it therefor a few minutes. They were then given a questionnaire which sought to probe their thinkingon a number of related aspects, in the order given below;

a) What the big bubbles in boiling water are made of and whether the response wassomething the pupil Icnew already', or had just 'thought up'.

b) What happens if a beaker of water is left boiling for some time.

c) What steam is.

d) What the little bubbles are when water is first heated.

Specific opportunities were also given, after (b) and (c), for pupils to reconsider earlieranswers in the light of thoughts prompted by later questions.

Interview 1

The pupil was asked to predict the effect of heating a beaker of water for some time, and thenwhat the t i g bubbles' at boiling were.

Interview 2

This was the last section of this interview, and followed a section on particle ideas, whichincluded water.

a) Using the diagram depicting bubbles in boiling water (below) the pupil was questioned onthe big bubbles and wnat was happening to the water.

b) The pupil was then referred to his or her previously drawn particle diagram for water andasked if he or she could use particle ideas to give an explanation.

c) The pupil was asked if he or she had changed his or her mind on this matter. After a reminderof the balloon in the glass oven, an explanation of this event, using particle ideas if possible,was invited.

Interview 5

This was the first section of the interview, before ideas of particles were 'formally' addressedin the section that was to follow.

a) The pupil was questioned as for part (a) at interview 2, but with many more supplementaryquestions . If a pupil volunteered ideas of particles, he or she was asked to draw particlediagrams for the inside of a bubble and a part of the liquid'.

b) The pupil was asked if he or she had changed his or her mind since year 7. The pupil wasreminded of the 'gas syringe experiment', and asked to explain what happened, and if therewas a connection to the beaker of boiling water and any previous particle diagrams.

c) The pupil was asked about 'steam'.

d) The next section of the interview moved onto particle ideas, which were then prompted for ifnecessary. At the end of this section, any pupil who had not invoked particle ideas for theboiling water was invited to do so.

Figure 4. Interview tasks and questioning on boiling water.

Heat: The bubbles were heat. This seemed to be merely a descriptive state-ment that heat was causing the bubbles.Air: The bubbles were either air, oxygen or gas (most said air). There wasno link between the interior of the bubbles and the loss of water.Air and water: The interior was said to be air, but with some water mixedwith it.Water as a gas: The water itself was changing into a gas to form the bubble.



Table 1. Pupils' views on the composition of the bubbles in boiling water(AT = 32).

Number of pupils

Composition of bubbles Survey* Interview 1% Interview 2|§ Interview 5

Heat 4 0 0 1Air 23 21 11 9Air and water 0 0 8 2Water as a gas 4 11 12 20

Notes: * One pupil was absent% One pupil gave oxygen from the breakdown of water§One pupil gave 'energy', which was not the same as 'heat'

The interpretation of pupils' responses to this kind of questioning is not a simplematter. To begin with, one must assume that a pupil shares a basic perception ofthe phenomenon; i.e. that big bubbles form at the bottom and rise to the top, witha simultaneous loss of water from the beaker. For all but one pupil this appeared tobe the case. This pupil consistently failed to show an appreciation of the decreasein the amount of water (other than by 'boiling over') and therefore his responsesare not included in table 1 (hence N = 32).

The pupils' meaning for the terms 'air', 'oxygen' and 'gas' is particularlyproblematic. Unless one has good reason to think otherwise, these terms mustbe regarded as synonymous (Andersson 1984). Added to this is the everyday mean-ing of 'steam', which refers to a mist rather than water truly in the gas state. It washere that a pupil's use of such terms in other parts of the same interview, and inother interviews, proved most helpful in the interpretation (see Johnson and Gott1996 for an example of this kind of triangulation that could be carried out). Therewere a number of occasions where pupils used 'air' as a general term for the gasstate (as science once did) and did seem to mean 'water as an air' (and so wereplaced in the fourth category). However, many did not and did seem to mean air asof the air in the room. As far as the 'air' category is concerned, the key feature isthat the pupils seem to see no link between the interior of the bubbles and the lossof water — 'air' did not seem to mean 'water as an air'. Even so, an involvement ofthe bubbles was envisaged by some. This was a 'mechanical' association, in thesense that water was carried out by the rising hot air as the bubbles burst at thetop, e.g. for pupil 27:

I: Where in the beaker does the water come from?P: Well it just comes from all around here [points to diagram] as it comes from the

bottom .. . where it's beginning to form, the water sort of goes around the side ofthe thing.

