Students’ ideas about open system chemical events

Students’ ideas about these reactions have been probed by a number of workers including Andersson (1984, 1986 and 1990), Schollum (1981a and b, 1982), Brook et al (1984), BouJaoude (1991), Ross (1987 and 1993), Watson et al (1997), Barker (1995) and Barker and Millar (1999).

The origin of rust

Andersson (1984), Driver (1984) and Schollum (1981a) among others report a consistent pattern of responses among 14 -15 year olds about the origin of rust on an iron nail. A selection is given here.

A minority of students attribute the rust to a chemical reaction, not always seen as including oxygen, for example:

“Rust is the form of the chemical reaction after the nail has been taken apart by the rain.”

“…caused by water and an impurity in the nail reacting” (Schollum, p 13).

These students seem to have learned “reaction” and use it to describe production of rust. Even when oxygen was known to be involved, students did not necessarily associate this with an increase in mass, for example:-

“The iron had only reacted with the oxygen of the air which does not weigh anything.” (quoted in Driver et al, 1985 p 163).

In this case, the student does not think that gases have mass. More commonly, students thought that the mass of a rusty nail would be lighter than the original nail because the rust

“eats away” the metal, for example:-

“As the nail rusts away it will get smaller..”

“Rust rusts away” (Andersson, 1984 p 34)

Brook et al (op cit) found this response among one-third of 15 year olds. It is similar to the low-level macroscopic thinking reported earlier in that life-like properties are ascribed to the rust. About one-third think the mass of the nail would not change, because the rust was simply “part of the nail”, for example:-

“[The rust is] there all the time under the surface of the nail” (Schollum, 1981a, p 13).

Andersson (1990) calls this “modification”; the rust existed before the event, but became visible when the nail was left in water. A different type of modification idea is reported by Brook et al and Andersson (1984) who found that about one-third of 15-year olds thought the nail would be heavier after rusting:-

“Rust makes the nails heavier”

“Water is added when rust forms”

“Oxygen is added when rust forms”

“Oxygen and water are added when rust forms” (Andersson, 1984 p 34 - 35).

The reaction between copper and oxygen

Andersson (1984, 1986) and Hesse and Anderson (1992) studied students’ thinking about the reaction between copper and oxygen. Andersson asked 13 -15 year olds to explain how a dark coating forms on hot copper pipes. About 10% explained that “This is the way all copper pipes change” (1986, p 552), accepting the event as fact, or “it is just like that”.

Other suggestions included that water had seeped through the pipes and caused the coating, an explanation which Andersson describes as “displacement”; and that the copper was changed by the heat (“modification”). About 20% of 15-year olds recognised this, explaining, for example, that:-

“Copper and oxygen have reacted”

“It is oxidation. Air = oxygen reacts with copper, copper oxide is formed and that is the dark coating.” (p 556)

In Hesse and Anderson’s (1992) case study, one student (no age is indicated) explained that copper and oxygen reacted with “heat as the catalyst” (p 287). So, although some students have well-developed, accepted views of the copper/oxygen reaction, a majority at age 15 do not.

Barker (1995) asked 16 year olds beginning post-16 chemistry studies where the “black stuff” came from when powdered copper metal was heated in air, given that a mass increase occurred. 63% said that it came from a reaction with oxygen. A further 12% suggested from a reaction with “gases/air”, while about 10% suggested the black stuff was soot, carbon or carbon dioxide. At 18 years old, 75% of the same students gave the correct answer and about 8% gave the two main misconception-type answers.

Burning steel (or iron) wool

The rate of the reaction between iron and oxygen can be increased by heating the iron in the atmosphere. When external heat is applied, chemists say the iron is being “burned” or “combusted” in oxygen. Students’ ideas about this reaction are reported by Driver et al (1985), Andersson (1986) and Donnelly and Welford (1988).

