The particle model

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Johnson, P. (1998). Progression in children’s understanding of a ‘basic’ particle theory: A longitudinal study. International Journal of Science Education, 20 (4), pp.393–412.

Very often students will get asked to label a particle diagram or to draw the particle diagram for a given state, however this paper suggests that even if answering ‘correctly’ students may hold a range of misconceptions.

The particle model under discussion in this paper is described as a model with sufficient detail to account for the characteristic properties of the three states of matter. It is not, the author specifies, a model that distinguishes between the types of particle (atoms, molecules and ions). 

The author summarises the common findings of the existing literature about student understanding of the particle model, identifying five areas of difficulty:

1. The relative spacing between particles in the three states.

Showing the spacing between particles in the liquid state as intermediate between the solid and gas states.Typical diagrams for the gas state underrepresent the relative spacing of particles.

2. The intrinsic motion of particles

Many students showed little appreciation of the intrinsic motion of particles.

3. Ideas of forces of attraction between particles

Very few students used the idea of forces of attraction between particles.

4. The ‘space’ between particles

The idea that there is ‘nothing’ between particles seemed to cause a lot of difficulties for students. Some preferred to think that something must be there (often referring to this as ‘air’).

5. The nature of the particles themselves

Many students were found to give particles the same properties as the bulk material. For example, a copper atom was thought to have the same properties as copper metal.

The paper reports on a three-year longitudinal study following a cohort of pupils in a non-selective English secondary school as they moved from year 7-9 (ages 11 to14). Students were periodically interviewed following teaching of four planned unit designed to develop the idea of a chemical substance.

The responses were used to identify four distinct particle models held by the students.

Model X: Substances are continuous (and not made of particles).

Model A: Particles are found in the continuous substance.

Model B: Particles are the substances, but with the macroscopic character of the bulk substance.

Model C: Particles are the substance and the properties of the substance in a given state are a collective property of those particles.

The interview questions enabled the author to identify the model of thinking held by students at the time of the interview. Whilst there were some students were inconsistent and applied different models in different circumstances, the majority had complete models of either X, A, B or C. This method of categorisation enabled the author to explore how student thinking about the particle model changed over time.

In general, many students were found to progress in the model that they were using. The author identified two different ‘dimensions’ of this progression: a continuous to particulate dimension and a macroscopic to collective properties dimension.

BEST Diagnostic question

Imagine you could see the particles in a jar of methane gas.

Which diagram best matches what you would see?

 

The correct answer is C.

Reflective questions

When teaching the particle model, do you teach what is between the particles? How do/could you explain the concept of ‘nothing’ to students?

It is not uncommon to refer to the ‘particles in a solid’. To what extend could this language reinforce existing misconceptions and how could the language be changed to avoid this?

An atom is often defined as ‘the smallest particle of an element’. To what extent could that reinforce earlier misconceptions about the particle model? How could this be mitigated?

Useful links

BEST Topic 1 Key concept 1: Particle model for the solid, liquid and gas states

Diagnostic questions to check for student misconceptions about the particle model as part of a five-part progression (and including response activities)

University of York Science Education Group

Developing understanding: States of matter

A ramped student worksheet that aims to help students to deepen their understanding of the particle model and to strengthen their mental models.

Royal Society of Chemistry

Acknowledgements

The example BEST diagnostic question was developed by Helen Harden (UYSEG), from an idea by Andrew Hunt selected from a collection of ASK items devised for research by Philip Johnson (Durham University).

The particle model image is from the RSC Johnstone's Triangle Resource: States of Matter

 

 

 

 

 

Compounds: The importance of emergent thinking

 

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Talanquer, V. (2008). Students’ predictions about the sensory properties of chemical compounds: Additive versus emergent frameworks. In: Science Education. 92 (1). January 2008. pp.96–114.

This research investigated the reasoning used by students to predict the properties (colour, smell or taste) of the compound formed from the reaction of two substances, each with their own given properties. 

The opening of the paper introduces the idea of "commonsense reasoning" that can result in naïve explanations by novice learners.  The author shares findings from the research literature relating to the use of this “intuitive thinking”. It is suggested that an intuitive thinker is more likely to create an explanation based on a single mechanistic cause that produces a linear progression of events.

This study was carried out in the U.S. with over 400 students in their first year of a general introductory chemistry course for science and engineering majors. These first-year student participants were still regarded as “novices” for the purposes of the study.

The focus of the study was on the properties of compounds and so it is still potentially highly relevant to those teaching younger students as the properties of elements and compounds commonly features in the school chemistry curriculum for students aged 11 to 14. 

The paper describes one way of reasoning about the predicted properties of a compound (from given information on the properties of the reacting substances) as an additive framework. Using this framework, the properties of a compound are thought of as a linear combination of the original properties of each component.

This is to be contrasted with the use of emergent framework of thinking in which the properties of a complex system result from an interaction of its parts. This is the framework of thinking that should be applied to the properties of compounds. The properties of a compound arise (or "emerge") from the arrangement of atoms (or ions) and not from the addition of the properties of the substances from which it is formed. 