I: What, round the edge round the outside of the bubble?P: Yes.I: Not inside the bubble?P: Ah I'm not... the bubble is air.

Another association was as a heating mechanism for the surface water; 'hot airwarms water at the top . . . and then steam rises from the top of the hot water' (pupil19). When pressed on the source of the air (at interview 1), replies revolved aroundthree possibilities:


574 P. JOHNSON

• it just appears, with no specific source cited;• the water traps the air, already in the beaker, as it is poured in;• water has air in it (e.g. for fish to breathe).

Generally, the pupils did not regard the accumulating number of bubbles, as wateris kept boiling, as an issue of concern. All the while there was water there was alsoair; the two decreased proportionately. However, six pupils, perhaps seeing theneed for a greater supply, proposed ideas of a 'cycling process'; i.e. cold air entersand goes down to the bottom (unseen) as the bubbles of hot air burst. For thesurvey, 'water as a gas' represents a collection of somewhat imprecise responses.The telling point was that it seemed that a connection between the interior of thebubbles and the loss of water was being made. For one of these pupils, part (b)triggered a change of mind from 'air' to 'hot water' for the bubbles. This exem-plifies an important point; the frequencies in table 1 represent the final responsesof the children at each stage. There were a number of occasions where pupilschanged their responses during the questioning; i.e. after they had started tothink about the matter at issue. In most cases this worked in the direction of achange from 'air' to 'water as a gas'. However, for one pupil (who was in theauthor's teaching group at the time) this worked in reverse. She did not reallybelieve the bubbles were water as a gas. Pupil 4 (interview 2) had started by saying,'the heat's making it go into a water gas kind of, but could give no explanation ofhow this could happen. The interview continued:

I: What did you used to think?P: Air.I: Why don't you think that any more?P: Because you told us it wasn't.I: What would you like to think the bubbles are?P: Air.I: Where does the air come from?P: The air in the water.I: How does it get in?P: Not sure.

Finally, two anomalous responses should be noted. One pupil (at interview 1),using knowledge of the composition of water, suggested that the bubbles wereoxygen due to the breakdown of the water. He was quite confused as to whatmight have happened to the hydrogen, and suggested that this might be thesteam (the oxygen, as a clear gas leaving unseen). He maintained this line ofthinking at interview 2, but changed to the water itself as the gas at interview 5.There were no other cases of pupils using the idea of water breaking down into itsconstituent 'gases'. Also, at interview 2, one pupil gave 'energy'. From the wholeresponse this was clearly not the same as the 'heat' category.

Changes in the responses of individuals over the study

A general move towards the idea that the bubbles are composed of water itself, butnow in the gas state, is revealed by the statistics in table 1. Figure 5 gives thechanges for individual pupils; it looks somewhat complex, but a number of pointscan be made.



Waterasagas

Oxygenfromwater

Air & water

Air

heat

Survey Interviewl Interview 2 IrterviewS

Figure 5. Changes in pupils' responses on the composition of the bubblesin boiling water.

