Students’ ideas about specific chemical events: closed system reactions

Students’ poor understanding of the relative densities of matter and particle theory, creates problems for them in realising what happens during these changes. The main examples of such reactions used by researchers are discussed here.

Phosphorus and oxygen in a sealed container

This reaction has formed the basis for a question used in major studies exploring students’ misconceptions. The question features a piece of phosphorus placed under water in a sealed flask heated by the Sun. Students are told the phosphorus catches fire, producing a white smoke which dissolves in water. They are asked if the mass of the flask and contents together will be the same, greater, or less than the initial value when all changes are complete. Andersson (1984, 1990) and Briggs et al (1986) report that about 30% of 15 year olds give conservation-type answers, suggesting the mass would be unchanged because “the flask is sealed”, for example:-

“Despite a change of form or state, the same weight is present” (Driver, 1985, p 165)

“The flask is sealed. Nothing is added or leaves” (Andersson, 1984 p 40 - 42).

A further 16% thought the mass would decrease, suggesting that:-

“Smoke weighs nothing / is light / is lighter than a solid”

“The phosphorus/the smoke dissolves in the water [so becoming lighter]”

“The phosphorus burns up or is destroyed”

“Oxygen is used up when combustion takes place” (Andersson, 1984 p 40 - 42).

Only 6% thought the mass would increase, for example, because:-

“The smoke is heavier than the phosphorus”

“When the smoke dissolves in the water, the weight increases” (Andersson, 1984 p 40 - 42).

Thus, about one-third of students aged 15 do not conserve mass in this reaction. Andersson (1984) suggests that:-

“If a pupil is to be able to decide whether an amount of matter, or more exactly, mass, is conserved or not, s/he must be able to distinguish between what is material and what is not.” (p 45)

If students do not focus immediately on the sealed flask, their response depends mainly on their thoughts about the smoke. Students who think smoke is “material” may offer a conservation response, or suggest the smoke is heavier than the phosphorus. Those who associate “smoke” with the term “gas” and do not think that gases are material will give non-conservation responses. Alternatively, students may also think that matter is used up when a reaction occurs, and hence suggest the mass decreases.

Barker (1995) (reported in Barker and Millar, 1999) used a slightly adapted version of the same question in a longitudinal study of 16 year olds beginning UK post-16 chemistry courses. About 75% of the 250 students involved gave the correct answer, while around 6% confused mass and density, reasoning that the mass would decrease because gas / liquid “weighs less than solid”. 11% thought that mass decreased because the phosphorus dissolves or is used up. By the age of 18, about 81% of the same sample gave the correct answer, while only around 3% confused mass and density and 5% thought the mass would decrease.

Precipitation

Mixing two aqueous solutions may produce a precipitate, for example in tests for reducing sugars and sulphate ions. de Vos and Verdonk make use of precipitation reactions in their teaching scheme, but little other work has been done on students’ understanding about this type of reaction. Barker (1995) and Barker and Millar (1999) probed 16-18 year olds’ thinking about the conservation of mass in a precipitation reaction over a two year period. They found that about 44% of 16 year olds conserved the mass, agreeing that the mass of solid precipitate and liquid has the same mass as the two original liquids. By the end 70% gave this response. Some confusion between weight and density was apparent. About 17% of 16 year olds thought the mass would increase because a solid “weighs more than an liquid” a figure which decreased to about 10% by the end of the study. A third finding was that about 14% of beginning students suggested a gas was produced so the mass would decrease, while 7% gave this answer at the end of the course.

Happs (1980) and Schollum (1982) interviewed students aged 10 - 17 about the formation of a precipitate made on mixing lead nitrate and sodium chloride solutions. Students of all ages tended to describe, rather than explain what they thought had happened, for example:-

“It’s gone all murky” (Happs, 1980, p 10)

Others used scientific language, such “solvent”, but very few used “precipitate” to describe the white solid. Older students thought the precipitate was a new substance, while the younger ones described the reaction as substances joining together. However, some older students thought no reaction had occurred:-

“If those two (sodium chloride and lead nitrate) had reacted, it would have gone clear.” (Schollum, 1982, p 12)

Dissolving

For this purpose, dissolving is regarded as a chemical change.