The author devised a series of questionnaires using multiple choice questions and black and white particle diagrams. The questions asked students to select the answer that best predicted the colour, taste or smell of the compound resulting from a reaction between the two substances depicted. Some students also took part in follow-up interviews to determine the reasons for their answers.

The researchers used a range of examples in the multiple-choice questionnaires including:

• varied ratios of reacting substance particles
• different sizes of reacting substance particles
• reacting substances with no property (e.g. no colour)
• the answer options of “other” and “more information needed to make a prediction”

A student who was confident in using an emergent framework for their thinking would be expected to consistently answer “other” or “more information needed” across all questions.

Less than 3% of students were found to consistently respond in this way.

In the initial question in which a blue substance reacted with a yellow substance with a 1:1 ratio of particles, 90.4% of participants selected the answer “green” as being the property of the final compound.  This suggests that the vast majority of students in the study were applying additive rather than emergent thinking.

The questions which included a non 1:1 ratio of reacting substance particles provided further evidence of this additive thinking. In a similar question to above (but with a 4:1 ratio of particles), 79.6% of students answered blue (rather than green).

The interviews revealed that even when some students did select “other” it was not necessarily due to emergent thinking. Sometimes students said that there was a need for more information on the “dominance” of a particular property. For example, if a blue substance reacted with yellow substance (in a 1:1 ratio of particles) might the blue dominate over the yellow rather than answering “green”?

If additive thinking is indeed present in other educational contexts, then this raises questions about the teaching of this topic. The reflective questions below raise some points to consider about the way in which this topic is commonly taught which could inadvertently encourage additive thinking.

The final recommendation of the author of the paper is that “helping students recognise the existence of emergent properties in chemical systems is crucical if we want them to develop meaningful understandings of a variety of topics”. This  raises questions regarding curriculum priorities and the importance of getting the earlier years (age 11-4) right to provide a sound foundation for later chemistry learning.

BEST Question

A compound Is made up of a combination of atoms from a blue substance and a yellow substance.

What colour is the compound?

A blue

B yellow

C green

D other

The expected answer is D "other". 

The colour of the compound is not related to the colour of substances made from its constituent atoms. This information is not sufficient to predict the colour of the compound.

Students who are using an additive approach are likely to predict that the compound is green (option C). A prediction of blue (option A) may mean that the student thinks that a darker colour may overwhelm the yellow.

Useful links

BEST Topic 2 Key concept 1: Atoms and molecules

Diagnostic questions to check for student misconceptions about the atomic model as part of a five-part progression (and including response activities)

University of York Science Education Group

Reflective questions

This study was undertaken with students in their first year of a general chemistry course in the U.S. To what extent do you think an additive framework of thinking is applied by students in your school context?

Look at some particle diagrams that you use to teacher elements and compounds. To what extent could these reinforce additive thinking and what adjustments to the diagrams or your use of the diagrams could better emphasise the need for emergent thinking?

A compound is often defined as “a substance formed when two or more different elements are chemically bonded together”. To what extent could this reinforce additive thinking and what adjustments in the phrasing of the definition, or clarification when teaching, could reduce this?

 

 

Acids and bases: A curriculum in layers

 

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De Vos, W., & Pilot, A. (2001). Acids and bases in layers: The stratal structure of an ancient topic. Journal of Chemical Education, 78(4), 494–499

This paper poses challenging questions for contemporary curriculum developers. At what point does the curriculum evolve to account for new scientific developments? How long after a new development in science should it impact upon the school-level curriculum? Historically science curricula across the world have evolved to include new discoveries such as DNA. Are more recent developments appropriate and important to include or are they to be considered as “beyond” school science?

 A common concern of teachers, and their students, is that the science curriculum is “too full”. Does this thinking limit the addition of newer chemistry ideas? Does the methodology of constant addition rather than restructuring limit the contemporary relevance of what students study at school?

In this paper the authors explore the presentation of the topic of acids and bases in school textbooks through time. The textbook chapters are effectively a proxy for chemistry curriculum content at the time of publication.

The authors propose a connection between developing scientific views about the nature of acids and bases through history with the presentation of this core topic in school textbooks.

The authors use the metaphor in the title of this paper “A stratal structure of an ancient topic” to encapsulate how evolving ideas about acids and bases became gradually added (and not superseded) in the chemistry curriculum. It is likely that the following quote will resonate with anyone involved with curriculum development to this day.

“Apparently it was much easier to add something to the existing curriculum than to remove something from it or to restructure it”.

Rather like an archaeologist digging into the layers of history, ideas about acids and alkalis can be traced through time from the earliest to the most modern layer at the time in which the paper was published.

Image by kp yamu Jayanath from Pixabay

In each layer the authors describe a “context”. This puts into perspective the way in which acids and alkalis were perceived.

The authors categorise six different layers which are outline below.

