Students’ ideas about the particulate nature of matter

Findings from these studies lead to the view that particle ideas are poorly grasped, as even with prompting around 25% of students of mixed age used only continuous ideas of matter in their answers.

Matter is made of discrete particles

Children’s naive view of matter is based on the “seeing is believing” principle. Particles cannot be “seen”, so they do not need to exist in a functioning model to explain the behaviour of matter. Novick and Nussbaum (1981) describe the basic learning problem as requiring a learner to:-

“…overcome immediate perceptions which lead him to a continuous, static view of the structure of matter. He must accommodate his previous naive view of the physical world so as to include a new model adopted by scientists. Internalising the model therefore requires overcoming basic cognitive difficulties of both a conceptual and a perceptual nature.” (p 187)

Evidence indicates that teaching does prompt change in children’s thinking. In their 1978 study, Novick and Nussbaum used interviews to probe the understanding 13 - 14-year-olds had about gases after teaching, finding that about 60% consistently used particle ideas. This figure increased to more than 90% at age 18+. CLIS project involving 15 year olds (Brook, Briggs and Driver, 1984) reports that over half the sample used particle ideas consistently in response to a wide range of questions covering all three states of matter.

Recent teaching, as in the Novick and Nussbaum study, generated even higher proportions. Johnson (1998a) reports results of a longitudinal interview-based study of 11-14 year olds’ understanding of particle ideas. He found that over a two year time span most of the thirty three pupils moved to a particle model for matter which included scientifically accurate aspects.

Students who do not use particle ideas may use the bulk properties of substances instead. For example, the CLIS study (Brook et al 1984), includes this response in answer to a question concerning the change in temperature of a block of ice:-

“As the temperature rises to -1 oC the ice will melt causing the block of ice to get smaller” (p 57).

And about car tyre pressure during a journey:-

“When a car goes on a journey, the tyres start to warm up and this causes pressure”. (p 35)

Brook et al call these “low-level macroscopic” answers, given by children who think of matter as continuous. Many children who appreciate that matter is particulate do not relinquish all their naive view, so ascribe bulk properties to particles themselves:-

“[particles can] change their form [solid to liquid]; explode, burn, expand, change shape and colour, or shrink” (Happs 1980 p 9 - 14).

Griffiths and Preston (1992) found similar ideas. Their small-scale study reports that about 50% of 18-year olds think water molecules in steam are larger than those in ice. This type of explanation seems to be an “intermediate” stage between full appreciation of the particulate nature of matter and naive ideas. Although some students may develop a scientific view, many people may not move from this intermediate stage.

Particles are in constant random motion

Evidence indicates that random particle motion in liquids and gases is difficult to appreciate. For example, Westbrook and Marek (1991) carried out a study involving about 100 undergraduates, none of whom attributed dye diffusion to random motion of particles.

Students aged 16 and above seem to accept that gas particles are uniformly distributed in a vessel (Novick and Nussbaum 1981), but when asked, “Why don’t the particles fall to the bottom?”, only around half thought that the particles were in constant motion.

Space between particles is “empty”

Novick and Nussbaum (1978, 1981) investigated this notion in studies involving Israeli 13-14 year olds and 10-20 year old Americans. They showed that the notion that empty space exists between particles causes students considerable difficulties. They found that 25% of the younger group suggested that although the particles were themselves discrete entities, the space between them was either filled, for example, with:-

“Dust and other particles; other gases such as oxygen and nitrogen; air, dirt, germs; maybe a liquid; unknown vapours..” (Novick and Nussbaum, 1978 p 276)

or was non-existent, for example:-

“The particles are closely packed - there is no space between them” or “No place is completely empty”. (p 276).

About 40% of 16+ year olds responded to the question “What is there between particles?”, with “vapour or oxygen”, while a further 10 - 15% thought “a pollutant” was present. University science students also use this “space-filling” model (Benson et al 1993), of whom about 33%

“seriously underestimated the relative amount of space between the gas particles themselves.” (p 596).

Students of all ages find space difficult to imagine and intuitively “fill” it with something. Since students depend on visible, sensory information about solids and liquids to develop their naive view of matter, their difficulty accepting a model proposing there is “nothing” in the spaces between particles is unsurprising.

‘Bonds’ or ‘forces’ exist between particles

Students seem to use the notion of forces between particles rather than constant motion to explain gas behaviour. Novick and Nussbaum (1978) asked 13 - 14 year olds to draw a picture to represent air in a partially evacuated flask. A significant proportion drew air around the sides of the flask, or in a mass at the bottom. Others, who indicated that air was composed of tiny particles, showed the particles in clumps or occupying only part of the flask. Explanations offered for these pictures included, “They are held in place by attractive forces…” (Novick and Nussbaum, 1978 p 277). Their 1981 study revealed that about 20% of 16+ year olds think “repulsive forces between the particles” prevent particles falling to the bottom of the flask. The attractive and repulsive force ideas imply static particles, confirming that particle movement in a gas is difficult to grasp. The ‘attractive forces’ suggestion supports the “clumped together” model, while the notion of repulsive forces “explains” the uniform distribution of particles. No evidence exists to indicate whether any individual student changes from one idea to another between the ages of 14 and 16. However, on accepting that particles are uniformly distributed, the attractive forces notion becomes redundant, so a student may use a new explanation, repulsive forces, instead. The ideas are Brook, Briggs and Driver (1984) found that a significant proportion of 15-year-olds use attractive forces between gas particles to help explain air pressure. Some students suggest the strength of the forces is temperature dependent. Other 15-year-olds did not think forces existed between particles in the solid state (p 74). The report does not indicate if these students also think forces exist between gas particles. However, Engel Clough and Driver (1986) and Stavy (1988) among others report that students do not apply ideas consistently to problems, so the same student could imagine forces to be present between gas particles and not between particles of a solid phase substance.