The 'order' of the categories corresponds to the general movement betweenthe categories that was observed; i.e. it produces the least crossing of lines.The four pupils who started in the 'water as a gas' category at the surveyremained in this category over all three interviews. These pupils appear tohave started with the notion that water can change to a gas at the macroscopiclevel.After unit 1, where the macroscopic explanation for the bubbles was 'given',at best, only six of the pupils appeared to take up the idea. Of these, four 'fellback' at interview 2. However, rather than a change in actual thinking, itseems more likely that this was a function of the greater depth of probing atthe successive points. At interview 1 the questioning was still at a macro-scopic level, and as a consequence more restricted and inevitably superficial.Pupils could give a 'taught answer' with no real way to explore their answerin any depth. Pupil 4 (one of the 'fallers'), quoted earlier, admitted the pointas such. It seems likely that the other pupils were also recalling an 'answer'without real understanding. Therefore, the 'downward movement' betweeninterviews 1 and 2 is probably the result of an overestimation at the earlierpoint. This leaves only two pupils who might have taken on board a macro-scopic idea of the bubbles being water as a gas.With all but one of the pupils in the 'air and water' category at interview 2moving on by interview 5, it seems as if this category might serve as somekind of an intermediate stage.There are examples of pupils making the same 'moves' as others, but at latertimes.


576 P. JOHNSON

Based on the literature, the survey responses had been anticipated and the sub-sequent teaching in unit 1 aimed to address the 'facts' of boiling water (figure 3).Naively, it had been reasoned that the bubbles were not usually a focus of teachingand so due attention to what was happening to the water was all that was required.(It is difficult to find a standard school text that deals directly with this matter.)Interview 1 showed how wrong one can be! Why did so many of the pupils stillmaintain that the bubbles were air? Did the pupils really think the bubbles inboiling water were air, or was it that the pupils had no conception of a liquidturning to 'a body of gas' as something that can happen? Without this notion,the pupils would have no option other than to bring in a 'ready-made gas' (air),by whatever means, to account for the bubbles. The most telling response, givenby one of the most able pupils, when pressed on the source of air was: 'I honestlydon't know' (pupil 22, interview 1). Such an interpretation led to the constructionof the glass-fronted oven, and its use in units 2 and 4 (figure 3). Unless one hasalready accepted the bubbles as water in the gas state, an open beaker of boilingwater, where water seems to merge into the air (via visible 'steam') does notprovide strong perceptual support to the idea of liquid water itself becoming a'body of gas' just like the air. However, this on its own appeared to make littleimpact (very few of the pupils freely recalled the 'demonstration' at interview 2).What did seem to make a difference was the introduction of particle ideas.

Pupils' views on boiling and their developing particles ideas

From other parts of the interviews, four particle model categories were identified(see Johnson 1998 for an account of the development of the pupils' particlemodels). These are defined below.

Model X: Continuous substance.Particle ideas have no meaning. Nothing that resembles havingparticles of any description is drawn.

Model A: Particles in the continuous substance.Particles are drawn, but the substance is said to be between theparticles. The particles are additional to the substance. There canbe varying degrees of 'profile' for the particles (weak to strong) andof association with the substance (none to close).

Model B: Particles are the substance, but with macroscopic character.Particles are drawn and are said to be the substance. There isnothing between the particles. Individual particles are seen asbeing of the same quality as the macroscopic sample — literallysmall bits of it.

Model C: Particles are the substance, properties of state are collective.Particles are drawn and are said to be the substance. The propertiesof a state are seen as collective properties of the particles.

Before proceeding, four points of detail need to be made.

• A few pupils used model B for the liquid state but model A for the gas state.These were placed in a category called AB, but for this paper these have beenmerged with model A.



• Some pupils used model B for different substances in different states butused model C for the same substance in different states; i.e. the particles didnot change on a change of state. These were placed in a category called BC,but for this paper these have been merged with model C.

• Some pupils used a variation of model B where 'air' was said to be betweenthe particles. These pupils have not been distinguished as a separate cate-gory, but will be noted in illustrations.

• The questioning at interview 2 did not allow specifically for the differentia-tion between models B and C. This undifferentiated category has been calledW. Overall, the evidence suggested that only three pupils might have been atmodel C.

