Introducing “chemical reactions”

For the purposes of this discussion, a chemical change occurs when atoms (or ions) in reactants are rearranged to form new substances. Often, chemical changes are accompanied by alterations in physical appearance and / or colour, the production of a gas, light, heat, or a cooling effect.

Students experience difficulty in recognising when a chemical reaction occurs. Many do not discriminate consistently between a chemical change and a change of state, which chemists call a “physical change”. Evidence for this comes from a number of studies. For example, Ahtee and Varjola (1998) explored 13 - 20 year olds’ meanings for a textbook definition of ‘chemical reaction’. Students were also asked to state what kind of things would indicate a chemical reaction had occurred. They found that around one-fifth of the 13 -14 year olds and 17-18 year olds thought dissolving and change of state were chemical reactions. Only 14% of the 137 university students in the study could explain what actually happened in a chemical reaction.

Students’ thinking about the characteristic evidence supporting a chemical reaction was probed by Briggs and Holding (1986). They report 15-year olds’ responses to a question about a “chemical” which loses mass, expands in volume and changes colour on heating. Students were asked if they agreed that a chemical change occurred. About 18% gave responses indicating agreement, for example:-

“The substance changes in colour, mass and state, so it would appear to be obvious that a chemical change has taken place.” (p 63)

About 23% offered other responses including:-

”..The mass has melted and has fild (sic) the tupe (sic) but the grams have decreased. The substance has melted so the mas (sic) has gone higher.” (p 63)

“The colour has changed. It has dissolved.” (p 64)

These explanations use the terms “melt” and “dissolve”, suggesting confusion with state changes.

Schollum (1981a) reports similar confusion of state vs chemical change. He found that around 70% of 14 year olds and over 50% of 16 year olds thought diluting a strong fruit juice drink by adding water was a chemical change. Schollum also found that 48% of 14 year olds and 55% of 16 year olds thought sugar dissolving was a chemical change. In defining the terms “physical change” and “chemical change”, three students described a physical change as:-

“When something changes its form from what it was before.”

“One where a reaction doesn’t break up the compounds.”

“Change of properties…Can be easily reversed back to its original form.” (p 20)

The same students defined a chemical change as:-

“… when the molecular form is changed by doing something, e.g. adding or removing water.”

“One where the compounds are broken to form new compounds.”

“Change to a different form or state. Is not easily reversed.” (p 20).

Applying these definitions, the first student would classify dissolving as a chemical change as this involves adding water. The second distinguishes the changes on the basis of whether compounds are broken or not, while the third focuses on changes of “form”. All three thought that sugar dissolving in water was a chemical change.

What is a ‘chemical reaction’ anyway?

What should be considered a physical or chemical change? Gensler (1970) dismissed students’ difficulties as artificial, saying that chemists were at fault. He disagreed that the traditional phase changes of water should be taught as standard “physical” changes “because the water does not change”, saying,

“Through first hand experience, everybody knows that, in fact, ice is not water; to maintain otherwise smacks of double talk.” (p 154)

He continues,

A detailed description of the processes …is surely best given in terms of changes in intermolecular “chemical” bonding.” (p 155).

Dissolving sugar or salt and recrystallising the solid from solution is commonly done at Key Stage 3 (11-14 year old). Gensler suggests this cannot truly be termed a “physical change” because recrystallised solute requires an act of “blind faith” on the part of the learner to believe this is identical to the starting material. The intermolecular bonds in the solute will differ from the original, and the solid may be hydrated. Gensler says that

“…in a discipline where experiment is paramount, the novice is being asked to distrust and discard his own experimental results and to place his faith in authority.” (p 154).

Thus, he suggests that students’ sensory information conflicts with what is taught, creating confusion. Recrystallised sugar, to a student, is not the same as the stuff which was added originally, so by the teacher’s own definition, a chemical change must have occurred. Redefinition of “chemical change” may help. Strong (1970) suggests that a chemical change be defined by these four characteristics:-

“(1) Identity of product determined by identity of initial materials

(2) Mixing of initial materials is essential when more than one reagent is involved

(3) Discontinuity between properties of initial materials and final product

(4) Invariance of product properties when temperature, pressure and initial composition are varied.” (p 689).

These criteria could be related to sensory characteristics helpful to students developing an understanding of the actual changes occurring on the microscopic scale. Gensler surely has a point worth considering. The wisdom of distinguishing between these two types of change for young students with mainly poor particle models of matter who rely heavily on sensory evidence must be questioned. Ahtee and Variola (1998) note that

“Only after the concept of atom is introduced is the difference between chemical and physical change obvious.” (p 314-5)

They suggest that to help students formulate a clear understanding of ‘chemical reaction’, a

range of phenomena should be presented within an approach stimulating observation,

questioning and argument. The authors also suggest that the atomic description should not be “given too soon” (p 315), but rather wait until students perceive a need for a general explanation in terms other than their own.