Students predicted how the mass of iron wool would change once burnt in oxygen. About 40% of 15-year olds (Driver, 1985) who had studied chemistry for two years thought the mass of iron would increase because of a reaction with oxygen. These students realise the mass of oxygen must be taken into account. A further 6% thought the mass would increase, but explained that this was due to soot from the flame adding to the dish, possibly influenced by the black appearance of the iron wool after heating. Around 40% thought the mass of the iron would decrease. This group included 19% who suggested gas or smoke would be driven off and 10% who thought that the “burning” would leave ash, which would be lighter than the iron. These students do not recognise the role of oxygen in the reaction, and are using the term “burn” in a non-chemical sense, not “reaction with oxygen”. Students’ familiarity with ash remaining after burning coal or wood, which is less bulky than the starting material, may contribute to this. About 5% thought the mass of the iron would be unchanged, for example:-

“It would stay the same because the powder is in the wool but heated up so there is really no difference.” (Driver et al, 1985, p 160)

This response conserves the amount of starting material, recognising that the iron present at the beginning would remain at the end, although this student does not see a role for oxygen in the reaction.

Andersson (1986) reports one other “transmutation” response among 15 year old chemists:-

“The steel wool that has burnt has turned into carbon. Carbon weighs more.”

“It forms carbon after being red-hot, which makes it heavier.” (p 555)

In a previous study (Barker, 1990) found that some 11 and 12 year olds used this reasoning in explaining how “the white stuff” from burning magnesium was formed:-

“[It] is from burnt carbon/is the soot left after burning” (p 69).

This response is perhaps based on students’ experiences of burning fuels, which are widely known to contain carbon. In the cases of metals burning, students who do not think as chemists use this information, instead suggesting that one substance can change into another.

Students’ ideas about iron burning in oxygen are consistent with those about rusting. We see confusion about conservation of mass and the involvement of oxygen. Next, we will examine students’ thinking about fuel / oxygen reactions.

Burning a candle

Students’ ideas about burning candles explored by various workers (Meheut et al, 1985; BouJaoude, 1991; Schollum, 1981a, b and Watson et al, 1997) reveal similar response patterns. Around 25% of 14 year olds describe a candle burning as a state change. Meheut et al (1985) found that about 25% of 11 and 12 year olds describe the change as “melting”. BouJaoude (1991) found 14 year olds thinking that a candle decreases in size because the wax evaporates, ignoring the role of the flame. As the oxygen is invisible, students’ senses suggest that only state changes occur. Some students think the candle flame is caused by the “wick burning”, not the wax (BouJaoude, 1991). This may help explain the state change response, because students could reason that heat from the flame (which is the wick burning) causes the candle to melt.

Students’ poor particulate models of matter may contribute to the “change of state” model for burning. Schollum (1981b) reports that a significant proportion of students aged 14 upwards do not perceive either the wax or the flame to be particulate in nature. Those who think the flame is composed of particles describe these as

“burnt little bits…pretty small bacteria…oxygen from the air …hydrogen particles from the air.” (p 12).

Only two students in thirty-six perceived the flame as particles of hydrocarbon. This finding supports the continuous view of matter discussed earlier.

Meheut et al (1985) report ideas about the role of oxygen in burning a candle. Although most 11 - 12 year olds knew oxygen was needed for burning, they could not explain exactly how the oxygen was used. A number thought the oxygen was “used up” or “burnt away”. In BouJaoude’s study (1991), 14 year olds were interviewed about the involvement of oxygen in a candle burning. One student said:-

“Oxygen feeds the fire and keeps the candle burning” (p 695).

Thus, the role of oxygen in burning candle wax is not well known. Instead, students may think that a state change is occurring, decreasing the candle mass by evaporation of wax. This thinking conserves the amount of original material. The view that oxygen is “used up” also appears prevalent, indicating that some students think oxygen is destroyed on burning.

