The Association for Science Education

K3.1 Classifying Materials


This unit explores the applications and implications of the ‘particulate theory of matter’ including both the kinetic theory of matter and the ways in which atoms of various elements can link together to form ‘substances’ be they elements, compounds or mixtures. A key aspect of the understanding of chemistry is that its processes involve the conservation of atoms of all the elements involved.

The introduction reminds of some aspects of learning theory outside the general perview of constructivism (notably the work of Alex Johnstone) that are particularly relevant to the learning of chemistry. The unit then covers general issues in the classification of substances including solids, liquids and gases, changes of state, particle ideas and elements, mixtures and compounds. An expected progression of children’s ideas is provided as are some useful references.

This is one of 17articles whose main aim is to support the processes of teaching/learning between the science education tutor and the trainee science teachers with a focus on “teachers’ knowledge and understanding”. During a primary or secondary BEd, PGCE or GTP we hope that those learning to become science teachers will be able to challenge their own understanding of science and scientific concepts. Unit K0 specifically explores general issues relating to all the knowledge units - to the learning of science.

Standards: This unit specifically addresses Q14 but, appropriately used can contribute to and provide evidence of competence for many others of the standards especially Q4,6,7,8,18, 22 and 25.

Keywords: Secondary, Primary, Teacher knowledge, Chemistry, Practical work, Demonstrations, PowerPoint presentation.


1.0 Introduction
2.0 Issues in understanding the nature of materials
2.1  Substances
2.2  Solids, Liquids and Gases and Changes of state
2.3  Particle Ideas
2.4  Elements, Mixtures and Compounds
3.0 Progression in children's ideas about the nature and classification of materials
4.0 Useful References

1.0 Introduction

Richard Feynman (See Gleik J, 1994) suggested that should all scientific knowledge be destroyed and only one sentence were available to pass on to the next generation - that sentence should be:
‘That all things are made of atoms - little particles that move around in perpetual motion attracting onfe another when they are a little distance apart, but repelling upon being squeezed into one another.’ In this one sentence there is an enormous amount of information about the world, if just a little imagination and thinking are applied. (p358-9)

Of course there may well be other things you feel it would be helpful or necessary to know if you were in the process of trying to reconstruct the basic useful ideas of science. Some of these, we hope, are explored in the science knowledge units. However, the complete list should come close to what you believe should be the science curriculum for pupils in our schools. Download k3.1_1a provides a beginning list of ‘other useful things to know about atoms and particles' - there is plenty of room for additions (and subtractions) but remember that there have to be good reasons and evidence of both reasonableness and utility. Above all on this planet atoms are (except in nuclear reactions) indestructible and are constantly cycled by the degradation of energy from the sun and from within the earth.
Download K3.1_1a 'Other useful things to know about atoms'

Some views complementary to ‘constructivism’

Much of the science material currently covered on this site takes an unashamedly constructivist view of learning - there are, however, a number of valid and important psychological insights and perspectives available from other areas of learning/teaching theory that we sometimes tend to undervalue. A useful, complementary (and provocative) view of issues in research into the learning of chemistry is given in Download k3.1_1b.  This is a summary of the 1999 Chemistry Education Research Lecture given by Alex Johnstone to the Royal Society of Chemistry. (Alex has researched and written extensively about the teaching and learning of chemistry and his publications are widely distributed in appropriate journals.)

The paper (Download k3.1_1b) covers:

  • A short critique of the emphasis, in science education research, on pupils’ alternative conceptions.
  • Particular difficulties for chemistry learners (from KS3 and especially beyond) caused by having to switch attention rapidly between the macro-scale (What is seen and experienced directly), the sub-micro-scale (atoms, molecules, ions, electrons etc. that are essentially invisible) and the representations (Symbols, formulae and diagrams) chemists use in their writing and explanations. There are issues also with developing specialised vocabulary, but these are similar in other subjects.
  • Learners are essentially ‘information processors’ - and the system is prone to overloading and the learner to ‘confusion’
  • Specific chemistry issues are explored including the mole, chemical equilibrium (learners usually come first to an understanding of equilibrium in physics - it seems that here lies the source of many of the problems we have in thinking about the dynamic process of chemical equilibrium in the face of the static situation in physics.)
  • Practical work in science is seen as leading to an unstable information overload for the learner almost by definition. This can be overcome by very careful preparation and follow-up to practical sessions, but in practice this is rare. 