“What is a ‘substance’?”: understanding chemical terminology

Chemistry in common with all sciences has a distinctive vocabulary of words with very specific meanings. A major part of teaching and learning chemistry is to approach this language in a way that assists students in development of their understanding of chemical concepts. Evidence suggests that difficulties may arise because teachers are unaware of the meanings and problems beginning chemists have with these terms, contributing to poor learning of the basic concepts they represent.

To assist with this, Loeffler (1989) suggests a strategy for teaching about the terms “element”, “compound” and “mixture” based around students learning differences between the macroscopic and microscopic worlds. He acknowledges it is chemically incorrect to think of particles behaving individually as large pieces of a substance. He therefore avoids using the word “element” in favour of “substance”, which could be used in describing macroscopic properties of any chemical normally named as an element, compound or mixture. The word chemical “species” is used to describe the particles present. So, for example, “water” comprises the species “water molecules”. The properties of the substance are taught very specifically as bulk properties, without mentioning particles. This would help students learn about the properties alone, without associating these with the particles present.

After encouraging use of separate terms Loeffler suggests gradually integrating them, making names of substances more precise, for example,

“Na, atomic sodium .. O₂ molecular oxygen … S, elemental sulphur” (p 929)

Although this is a good idea, as the macro-microscopic distinction is vital to address, it seems problematic to describe sulphur as “elemental” in contrast to sodium and oxygen which are also both chemical elements. The strategy adds an extra meaning to “element” beyond the traditional chemists’ view, so may cause confusion later.

Vogelezang (1987) also thinks that the notion of “substance” should be taught before learning about atoms and molecules because this relates more closely to students’ own experiences. As students tend to think of matter as continuous, the term “substance” is closer to their notion of “stuff” than are particle-oriented words “atom” and “molecule”. Vogelezang acknowledges that students still need to know about atoms and molecules and advocates de Vos’s and Verdonk’s (1985a, b, 1986, 1987a, b) strategy for this (discussed later). Nevertheless, the proposal supports the views of Stavy (1990a, b) and Novick and Nussbaum (1981) who believe that visual images help students learn the accepted scientific view of matter presented in science lessons.

However, Johnson (1996) points out that “substance” does not stand alone as a concept, but relates to other ‘component’ ideas such as material/object, purity, and chemical change. He found that 11-14 year olds misapply these component ideas so do not have a chemist’s view of “substance”. For example, the students in his study did not classify an iron nail and iron wool as “solid”, because they thought of solids as “having no holes” or existing in “lumps”. A chemist focuses on the material, rather than the shape, so regard both forms as “solid”. Use of “pure” is also problematic, because in the everyday world this implies “untampered with”, or “natural”. Children think of rock salt as “pure” but extracted salt as “impure” because it has gone through a chemical process. Similar reasoning is applied to distilled water. These ideas contradict with the chemist’s view that a pure substance comprises one single substance, rather than more than one.

Ahtee and Variola (1998) also found that students of all ages find the term “substance”

problematic. Students interchanged “substance” with words like “element” or “atom, for example:-

“Substances change outer electrons between them…” (17-18 year old).

These findings suggest that although using “substance” may be good in principle, clear foundations must be laid about chemists’ meanings of this term before it can be used in a strategy for teaching about chemical and physical changes.

Summary of key difficulties

Common practice is to develop chemistry in a hierarchical way building from particle theory, through separation of mixtures and the distinction between elements, compounds and mixtures towards chemical reactions and then features like chemical bonding, rates of reaction and so on. The success of this strategy is limited. Four key difficulties are presented.

1. Student’s thinking is not consolidated

The traditional approach does not permit time or space to develop and consolidate children’s learning about one idea before the next is presented. Assumptions are made at each stage that students have learned as the teacher intended. Little time is given to discovering children’s ideas and to addressing these. As a result, students exhibit very muddled thinking as they attempt to assimilate new scientific views about the world into their own structures.

2. Reasoning about reactions does not involve particles

Students’ reliance on continuous matter models leads them towards thinking about chemical reactions in the same way. Thus, bulk properties of a substance are attributed to particles – a copper sulphate particle would look blue, an atom of copper would conduct electricity and so on. Students may regard two forms of the same chemical element as different substances, due to structural variations such as those between iron wool and an iron nail. However, students show understanding of the differences between elements, compounds and mixtures when presented with diagrams, suggesting that visual images are helpful.

3. State changes are often thought to be chemical reactions

Students confuse state changes and dissolving with chemical changes. Chemists themselves do not help by arguing over details, missing the point for students! The key point required is that a chemical reaction involves making a new substance. Visual images are needed which make this clear and unambiguous rather than requiring the “leap of faith” suggested by Gensler (1970) in believing that a substance recovered from solution is the same as the starting material.