Tables 2 and 3 show the three main bubble composition categories cross-tabulatedagainst the pupils' particle models at interviews 2 and 5 respectively. At interview2 the boiling water section followed the particle section and the pupils were invitedto use particle ideas. In eight cases the opportunity was declined and the figures inparentheses in table 2 give those pupils who attempted to apply particle ideaswhere this was a subset of the pupils. At interview 5 'boiling water' precededthe section on particles. The intention was to gauge the readiness with whichpupils used particle ideas in their thinking. A gradation of 'trigger' points wasfound and these are given in table 4, together with their frequencies. The datain tables 2, 3 and 4 bear detailed analysis and for this the reader is referred toJohnson (1995) — with unmerged categories! Only the key points will be made here.This will be done for each particle model in turn and then the 'air and water'bubble category.

Table 2. Composition of bubbles in boiling water against particle models(interview 2).

Composition of bubbles

AirAir and waterWater as a gas

X

3(0)

Particle model

A

5(1)15

W (B or C)

3(2)76

Table 3. Composition of bubbles in boiling water against particle models(interview 5).

Composition of bubbles

AirAir and waterWater as a gas

A

4

Particle model

B

314

C

21

16


578 P. JOHNSON

Table 4. Points at which pupils started using particle ideas (interview 5).

Point at ivhich a pupil invoked particle ideas Number of pupils

Spontaneous use to explain the composition of the bubbles 18Particle ideas invoked to explain why water level goes down 6Particle ideas invoked to explain expansion of syringe 3Particle ideas used, when revisiting after 'particle section' 1Particle ideas not used, even after 'particle section' 4

Model X: Continuous substance — no particles

By definition, for those pupils at model X, particles ideas will have no involvementin their thinking.

Model A: Particles in the substance

Model A, which is still essentially a continuous view of matter, does not seem tooffer much in the way of helping a pupil to envisage water changing to the gasstate. It is not surprising that four of the pupils at 'air - model A' (interview 2) didnot apply particle ideas. The four pupils who did not apply particle ideas at inter-view 5 (table 4) were also in the model A category.

The 'water as a gas' — model A combinations (table 2 only) seem, then, on theface of it, somewhat contradictory. However, of these five cases, three were ofthose that had started with the macroscopic idea of the composition of the bubblesbeing linked to water right back at the first survey, long before any mention ofparticles. It is possible that the other two pupils arrived at a macroscopic concep-tion for this change, but a little later. It is worth noting that one of these pupils hadgiven a particularly ambiguous response at the survey which might have beenbetter placed in the 'water as a gas' category anyway. For the other pupil, it alsoseems possible that her particle ideas did actually help in some way. This pupil hadan unusually strong collective dimension to model A. In this early application ofher particle model to the formation of a bubble we see the particles used to repre-sent behaviour, and it is possible this was of assistance. Pupil 13 (interview 2), said:

... they've got hotter and the attraction's weakened, can't pull them together so muchso they sort of fly apart... go where they want.They can go where they want... with the liquid they're just restricted to where thewater is —the liquid.

It is interesting to see how the attractions are interpreted as the intervening sub-stance operating on the particles, rather than particle to particle. The other pupilsat 'water as a gas' — model A also showed signs of the particles helping to visualizetheir existing idea, with the movement of the particles being used to represent theprocess of the change of state, but not the substance itself.

Models B and C (W): particles are the substance

The coincidence of these models with the 'water as a gas' category is not surpris-ing. With the 'particles as the substance', the particles moving apart from eachother (due to increased energy) is the crux of the event. Figure 6 gives an example.



you heat it to the temperature so it then produces steam ...andthen that is the air - well gas from the water rising. .. thenwhen it bursts... when it starts to cool down then you can seethe steam.

when heated particles separate more

they're the same sort of particle apart from they're spread outmore so it rises

(Note the continued use of 'air" but with a different meaning!)

Figure 6. Pupil 19 (interview 2).