Download K3.1_1b 'Alex Johnstone (1999)'

There are other specific ‘teacher behaviours’ that can be employed to enhance learning (not evidenced here):

  • Engagement and involvement of the learners.
  • Mutual trust between teacher and student - and by the ‘system’ in the teacher.
  • Importance of ‘mastery learning’ of basic ideas and skills so that ‘chunking’ can occur and so reduce the demand on information processing. e.g. For a beginner a chemical formula such as H2SO4 acts as a number of separate bits of information - two atoms of hydrogen, one of sulphur and four of oxygen somehow joined together. With experience, use and understanding this formula can carry with it the chemistry of sulphuric acid as a single 'bit' or 'chunk' of information.
  • Importance of appropriate questioning and ‘wait time’ for the students to answer. This is to encourage thoughtful and considered answering rather than guessing and rote learning. It can be extended by using talk-partners to give pupils the time to rehearse an answer before having to go public etc.

2.0 Issues in understanding the nature of materials

2.1 Substance

Download k3.1_2.1a: This is a power-point presentation from a lecture on ‘Matter’ by Keith Ross. It is based on the idea that when we throw something away we tend to think it has ‘gone’. But matter cannot be destroyed, at least not at an atomic level during normal chemical and biological change. But to understand the fate of things thrown away we need to know their chemical make-up, and how they might interfere with (or help) living things. A study of recycling is a wonderful starting point to help children understand the structure of materials.

From a very early age we become familiar with the presence of ‘things’ and ‘other people’ in our environment and, well before we begin formal schooling, are able to differentiate between ‘things’ and ‘living things’ (Although it is a good while later that we begin to accept plants and seeds as ‘living’ and later still for bacteria and algae! What about viruses?)

Substances are the ‘stuff’ or ‘matter’ that all things are made of. Initially we need to learn to differentiate the things themselves (eg a chair) from the substance(s) or material(s) of which they are made (eg wood or plastic). In fact most ‘things’ are made of many different substances so learning about this is rarely complete and only sometimes important:

  • Rivers, ponds, seas and lemonade (and people!) are made mainly of water
  • Books are made of paper (paper is made mainly of cellulose - an organic polymer - based on the element carbon)
  • Coins are made of metals (Almost always ‘alloys’ that are mixtures of different metallic elements.)
  • Clothes are usually made from fabrics (woven from: cotton threads, silk, wool, Terylene, nylon etc. Sometimes the threads are made of mixtures of different fibres. All these fibres are organic polymers)
  • Mountains are made of granite, limestone, dolomite, sandstone (and many more rocks and soils - mostly made of compounds of the element silicon)

Download K3.1_2.1a 'Matter and Recycling'


It is often useful to classify substances into different groups. All such classifications have their uses and drawbacks. Download k3.1_2.1b is an analysis of some of the ways we try to classify matter and some of the difficulties associated with each of them.

Download K3.1_2.1b 'Classification of Materials'

2.2 Solids, Liquids and Gases

These are often called the three states of matter (Sometimes a fourth is added - the plasma state - but this rarely of interest before A-level studies.). Using Solid/Liquid/Gas as a way of classifying materials is not actually very helpful since most substances are solids at room temperature - this is discussed in download k3.1_2.1b above.
Three key issues are:

  1. It is easy to distinguish between the three states: solids maintain their own fixed shape and volume; liquids have their own volume but take the shape of their container and retain a level surface. Gases take the shape of their container and its volume since they will fill it. These are fairly straightforward but the ideas need to be practised and there are difficulties: e.g. finely powdered solids seem to behave like liquids, they take the shape of their container and can be poured - but the surface does not remain horizontal when the container is tipped; very viscous liquids flow very slowly and may seem more like solids. A good way to distinguish between powders (solids) and true liquids is to ask, “Can they be piled up - will they stay in a pile?”
  2. Depending on the temperature most substances can exist in all three states. The typical example is water; at normal room temperatures it is a liquid, below 0oC it freezes to become ice and at 100oC (and at normal atmospheric pressure) it boils to become steam/water vapour. Actually water does slowly evaporate at ALL temperatures if its surface is exposed and the ‘air or space’ above the surface is relatively dry - even frozen wet clothes will ‘dry’ on a washing line without the ice melting first. The main exception to other substances melting and boiling as the temperature rises is when they change chemically before one or both of these happen, for example wood.
  3. These states and changes in state are generally called ‘physical changes’ and are usually modelled or explained using the Kinetic Theory. (See next section)
2.3 Particle Ideas