Students thinking about attractive and repulsive forces may find it hard to learn scientifically correct ideas about changes of state and chemical bonding, both of which involve interaction between particles.

Summary of key difficulties

Four key misconceptions about the particle theory and matter are:-

1. “Matter is continuous”

A small proportion of 16 year old students are likely to use a developed particle model to explain physical and chemical phenomena. The continuous model of matter is powerful, such that despite teaching most students use only a primitive particle model, retaining aspects of this naive view. For example, some 16-year olds think the space between gas particles is non-existent or filled, or that particles expand when they are heated. Other students who understand that the gas particles are distributed uniformly explain this by suggesting that repulsive forces exist in between them so implying they are static. A small proportion of students do not use taught particle ideas at all, offering only low-level macroscopic responses to questions involving particle behaviour retaining their naive view of matter in a more complete form.

2. “The space between particles is filled”

Novick and Nussbaum (1978) concluded that:-

“The aspects of the particle model least assimilated by pupils in this study are those most in dissonance with their sensory perception of matter” (p 280).

The notion that empty space exists between particles is problematic because this lacks supporting sensory evidence. Stavy (1990a) and Benson et al (1993) suggest that visual evidence may help to change students’ ideas, since only then is the inadequacy of the naïve model made apparent.

3. “Bonds or forces explain how particles move”

Students may reason that attractive forces are present between gas particles and that these explain why gas particles may clump together. A student may modify this later to explain the uniform distribution of gas particles in terms of repulsive forces. In contradiction, forces may be present when the substance is gaseous, but not when solid. These ideas may contribute to difficulties for students in understanding chemical bonding.

4. “Particles can change form”

Students ascribe macroscopic properties to particles. For example, particles may explode burn, contract, expand and / or change shape. This primitive reasoning prohibits understanding of the nature of a chemical reaction.

Suggested activities

In the UK, 11–14-year-olds receive formal teaching about the particle model. Their background is usually a range of ideas about materials gained from primary school. Teaching should allow children’s ideas to develop by revisiting the topic and by providing opportunities for misconceptions to be expressed in a “safe” environment.

1. Be “up front” about the problem

Be honest that the invisibility of particles to the naked eye means our minds “see” materials as continuous. Explain that even scientists themselves did not understand about particles until quite recently – they had been at work for nearly 2000 years before the idea of atoms was accepted in the early 19th century.

The implication from this is that we cannot expect children to change their thinking overnight if scientists took this long to make the “discovery” themselves! Children may accept the existence of particles readily, but take a long time to assimilate the implications of this model for the behaviour of matter.

2. Make particles visual

Give children an idea of the scale of particles “smallness” by showing a range of microscope images of small items which we normally cannot see, for example, details of insects, bacteria, viruses. Ask what these organisms or items are made from. Atoms must be smaller than these! Introduce the idea of an “atomoscope” – a special microscope which can be used to look at atoms, or the idea that they have “molecular spectacles” so can “see” atoms. What might they look like? Ask children for their ideas, perhaps drawing pictures.

Then introduce the scanning tunnelling microscope (STM) as a real life “atomoscope”. Show pictures. Invite children to look at a sheet of copper metal and imagine the atoms comprising the structure. Ask them to imagine the sheet stretched to cover 50 miles from end to end and to estimate the size of the atoms in the sheet using a football, tennis or golf ball as a guide. When everyone has guessed, reveal that an atom would be approximately 1cm in diameter (in the UK the sweets called “Maltezers” are conveniently this size), much smaller than anyone will have thought!

3. Integrate particle ideas into other topics

Particle theory is often taught in isolation. This does not help students from appreciating particle behaviour in other situations. Use particle terminology when talking about chemical reactions or changes of state, for example, referring to “sodium particles” and “chlorine particles”, rather than just using the element names which refer to bulk substances. For the moment, this will suffice – the differences between atoms and molecules can be introduced later. Support this with models or images of the particles. Introduce simple symbol equations as soon as possible rather than using word equations that emphasise bulk materials rather than particles.

4. Use diagnostic questions

Explore students’ thinking by giving situations to explain, such as “If you pump air into a soft bicycle tyre or football, does the mass increase, go down or stay the same?”. Students should select the answer they think is correct. Then actually do the experiment. A sensitive balance is required to demonstrate that mass would increase. Students will usually respond that the mass would stay the same because gases have “no mass”. They will be surprised to see the mass increase, so be ready to help them adjust their thinking by encouraging the idea that particles have mass!

Examples like this can be extended to encourage thinking about particle movement. Ask what would happen to the pressure inside a tyre or football left in the sun on a hot day. Ask again if the mass would increase. This time, there is no change in mass, as no more gas has been added, but the tyre/football has become harder, just as before. So why has this change occurred? Questions and answers can be used to lead students to the idea that the pressure inside has increased and that this is caused by increased particle movement.