This view renders the nature of the particles redundant, but those at model B stillseemed to need 'the support' of thinking that the nature of the particles alsochanged. Nearly all of the pupils using either models B or C did talk of theparticles moving apart but for some the connection to the bubble was a problem.Pupil 5 (interview 2) provides a salutary reminder of the need for care and cautionwhen interpreting pupils' words. His first response seemed to represent a soundunderstanding: 'well the heat give the particles more energy and they can breakaway from the attraction... and um.. . go ...and and turn into a gas. They getfreedom from the others.' However, for him, the bubbles were still air frombetween the particles, and he went on to explain: 'as the particles break apartthe air is able to come out'. The turbulence of the water was interpreted as theparticles 'pulling apart', and it was the released air that formed the bubbles, not theseparating particles. When asked if one could ever see the gas he had referred to hereplied, 'yes... steam', which could be seen 'coming out', but not when it was inthe water. (This is one of the instances of a particle model of continuous airbetween the water particles.)

By definition, one might expect to find all model C pupils in the bubblecategory, 'water as a gas', therefore, the air-model C combination seems anom-alous. One of these pupils was pupil 13, again! In all of the later particle tasks sheshowed a model C, indeed her explanation of water changing to the gas state in the'syringe' was particularly good. However, she seemed to have a block in translatingthis to the formation of the bubbles. For her, the loss of water was due to particlesleaving at the surface, independently of the bubbles, even though she was quitespecific about the separation starting at the bottom as the heat 'worked its waythrough'. However, when asked if one could notice the particles separating withinthe water of the beaker, she replied: 'no, not in a beaker of water... you can't seeit.. . all you can see is the steam ... steam comes off the top'. This, then, still leavesthe need for something like air to form the bubbles. Linking the actual bubbles tothe separation of the water particles was a problem for four other pupils at inter-view 5 — it was not a problem for the syringe. The formation of bubbles within thebulk of a liquid might pose an additional demand on pupils, as well as the changeof state itself.

Air and water category

This category represents an incorporation of the idea that the bubbles are linked insubstance to the water, but still without the conception that a liquid itself can


580 P. JOHNSON

Figure 7. Pupil 14 (interview 2).

become a body of gas. A ready-made gas (air) is used to provide the 'body' of thebubble for the water to mix in to. There were two variations on this theme (ofequal frequency at both interviews 2 and 5), reflecting differing degrees of depen-dence on the air.

The inversion of continuous air and particulate water

This used the idea of water particles being dispersed throughout continuous air. Inthe liquid state the air was said to be between the particles, heating then forcing theair out, taking water particles with it. The bubble is the inverse of air in water -water in the air. The water particles by themselves could not be conceived of as agas.

Particulate air and water

The second variation was a bubble composed of both water and air particles, again,the air particles already being 'in' the water. Figure 7 gives an example. Althoughjust using particles, it is interesting to see the air particles being invoked by thepupils to account for the bubble formation. It seems they needed to have a sub-stance which was a gas in order to do this. The water particles could not do this ontheir own. When asked if there would still be bubbles if there was no air, pupil 14thought probably not.

Discussion

The results show that half of the pupils had moved to an acceptance of the idea ofthe bubbles being 'water as a gas' by the end of the study, with four (possibly five)pupils having started with this conception — at a macroscopic level. For almost allof those who changed this was not a result of 'teaching' a macroscopic descriptionof boiling water — instead, the move to water as a gas coincided with the introduc-tion of particle ideas. Did the particle ideas actually help the pupils to conceive ofthe bubbles as being water, but in the gas state? Of course, given time, more of thepupils might have taken to a macroscopic view but there do appear to be groundsfor suggesting that the particle ideas did help. First, it would seem to be more thancoincidence that all but one of the pupils with models B or C invoked particle ideasin their responses at interview 5. The one pupil who did not was one of the pupilswho had started with macroscopic ideas. She was quite able to apply particle ideaswhen asked to do so, but, presumably, did not need to for her own benefit. Whenasked about any change of mind, some pupils admitted the role of particle ideas:



e.g. pupil 17 (interview 2), 'I used to think air — didn't realize the particles'. Thefollowing exchange gives more direct evidence. Pupil 1 (interview 5):

P: When you put the water in the... well that gas that's been trapped... comesup ... when you boil it it comes up.I: Which gas?P: The air.I: Where was it?P: Inside the beaker...I: Can you explain how the water level goes down?P: When you heated it the particles get given more energy and their attractionbecomes weaker... so they can turn into a different state which is gas and theycomes up. . . hang on just thinking about that - is that when the particles turn intoa gas - the bubbles?