4. The language of chemistry causes confusion

Students meet many different terms in chemistry each with a specific meaning to chemists. In learning the basic ideas, these are often confused. The word “substance” for example, may be interchanged with “element” and “atom”. Introduction of the terms “element”, “compound” and “mixture” before students understand what happens in a chemical reaction may also create problems. Other descriptive words also cause problems. For example, the extraction of “pure” salt extracted from rock salt is not considered as purification, but producing a chemical product (Johnson,1996). Children need to be given opportunities to learn chemists’ meanings rather than to be told the terms alone.

Suggested activities^[1]

The Dutch educators de Vos and Verdonk (1985a, b, 1986, 1987a, b) propose a strategy for introducing chemical reactions entitled “A New Road to Reactions”. This five-stage technique requires teachers to avoid a traditional approach based on understanding detailed terminology and instead to present chemical events so students must think of explanations for what they see.

This sequence of steps describes a valuable way of providing visual images to help students form an accepted view of chemical changes. Students are assisted at the outset to make the physical/chemical change distinction and thereafter to realise that chemical changes occur on a microscopic scale between atoms. The approach challenges the sequence commonly used to teach about basic chemical ideas appears to create confusion for many secondary-age students.

1. Acknowledge a new substance is formed

Students grind potassium iodide and lead nitrate separately using pestles and mortars prior to tipping one solid into the other. Immediately on mixing, the powders produce a bright yellow solid (lead iodide) mixed with a white solid (potassium nitrate). The teacher fakes anger asking, “Who put that yellow solid in the mortar?” This leads to indignation: “I don’t know, it just appeared”, “It came from nowhere”, “It wasn’t me!” The teacher response is “Well it can’t have just appeared, it must have come from somewhere! Where did it come from?” Eventually, students may say that the white powders are like tiny eggs, that the yellow powder was inside, so mixing them broke the “eggs” and caused the yellow stuff to appear. Andersson (1990) suggests this arises because:-

“It seems that most children at the age of 14 still firmly adhere to an unspoken and unconscious idea that each individual substance is conserved, whatever happens to it.” (p 4)

Recognition of the yellow stuff as a new substance is the key point - hence they are reminded that if a white substance was made of “tiny eggs”, the yellow stuff would have appeared during the grinding prior to mixing. Students prefer intuitively to think of the two original substances as existing with the yellow stuff, but something stopped them from seeing the yellow material at the start.

With persistence students admit the substance is new and “just appeared”. The event creates cognitive conflict, as the result and questioning challenges students’ thinking. De Vos and Verdonk note:-

“The role of the teacher is to make it harder not easier [italics added] for the student to abandon his or her former idea. The new view on substances should be a personal victory of the student and something to be proud of…” (p 239)

2. Extend this thinking to other reactions

Students carry out the same reaction, but add small quantities of solids to water in a petri dish. Small amounts of the lead nitrate and potassium iodide are placed at opposite sides of the dish. After a few moments, a line of crystalline yellow lead iodide appears in the centre of the dish. Students may explain this using the idea that “molecules” of the substances “attract” one another. This is dispelled when students repeat the experiment by adding one reactant to the dish a few minutes before the other, resulting in instant formation of the precipitate. Other combinations of substances including sugar and salt and salt and lead nitrate help students to realise that precipitates do not always form, even though “molecules” of the substances collide with each other. At this stage, students can be encouraged to think that the particles are very small, otherwise they would be seen moving through the water.

3. Show reactions involving heat generation

Allow students to feel the temperature rise occurring when steel wool is placed in copper sulphate solution. The authors point out:-

“[Students] are not looking for a general statement [to explain events] and they have no reason to generalise about chemical reactions on the basis of one particular experiment.” (p 973)

This is important, because if a teacher gives a general explanation, students may think that all reactions produce heat. Next, students measure the temperature change occurring when sodium hydroxide solution is titrated against hydrochloric acid. Students are asked to explain where the heat comes from. Work with the students towards the answer that new chemical bonds are formed.

4. Introduce the idea that particles are rearranged when chemical reactions occur

The fourth step introduces students to the idea that chemical reactions occur because particles in substances are rearranged. At the start, in stage one, the students thought that the white solids remained unchanged, and that the yellow substance already existed. They were conserving the identity of the white substances and did not realise that these changed in the chemical reaction. de Vos and Verdonk (1987a) note:-

”..most students attribute a particular identity to a molecule and suppose the molecule keeps this identity throughout chemical reactions… According to this view … a molecule can go through many radical changes and yet retain its identity and belong to the original species.” (p 693)

At this stage the students’ tendency to conserve identity of substances is dealt with. Students need to learn that although an atom retains its identity during a chemical reaction, a molecule does not. The making of new bonds implies that new molecules are made from the original particles. The term “atom” can be introduced later. The authors acknowledge that changing students’ thinking is difficult.

5. Illustrate the principles by decomposing malachite

When malachite is heated the “molecule” is broken down into two other substances. de Vos and Verdonk (1987b) propose using the decomposition of malachite to introduce the idea that a “molecule” of malachite can be “broken”. After this, using a copper cycle, they introduce the idea that a chemical element, copper, cannot be decomposed into anything else. Only then is the term “atom” introduced.