These will be found represented and explained in almost every science or chemistry text-book written for pupils at KS3 or above. It is instructive to see how well they make sense to our teachers in training.  It is not a trivial task to come to an understanding of the nature of the particles involved in any particular case - how the forces between the particles and their size (and sometimes the shape) of the particles affect their movement at particular temperatures and how the conversions from solid to liquid (and reverse) and from liquid to vapour (gas) and the reverse are controlled. (Note that the freezing temperature and the melting temperature of a substance are the same. It is also quite a sophisticated task to distinguish between evaporation and boiling.) Because the particles are able to move about in liquids and gases - diffusion is possible and, if more than one type of particle is present, the system becomes as mixed as possible, with high concentrations of any particle tending to spread out into places where the concentration is lower. (Remember: even when the system is completely mixed the particles continually move about - it is just there is no overall change on a large scale.) It is interesting to note that solid state diffusion occurs (very slowly) in metallic substances, which is why annealing, for example, can change the properties of a metal.

The particle model was identified as one of the key scientific ideas in the KS3 strategy. Unit 7G of the KS3 schemes of work covers this topic.

See Download k3.1_2.3a for a discussion of two important misconceptions concerning the particle representation of the states of matter:

  • Pupils (and text books) sometimes think particles representing the liquid state are more spread out than in solids - but solids and liquids have similar densities and they are both virtually incompressible, so their particles must be touching in liquids as well as solids.
  • they often think that particles representing the gaseous state are moving with more energy than in the liquid and the solid (this will be true only if the gas is hotter than the liquid or solid). 

Download K3.1_2.3a 'Particle problems'

Further misconceptions

  • Pupils often think that there has to be ‘something’ (usually ‘air’) between the particles! Slide 20 of Download k3.1_2.1a also shows a diagram drawn by post-GCSE student to illustrate a ‘story’ of a water molecule. This demonstrates a number of misconceptions - especially how difficult it is to visualise the ‘nothing’ that is between the particles.
  • Download k3.1_2.3b is the manuscript for a paper on Science Graduates’ understanding of evaporation and boiling which was later published in School Science Review (Goodwin (2003)). This piece of research looks at a more sophisticated level of understanding.

Download K3.1_2.3b 'Evaporation and boiling'

2.4 Elements, Mixtures and Compounds

This classification often arises early in KS3 - and sometimes at KS2 (with more able /interested pupils). It introduces the various sorts of ‘particles’ that we may be talking about and illustrates the changing meanings of some of the important ‘particle’ words as we learn more and are able to differentiate more as ideas develop.

The scientists’ views of ‘atoms’ has developed hugely over the past 100 years - although the older view of atoms at tiny indestructible spheres still serves for many purposes. (In the simple kinetic theory the indestructible sphere model also works for molecules - providing that no chemical reactions occur.) During the first half of the 20th century a model of the atom developed - with a very small very dense nucleus (made of protons and neutrons) and surrounded by a cloud of electrons. Most of the inside of the atom is ‘empty space’ - it is the forces of attraction between positively charged protons and negatively charged electrons that make atoms seem to be hard particles. The rearrangements of the electrons on the outer edges of atoms - give rise to chemical ‘bonds’ by which atoms sharing, transferring or letting electrons move relatively freely become linked in various ways to form molecules, ionic or metallic structures. Without exception these ‘bonds’ are formed by electrostatic attractions between oppositely charged particles - arranged so that the overall structure has the minimum energy. When these different substances (chemicals) interact such that the bonding is rearranged we have a chemical reaction and new substances are formed. (Note: in real life it is not always obvious when new substances are formed - this requires both careful observation, tests and sometimes some ‘chemical intuition’). A description of chemical bond formation will be found in most science text-books at GCSE and above but they frequently give ‘rules’ based on ‘octets’ of electrons without much mention of electrostatic attraction (except between oppositely charged ions) or the system having to find an energy minimum within which to ‘rest’ if it is to be stable.

A significant resource, produced by Keith Taber and published by the Royal Society of Chemistry (Taber - 2002) explores many of the misconceptions experienced in the learning of chemistry. Copies were distributed to all secondary schools when the materials were published, should be available in libraries and are still available from RSC.). Trainee teachers who will be covering the chemistry curriculum at GCSE and above really should have access to this very important research, much of which is freely available on line:

There are only about 100 different sorts of atoms (a version of the Periodic Table of Elements is shown in Slide 13 of Download k3.1_2.1a) A much more sophisticated version is available at ( ). An element is a substance that is made up of only one kind of atom. (It is possible for the number of neutrons in the nucleus to vary, within limits. This allows for atoms of the same element to exist with slightly different atomic masses. These are called isotopes and the measured relative atomic mass (RAM) of an element is the weighted average of the isotopes present. For this reason the RAMs of most of the elements are not whole numbers. It is the number of protons in the nucleus (and hence the number of electrons in the neutral atom) that determines the actual element and defines the Atomic Number.