Therefore, it would seem that most of the pupils did not start with the idea thatwater could be a sample of gas and it was the particle ideas which helped them tothink of this as something that could happen. Informing a pupil that the bubbles inboiling water are water in the gas state is not enough; the pupil needs a means ofseeing why such a happening is a possibility. Particle ideas seem to provide a wayof starting to think about this possibility. Model A, as might be expected, wouldappear to be of little use. However, model B does seem to be of value in providing away of starting to think about this possibility - even if the 'support' of ascribing themacroscopic character to the particles is still needed. The aim, of course, is for amodel C; an entirely paniculate view, which, essentially, 'only' requires a separa-tion of the particles.

The 'air and water' category is an example of a kind of thinking that surfacedin many parts of the wider study. Pupils appear to be thinking in terms of threetypes of substance — 'solids', 'liquids' and 'gases'. Although the water particleswere apart from each other, the pupils needed a substance that was 'a gas' to formthe 'gas' of the bubble - either continuously or as particles. The water particlesthemselves could not 'be the gas'. Whether this category represents some kind ofprogress from an 'air' category is difficult to say. It does not concede that the wateritself can be 'the gas' but seven of the eight pupils at interview 2 did move on.Perhaps, linking the loss of water to the interior of the bubbles is an important firststep towards appreciating what is happening. A response of oxygen and/or hydro-gen, so commonly reported in the literature, is also consistent with 'types of sub-stance' thinking — oxygen and hydrogen are known 'to be gases'. In this respect, afeature of this study was that only one pupil gave such a response. From hisreading out of school, this pupil was already aware of the composition of waterin year 7 - the others were apparently not (a salutary reminder of just how different11-year-olds' worlds are from a science teacher's). In the teaching, water as acompound of hydrogen and oxygen was not introduced until unit 4 and it seemspossible that this 'holding back' might have given the pupils a better chance todevelop the idea of this change of state without the distraction of this extra 'knowl-edge' at an early stage.

Conclusion

If pupils do not appreciate that a substance, such as water, can exist as its ownbody of gas (i.e. be a sample of gas) one has to ask what they are supposed tounderstand when told of 'gases' such as oxygen or carbon dioxide. As noted earlier,


582 P.JOHNSON

the literature suggests that children find this third state very mysterious. That a'gas' is a sample of a substance, just as much as an iron nail or a pool of water, is byno means an obvious notion. And yet, of course, it is vital to an understanding ofthe material world — chemistry rests upon this. The significance of the bubbles inboiling water, then, is that attention to this provides a way in to understandingwhat 'a gas' might be. If one can see that a substance such as water can exist 'as agas' one might then be able to think 'a gas' such as oxygen is equally a substance -the only difference is that it has already boiled. One can also see how it is possibleto have different gases — these are different substances. A most fundamental under-standing of science is at stake here. A treatment of boiling water which ignores thecomposition of the bubbles and just considers a general leaving of water into the airmisses the point: one suspects this is common practice in our schools. Waterdispersing into an existing body of gas does little, if anything, to suggest whatthe gas state might be. On this basis, even allowing for a distinction betweenvapour and condensation, the claim by Bar and Travis (1991) that children froma young age 'understand that during boiling liquid is changed to gas' must bequestioned.

If we are to develop pupils' understanding of the bubbles in boiling water, andhence an understanding of the gas state, the particle theory would appear to be anecessary way in (for most pupils of this study, at least). The particle model mustbe seen as a means of first establishing the possibility of a sample of substance beingin the gas state. Given that pupils' understanding of particle ideas is generally verypoor, it is important that our teaching here is improved (see Johnson 1998). It issurely no coincidence that understanding in both of these areas is so lamentable.Even with a model B the possibility of a substance in the gas state can start to beentertained and once this has happened the particle model might then be able tolose its 'macroscopic supports' and develop to model C.