Layer 1: Craft context

Acids, bases, alkalis and neutralisation have been known about for hundreds of years. Modern nomenclature was not used and this early knowledge related more to forms of "recipe”. For this reason, the authors named this layer as being based in a “craft context”.

Acids and bases have been included in the chemistry curriculum since the early 19^th century. This “craft layer”, according to the authors, forms the bottom (first) layer.

Layer 2: Synthesis context

The trigger for this second layer was the work of Antoine Lavoisier. Following discovery of the critical role of oxygen in combustion there was as shift in thinking to consider the synthesis of acids. Lavoisier introduced a systematic naming approach for inorganic acids by linking the name to the element from which each acid could be formed. For example, sulfuric acid was formed from sulfur.

Lavoisier categorised bases as deriving from metallic elements via their oxides. Acids reacted with bases to form salts and water.

The authors contrast the highly practical approach of the craft context with the more theoretical approach of the “synthesis context”.

19^th century chemistry courses adopted this synthesis context through the inclusion of a chapter on acids, bases and salts. This provided a systematic overview for students of then contemporary inorganic chemistry.

Layer 3: Analytical context

The authors describe how the introduction of the periodic table in the second half of the 19^th century led to acids and bases losing their central position in the theory of inorganic chemistry. However, a new reason was found to be important to keep them in the chemistry curriculum.

Half-way through the 19^th century (and later in the chemistry curriculum) the understanding of reactions occurring in fixed proportions resulted in the development of analytical chemistry.

Ideas such as neutralisation and end point (as well as gram-equivalent weights and normality) became a core part of chemistry education. Apparatus such as pipettes, burettes and conical flasks became key to school studies of chemistry.

Layer 4: Arrhenius context

Towards the end of the 19^th century, the authors explain, physical chemistry began to emerge. The ionic theory was developed by Arrhenius. This familiar theory stated that in aqueous solutions acids and bases are ionised (completely if strong and partially if weak).

As a result, in the first half to the 20^th century the topic of acids, bases and salt became used as an illustration of ionic and equilibrium theories.

A new definition of an acid emerged as a “hydrogen containing substance that in aqueous solution produces hydrogen ions in solution”. The new definition of a base was a substance containing a hydroxyl group that in aqueous solution produces hydroxide ions.

New concepts were introduced to the curriculum such as K_a, pH, pK_a, pK_b and pK_w alongside calculations involving logarithms.

Layer 5: Brønsted context

The authors decided to make this as a separate layer due to the Brønsted-Lowry and Lewis theories referring to protons and electrons (atomic structure).

The authors suggest that the shift from the Arrhenius context to this context was substantial.

In this context, acids and bases are no longer described in terms of substances. For example, sodium hydroxide is no longer the base. The base is the hydroxide ion because it can accept a proton.

The authors take this further to explain that a neutralisation reaction is now defined in terms of its mechanism (transfer of a proton from the acid to the base).

Layer 6: Application context

This layer was contemporaneous with the writing of the paper. The authors describe how in recent years there have been no new theoretical layers added to chapters in chemistry textbooks. They suggest that this is because acids and bases are no longer a key area of modern chemical research.

The new layer, the authors suggest, arose from an educational viewpoint of the importance of the making clear the relevance of the acids and bases topic to students and society.

At this point textbooks started to make connections to the neutralisation of acid in the stomach or the environmental issue of acid rain. Analytical chemistry became set in more relevant topics such as analysis of products from a supermarket.

Implications for teaching and the curriculum

As a result of their detailed analysis and categorisation the authors’ discussion revolves around the problem that these different contexts are not always clearly distinguished in school textbooks. Even more problematically the authors give examples of where mixed contexts are used.

A very simple example of this is the use of the word “acid” as a noun. It can have three different meanings depending upon the context. In the craft context an acidic solution IS the acid. A bottle contains “hydrochloric acid”.

In the synthesis, analytical and Arrhenius contexts the substance in the bottle is a solution of an acid. Finally, in the Brønsted context an acid is defined in terms of particles so “hydrochloric acid” is said to contain the acid hydronium ion.

It is clear to see that unless these differences are made clear in teaching there is a lot of scope for confusion.

The authors conclude with the following remark.

“The stratal structure is not a result of a well thought out pedagogical strategy. Instead, it is a product of a historic process, and accumulation of successive, separate decisions.”

The authors caution that this stratal structuring is not restricted to the chapter on acids and bases and suggest that it could be a reason why so many people claim to have “never understood anything of chemistry”.

This issues a challenge to modern curriculum development. At what point in educational history will anyone have the courage to tackle the traditional and worldwide fundamental chemistry curriculum to ensure a properly planned progression for students of the future.

Reflective questions

What does the word acid mean when you are teaching students aged 11?

At what point in the curriculum does the context and hence meaning of an acid change?

Is a mixed meaning of the word acid ever used and how could this be avoided or be made more explicit?

What could curriculum developers do to support a more coherent approach to the teaching of acids and bases in school?