Watson et al (1997) describe the explanations given by 150 14 and 15 year olds to questions about aspects of combustion, including ideas about what happens when a candle enclosed in a gas jar burns for a few seconds. In exploring the consistency of explanations across a range of combustion reactions, the authors found three types of framework based on the categories “chemical reaction”, “transmutation” and “modification” in Andersson’s (1990) model. They note that students using a transmutation framework including ideas such as material being changed into heat, oxygen “feeding the flame” and non-conservation of mass in a combustion reaction tend to use this thinking consistently across a range of situations. The tenacity of this framework may in part be due to the limitations of student experience, as it “works” well for the carbon- and hydrogen-based fuels used commonly in pre-16 courses. A second group using modification ideas in which, for example, oxygen is not involved in the change, or the flame is the source of heat for the reaction tend to adapt their thinking according to the characteristics of the substance being burned. A third group use chemical reaction ideas and transmutation ideas. Watson et al suggest that students whose responses are inconsistent may be moving from one “theory” for explaining combustion to another. They indicate several aspects of combustion which are absent from students’ responses, including the formation of imperceptible products such as gases, the weight of gases and the existence of atoms or molecules. Success in making the transition to a “chemical reaction” framework may depend on the extent to which students understand these imperceptible aspects.

Burning butane

BouJaoude (1991) and Schollum (1981a, b) asked students to explain what they thought was happening when a gas burner was lit. Schollum (1981b) reports that students agreed readily that “burning” occurred. Noticeably, students did not use the change of state model, perhaps because gas cannot melt! 12 - 15 year olds suggested frequently that the gas was destroyed, for example:-

“The gas is eating up, no the flames are eating up the gas… It eats it up and then it goes up in little pieces.” (1981b, p 7)

One student in BouJaoude’s study used similar reasoning to explain that oxygen was “burned up”.

Schollum reports that many students aged up to 17 years think heat is produced, for example:-

“It turns into heat or heat waves.” (1981b p 7)

Some older students described the products as carbon dioxide and hydrogen, suggesting that the role of oxygen in producing carbon dioxide and water was not well known. Since students may use this reaction everyday in cooking or heating, the “gas becomes heat” response may be expected. However, these responses indicate that a high proportion of 14 - 15 year olds may think that gas or oxygen is destroyed when burning occurs.

Burning petrol

Andersson (1984) reports the ideas 15 year olds have about burning petrol in a car engine. Students were asked to predict the mass of exhaust gas formed when 50 kg of petrol was placed in a car that was then driven until the tank was empty. Their responses can be compared with those given to the conservation of mass in closed systems, reported in section 6.1.

Andersson found that only 3% of 15 year olds thought the mass of the fuel would increase. Although some gave the expected response, that petrol had reacted with oxygen, others thought the mass would increase because:-

“The petrol is mixed with the air and then it gets heavier.” (p 38)

This student recognised that air was involved, but did not appear to think that a chemical reaction had occurred. However, the terms “mixed” and “reacted” may, to these students, be synonymous, so this could be their way of saying that a reaction had occurred.

Over 50% of Andersson’s respondents thought the mass of the petrol would be unchanged. Many used the state change model, for example,

“Even if it doesn’t come out in liquid form it must weigh just as much.” (p 38)

This indirectly says that the petrol turned into gas, mirroring the candle wax “melts” response described above. These students do not perceive that oxygen is involved, but conserve the amount of petrol.

About 27% of respondents thought the mass of exhaust gas would be less than the mass of petrol for at least two reasons. First, gases “do not weigh as much as liquids”, so independent of what happened to the petrol, that gases are emitted means the mass must be less, for example:-

“Gas is lighter than petrol (water), so if you only have 50 kg of petrol and it’s transformed into gas, it must be lighter..”. (p 37)

This response confuses mass and density. They may conserve amount of stuff, but think that the measurable mass has changed.

A second explanation for mass decrease is that petrol has changed (“transmuted”) into energy, for example:-

“It’s less than 50 kg because part of the petrol has been changed into heat and kinetic energy.” (Andersson, 1986 p 555)

Similar responses were found in explanations about butane burning. These ideas suggest that although students are aware that burning generates heat, they do not know how the heat is produced.