Note

1. The term 'gas' will be used for a sample of material where the particles are dispersed,whether or not the sample is above its critical temperature or pure.

References

ANDERSSON, B. (1984) Chemical Reactions, EKNA report 12 (Gothenburg: Institutionen forPraktisk Pedagogik, University of Gothenburg, Sweden).

ANDERSSON, B. (1990) Pupils' conceptions of matter and its transformations (age 12-16).Studies in Science Education, 18, 53-85.

BAR, V. and TRAVIS, A. S. (1991) Children's views concerning phase changes. Journal ofResearch in Science Teaching, 28(4), 363-382.

BELL, B., OSBORNE, R. and TASKER, R. (1985) Finding out what children think. In R.Osborne and P. Freyburg (eds), Learning in Science (London: Heinemann), 151-165.

DRIVER, R., GUESNE, E. and TIBERGHIEN, A. (eds) (1985) Children's Ideas in Science (MiltonKeynes: Open University Press).

DRIVER, R. SQUIRES, A., RUSHWORTH, P. and WOOD-ROBINSON, V. (1994) Making Sense ofSecondary Science - Research into Children's Ideas (London: Routledge).

GARNETT, P., GARNETT, P. and HACKLING, M. (1995) Students' alternative conceptions inchemistry: a review of research and implications for teaching and learning. Studies inScience Education, 25, 69-95.

JOHNSON, P. M. (1995) The development of children's concept of a substance. UnpublishedPhD thesis, University of Durham, UK.



JOHNSON, P. M. (1996) What is a substance? Education in Chemistry, 33, 41-42.JOHNSON, P. M. (1998) Progression in children's understanding of a 'basic' particle theory: a

longitudinal study. International Journal of Science Education, 20, 393—412.JOHNSON, P. M. (1998) Children's understanding of changes of state involving the gas state,

Part 2: Evaporation and condensation below boiling point. International Journal ofScience Education, in press.

JOHNSON, P. M. and GOTT, R. (1996) Constructivism and evidence from children's ideas.Science Education, 80, 561-577.

JOHNSTON, K. and DRIVER, R. (1991) A Case Study of Teaching and Learning about ParticleTheory (Leeds: CLIS, Centre for Studies in Science and Mathematics Education,University of Leeds, UK).

MAS, C. J. F., PEREZ, J. H. and HARRIS, H. (1987). Parallels between adolescents' concep-tions of gases and the history of chemistry. Journal of Chemical Education, 64 (7), 616—618.

OSBORNE, R. J. and COSGROVE, M. M. (1983). Children's conceptions of the changes of stateof water. Journal of Research in Science Teaching, 20(9), 825-838.

PIAGET, J. (1929) The Child's Conception of the World (London: Kegan Paul, Taubner).PFUNDT, H. and DUIT, R. (1994) Bibliography: Students' Alternative Frameworks and

Science Education, 4th edn (Keil, Germany: Institute for Science Education).POSNER, G. J. and GERTZOG, W. A. (1982) The clinical interview and the measurement of

conceptual change. Science Education, 66, 195-209.RUSSELL, T., LONGDEN, K and MCGUIGAN, L. (1991) Materials, Liverpool King's SPACE

Project (Liverpool: Liverpool University Press).SERE, M. G. (1985) The gaseous state. In R. Driver, E. Guesne and A. Tiberghien (eds),

Children's Ideas in Science (Milton Keynes: Open University Press).STAVY, R. (1988) Children's conception of gas. International Journal of Science Education,

10(5), 553-560.STAVY, R. (1990) Children's conception of changes in the state of matter: from liquid (or

solid) to gas. Journal of Research in Science Teaching, 27, 247-266.WANDERSEE, J. H., MINTZES, J. J. and NOVAK, J. D. (1994) Research on alternative con-

ceptions in science. In D. Gabel (ed.), Handbook of Research on Science Teaching andLearning (New York: Macmillan), 177-210.


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