Piaget and Inhelder (1974) reported that young children think that sugar “disappears” when dissolved in water, and thus do not “conserve” the mass of material. They are content with the notion that the mass of water would not change, because the substance added to it simply no longer exists. A number of workers including Driver (1985) and Cosgrove and Osborne (1981) have explored the prevalence of this and other explanations among older children. Driver in her study (reported in Briggs et al, 1986) found that about two-thirds of 9 - 14 year olds thought the mass of a sugar solution would be less than the mass of the sugar and water. When a similar problem was given to 15 year olds (Andersson, 1984), over half of the sample thought the mass of the solution would be less. Students offered a variety of explanations, including:-

“When the sugar dissolves into the water the sugar has no mass so it is just like the 1000 g of the water.”

“The sugar will decompose and form a liquid with the water and so will weigh less.” (Andersson, quoted in Driver et al, 1985, p 154 - 155)

These students do not conserve mass, suggesting that their thinking about this process may not have changed from early childhood.

About 30% of the 15 year olds in the Andersson study predicted that the mass would be unchanged. This figure rose to about 50% of the students who had studied chemistry. Responses in this category clearly showed that students knew the sugar would still be present, for example:-

“Not one of the two substances would have gone anywhere else except in the pan … even though the sugar cannot be seem it is still present.” (Andersson, quoted in Driver et al, 1985, p 154).

Although this response does not use particle ideas, the student certainly conserves mass. Others achieved the same result by adopting an algorithmic approach, adding the masses of solute and solvent given in the question.

In the Cosgrove and Osborne study, about one-quarter of respondents used the word “melt” to describe what happened to sugar, for example:-

“The sugar is dissolving … the water is sort of melting the sugar crystals” (Cosgrove and Osborne, 1981, p 18)

The terms “dissolve” and “melt” seem here to be used synonymously, although its usage decreased with age.

In the Barker (1995) study (reported in Barker and Millar, 1999) 250 students were asked what they thought the mass of a solution of salt (sodium chloride) would be compared to the mass of solute and solvent. About 57% of 16 year olds thought the masses would have the same value. Several significant misconceptions were found, including 16% who thought that a gas would be released when the salt dissolves and 7% who said that mass was lost in dissolving. By the age of 18, the percentage giving the correct answer was 62%; 15% still thought a gas was produced and about 4% thought mass was lost. These data indicate that some students may think dissolving is a chemical reaction, and that release of a gas is a standard characteristic of this. Alternatively, students may have read “sodium” rather than “sodium chloride”, so misinterpreted the chemical event in the question.

Dissolving an effervescent tablet in water

Students’ ideas about the evolution of a gas from dropping an effervescent tablet in water have been investigated. Schollum (1981a and 1982) interviewed 11 - 17 year olds about the events occurring when a vitamin C tablet is dropped in water. Typically, students said the tablet “dissolved”, and that a gas, named by most as ‘air’, was produced. A few older students named the gas ‘carbon dioxide’. Students could not describe how the gas was formed. Some indicated the gas existed already, contained inside the tablet, and was released when the tablet was added to the water, for example:-

“When they made the tablet they put little air bubbles in”

“…it must have been some sort of airlock in it and the air that’s in it forces itself out and up to the top” (1981, p 5)

Others suggested the tablet had reacted with the water:-

“The tablet is reacting with the water, splitting up the hydrogen and the oxygen. That’s turning them into their gas forms and the gas comes out the top.” (1981, p 5)

No students explained the gas formed by rearrangement of atoms. The compounds in the tablet that react to form the gas were not named, which perhaps created extra difficulties. Many students described the event as a chemical reaction, but their explanations suggest that they did not really know what this meant. They did not understand that rearrangement of atoms to produce a new substance is involved. This supports the finding of Hesse and Anderson (1992), who note that:-

“… the term “reaction” was regularly found in students’ explanations, yet these students demonstrated little understanding that reactions involve the interaction of atoms and molecules. The misconception remained for most students that scientific explanations involved little more than the ability to ‘talk fancy’.” (p 294)

Students learn a scientific vocabulary, but not the ideas lying behind the words.