Barker (1995) and Barker and Millar (1999) report 250 16-18 year olds’ responses to a slightly modified version of Andersson’s “petrol” question. They found that only about 14% of 16 year olds beginning post-16 chemistry courses realised the mass of gas increased relative to the petrol. At the age of 18, this figure increased to 40%. The most frequent incorrect answer was the response “what goes in must come out”, given by 44% of 16 year olds and 30% of 18 year olds. Small proportions of students at both stages thought that petrol was converted to light, heat or energy; that the gas was lighter than the starting material so the mass would decrease; and that the petrol was used up or burned away.

The petrol question does not mention the involvement of oxygen, leaving students to realise this for themselves. So, as many may not know what occurs in a car engine, the question may invite the responses “what goes in must come out” and “gases are lighter than liquids”, as these are the only bases on which responses can be made from the information provided. Nevertheless, the range of responses was comparable to that for the fuel questions described above and there is certainly evidence to suggest that even where the fuel was burned in the students’ presence many still did not realise that oxygen was involved.

Although the petrol question appears to be problematic, it is still a valid way of probing students’ thinking about an everyday event.

Summary of key difficulties

The research evidence indicates that students develop a range of faulty models about open system chemical reactions.

1. The role of oxygen is poorly understood

The atmosphere is invisible to the eye - and students’ reliance on concrete, visible information means they therefore often avoid the role of oxygen in their explanations for open system reactions. Even if the role of oxygen is appreciated, the notion that gases do not have mass means that students do not realise that solid products of an oxidation reaction have more mass than the starting solid.

2. Fuels change state and do not burn

Some students develop the model that fuels change state on burning. In this model, candle wax melts and petrol turns to gas. The flame of a candle is an entity separate from the fuel and is not particulate.

3. Fuels are destroyed in burning or changed into something else

That many solid fuels produce a solid ash on burning which is much smaller in mass and volume than the original material leads to the impression that the fuel has been destroyed. Rust, produced in slow oxidation of iron, may be perceived as an active agent eating away the metal. Carbon is thought of as a product of a burning or oxidation reaction, even if this element was not present in the reactants. Similarly, fuels may transform into energy.

Suggested activities^[1]

1. Use diagnostic tasks

Find out what students are thinking by asking diagnostic questions such as those used in research studies. A good way of introducing a lesson on fuels is to provide three different examples – one solid, one liquid and the third gaseous. Show students that all three burn, and ask what the mass of the products would be. Find out if they use change of state or destruction models in their answers, and if they reason consistently across all three fuels. Ask them what they think are the products of combustion in each case. Be prepared to act on their answers, perhaps using the strategies described below.

2. Use molecular models

Divide a class into groups, giving each a different fuel to consider. The group must make a molecular model of the fuel and oxygen gas, then use these models to explain how the fuel burns. The models can be used to work out the equation for the reaction.

3. Present common ideas explicitly

Students need to realise that the products of combustion are consistent, regardless of the

fuel. Combustion always produces an oxide. Work with them on understanding that gases

have mass, so the products of combustion must always include the mass of oxygen.

Equations can be used to support this. Show that carbon can be produced by holding a tile above the flame of carbon-based fuels – that the same products arise regardless of the fuel being solid, liquid or gaseous. The difference may be in amount – a “clean” fuel such as ethanol will generate fewer carbon particles than candle wax. Repeat the procedure with metals being oxidised – molecular models can be used to show that the metal is reacting with oxygen gas, and that therefore no carbon is produced.

4. Go back to basics

Review students’ understanding of chemical reactions. Combustion is a chemical reaction resulting in the production of new substances in which mass is conserved. Using models, students can be led to realise that some energy is required to break bonds initially to start the reaction, but that after this the reaction is self-starting. Energy released when the product molecules are formed is used to break more bonds and to heat/cook/ drive a vehicle until the fuel or oxygen supply is exhausted.