When atoms of one element join together with atoms of one or more other elements the result is a chemical compound. The energy requirements usually require that atoms have only one or a limited number of ways in which they combine and these compounds can be represented by a definite chemical formula.

Even the smallest ‘bit’ of a substance that it is possible to see contains a huge number of atoms (and often molecules) - in every day language we may talk about a drop of water in a spray with a volume of .000000001cm3 (equivalent to a diameter of around .001mm - almost invisible without a microscope) as a particle. This would however, contain well over ten million million molecules of water! So the particles in our model really are very small. (See also Download k3.1_2.3a).

Different substances - especially gases, liquids and powdered solids - can often be mixed together in almost any proportions. Provided that no chemical reactions occur - these remain as mixtures.

3.0 Progression in children’s ideas about the nature and classification of materials

A fairly full picture of expectations can be found in the Science National Curriculum -

  • Pre-school children should develop language to describe and name a variety of living and non-living things and begin to recognise some common substances. They should begin to realise the difference between an object (eg a ball) and the substance it is made from (eg rubber)
  • KS1 some meaningful play and experience with solids liquids (and gases). Ideas of conservation of material during ordinary processes will be developing but most will be quite happy that the water simply ‘disappears’ when puddles evaporate and candles ‘disappear’ when they burn; also that crushing a solid to form a powder or pressing a lump of plasticene into a flat shape may decrease their weights. They may appreciate some reversible changes such as ice melting and water freezing but may not realise that water and ice are the same substance. They will develop some specialised language relating to common processes involving materials such as cooking food, weather, shopping, vehicles, washing etc.
  • At KS2 classifications of the states of matter should be becoming firmer and they should recognise that heating and cooling can cause reversible changes - whereas other changes are more permanent. An understanding of the water cycle will develop. The idea that water can exist as a gas, indeed that air is a gas and has weight are difficult concepts, and many pupils will reach GCSE still unclear about the massive nature of gases (see download k3.1_3a). Some substances can be separated from each other by physical processes. Very early beginnings of particle ideas - scale is unlikely to be appreciated - but many children will begin to pick up some of these ideas from TV, books, parents and peers, even if they are not emphasised at school.

Download K3.1_3a 'Air is a real substance'

  • KS3. Continuing development of ideas and experiences with materials - their properties, uses, changes with temperature and pressure, changes of state, chemical changes. Development of more systematic explanations in terms of particles and increase of movement energy at temperature increases. Qualitative understanding of melting, freezing, diffusion, evaporation, condensation, expansion. Gases have mass - an idea that needs to be developed carefully. Ideas of atoms, ions, molecules and metals. Trends in the Periodic Table of Elements. Atomic structure and radioactivity. It is at this stage that pupils must begin to appreciate the difference between matter at an atomic level (indestructible particles that change their arrangements but retain their identity) and matter at a bulk level (where things change, and matter seems to come and go). See Download k3.1_1b (Alex Johnstone 1999) again.
  • KS4 Here the idea becomes established of an element (with its own particular unchanging atom) their arrangement in the periodic table and the way their electronic structure helps to explain the sort of bonds they can make and the relationship between the types of bonding in materials and their physical properties and applications. Some pupils will become familiar with the mole concept.

4.0 Useful references

  • Gleik J (1994) ‘Genius - Richard Feynman and modern physics’, London, Abacus.
  • Goodwin A (2003) ‘Evaporation and Boiling - trainee science teachers’ understandings.’ School Science Review, 84 (309) pp 131-141.
  • Littledyke, M. Ross, K.A. and Lakin L. (2000) Science Knowledge and the Environment A guide for Students and Teachers in Primary Education. London David Fulton.
  • Ross, K.A. (1997) ‘Many Substances but only five structures’ School Science Review, Vol 78 no 284 pp79-87
  • Ross, Lakin and Callaghan (2004) ‘Teaching Secondary Science’ Second Edition London: David Fulton
  • Taber K (2002) ‘Chemical misconceptions - prevention, diagnosis and cure. Volume 1: Theoretical Background. Volume 2: Classroom Resources. London, Royal Society of Chemistry.
  • A very useful Chemistry resource developed recently by the Royal Society of Chemistry covers all stages of compulsory schooling and provides a wide variety of materials for use by teachers is: (Accessed 23/11/09)

Section Developed by: Alan Goodwin (MMU) with additional material by Keith Ross (UoG) May 2006                         

Published: 10 Apr 2007, Last Updated: 13 Sep 2008