Andersson (op cit) asked 13 - 16 year olds about the reaction occurring when an aspirin tablet is dropped in water. He found that about 25% of all ages reasoned the gas produced had mass. This suggests that although students cannot explain how the gas is formed, some are at least satisfied that gases are material.

Barker (1995) asked 16-18 year old students a similar question. Few students at any stage of the longitudinal study explained that the gas had not existed but formed in a reaction. About 37% at the beginning and end suggested the gas was already present in the tablet and around 10% described the gas as being “in solid form”. These data support the suggestion that students may think of gas evolution as a characteristic of chemical reactions, and that the chemist’s meaning of this phrase is not well understood.

Summary of key difficulties

1. Mass and density are confused

Reactions involving changes of states are difficult to explain correctly. Thus, students may reason that the products from a precipitation reaction are heavier than the starting materials; that when a gas is produced the reaction has lost mass overall.

2. Gases may pre-exist / be characteristic of a chemical reaction

Students are often shown reactions producing gases. Thus, they may connect gas production with the notion “chemical reaction” so the two are inextricably linked. Explaining how a gas forms is also problematic – gases may “pre-exist” in the starting materials for a reaction, simply being “released” when a tablet or other substance meets water.

Suggested activities^[1]

Demonstrating simple reactions can be a powerful way of prompting students to change their thinking about this type of reaction.

1. Precipitation

Place two 50 cm³ measuring cylinders on a balance. Add 25 cm³ barium chloride solution to one cylinder and 25 cm³ sodium sulfate solution to the other. Record the total mass. Put small amounts of the two solutions in separate test tubes. Tip one solution from one test tube to the other. Ask students to observe the white precipitate formed. Now ask them to predict what happens to the mass – if one solution on the balance is tipped into the other, what happens to the mass? Will it go up, down or stay the same? Expect that about half of a class of 14-15 year olds will say the mass will increase because a solid has formed. Next, tip one solution from one measuring cylinder to the other. Ask students to observe that the mass has not changed. Some will be disbelieving. Ask for their explanations, working towards the answer that no mass has been added or taken away, so no mass change is expected, while the density of the materials is irrelevant.

2. Making solutions

A similar strategy can be used to help students think about dissolving. Have ready fixed, identical masses of sugar and sodium chloride and two beakers of water. These solids dissolve to form solvated molecules (sugar) and ions (sodium chloride). Place one beaker and the sample of sugar or sodium chloride on the balance separately. Ask students to predict what will happen to the mass when the sugar / sodium chloride is added to the water. Some students may think that the mass will decrease either because a gas is produced or because the substance “disappears”. Others will think that the mass will stay the same, but will picture the sodium chloride remaining as molecules in the water, and sugar molecules breaking up into atoms. Demonstrate first that the mass remains unchanged, then use molecular models to show students what happens to the crystal structures in each case. This demonstration provides a good opportunity to discuss the behaviour of covalent and ionic compounds and intermolecular bonds.

3. Dissolving a tablet

Add an effervescent tablet to water in a conical flask. Ask students to predict what will happen to the mass – they may readily predict that this will decrease because a gas is being given off. Placing the flask on a balance and following the decrease in mass will support this. However, they may not realise that the gas itself has mass. To demonstrate this, repeat the experiment, but place a balloon on the mouth of the flask. This will inflate with the gas and the mass will remain unchanged. The experiment shows that the products of a reaction have the same mass as the starting materials, as well as that gases have mass just like matter in any other physical state.

This reaction can also be used to discuss the origin of the gas. Show the students the names of the ingredients in the tablet and ask which is the gas. They should find that this is impossible – therefore the gas cannot have existed beforehand, but must be made in the reaction between the compounds in the tablet.