K3.2 Changing Materials
This unit builds upon the particle ideas explored in Materials Unit 1. It explores the categorisation of change into ‘chemical’ and ‘physical’ as well as the conceptual barriers to an understanding of chemical change. The essential requirement for a chemical change is that ‘new substance(s)’ is (are) produced by rearranging the atoms of the reacting substances. Quantitative relationships are represented by balanced chemical equations and the energy changes that accompany the reactions are determined by the difference between the energy required to break existing bonds (links) between atoms in the reactants and the energy given out when new bonds (links) form in the products. The important relationship between these ideas and ‘sustainability’ issues is established. A summary of types of chemical reactions is given together with some useful references.
This is one of 17articles on www.scitutors.org.uk/ 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, Teacher knowledge, Chemistry, Practical work, Demonstrations, PowerPoint presentation.
2.0 Conceptual barriers to understanding of Chemical change
3.0 Sustainability and Science Education
4.0 Chemical Reactions
5.0 Progression in children's ideas about Changing Materials
6.0 Useful References
Change is at the heart of chemistry and in order to develop an understanding of chemical ideas learners need to experience the large numbers of examples of chemical change in everyday life, not just those in science laboratories. Many elementary science courses include, at an early stage, differences between chemical and physical changes - this may be useful but we only gather information and evidence ourselves about changes to materials over a long period of time and it is often not possible to decide definitely whether a particular change can be categorised as chemical or physical (Many everyday changes that take place during cooking, living, dying or making tea are very complicated and often consist of lots of different changes, some of which may be chemical and some physical!)
Even the criteria that are often taught to distinguish between the two sorts of changes are problematic. The following table is taken from my own science notes of almost 50 years ago and I certainly tried to use the ideas when I began teaching chemistry in secondary school:
|1||New substance(s) formed||No new substance(s) formed|
|2||Not easily reversed||Usually easily reversed|
|3||Often accompanied by a large energy change||Energy change usually small|
I could find examples that seemed to fit this categorisation - a candle burning, an egg being cooked (in boiling water) or a piece of iron going rusty seemed fairly good examples of chemical changes. All seem to form new substances - except the candle seems to disappear and it is some time before convincing evidence can be provided that water and carbon dioxide are the combustion products. None of the processes is easily reversed although the energy change for the egg and the iron are not obvious. Changes of physical state are usually cited as examples of physical change - e.g. water freezing or evaporating; candle wax or chocolate melting - it is however, not obvious that no new substance is formed. Children usually have to take on trust that ice, water and water vapour are the same substance (Actually I am no longer sure - since if one considers that ‘hydrogen bonds’ are actual chemical bonds then it really is probably more correct to suggest that the three are chemically distinct since the bonding linking the molecules is different in each case??!) Breaking a drinking glass and powdering coal are indeed physical changes (no new substance is formed - although some chemical bonds must be broken during the process.) but in these cases the processes are not easily reversed.
The only real criterion to distinguish between chemical and physical changes is whether new substances are formed but unfortunately it is not always clear whether this is or is not the case. A few examples are discussed in Download k3.2_1.0a - these are useful for fairly experienced scientists but should not be discussed in this way with beginners. Until we are sure whether new substances are formed it is probably better to be agnostic - and even when we think we know a change is physical there may be evidence to change our minds later! This makes a good discussion topic for your trainee teachers.
Particle theory and chemical change:
- Only a limited number of types of atoms exist (One type for each of the elements - just over 100) and most of these can be combined in various ways to form the huge range of chemical substances that can exist. (Most of these contain only a small number of different kinds of atom; at least 1 and rarely more than ten kinds.) The forces that hold atoms together are called ‘chemical bonds’.
- These atoms (Dalton’s ‘invention’ in 1810) are indestructible and cannot be created, destroyed or inter-converted except in ‘nuclear’ reactions such as those that take place inside stars, or during ‘supernovae’ explosions, in nuclear reactors or radioactive decay processes.
- When a chemical change takes place the atoms in the reacting substance(s) are rearranged to form the product substances. And if, as is usually the case, the bonds in the products are stronger or weaker than those in the reactants, energy is also given out or absorbed during the reaction. (The stronger, more stable a bond is the lower the potential energy it has so that more energy has to be provided to break a strong bond than a weak one. Conversely more energy is given out when a stronger bond is formed, as the atoms ‘rush’ together under the strong electrostatic forces. Many pupils have the misconception that energy is released as bonds break)
These are not trivial although they sometimes appear so to more experienced chemists/scientists. Some of the problems are explored below (and the work of Alex Johnstone and Keith Taber [see references in section 5] are useful sources of practical insight.)
In particular the following are pertinent - although some may be controversial. (Please consider these critically. The ‘story’ of chemical change, chemical reactions and materials is a developing one, not only from the perspective of pupils, but also for the science teacher and scientists themselves.):
- Children - and others - often think of atoms as ‘bits’ of ordinary ‘stuff’ that can melt or burn and have colour (after all we all know that carbon atoms are black and oxygen atoms are red!). These ideas are clearly meaningless for separate atoms and apply only to bulk matter. It is thus very important to help children distinguish between bulk matter (of everyday experience) and the world of these indestructible atoms. See Download 2a the particle concept.
- Atoms are only rearranged during chemical reactions - they can never be created nor destroyed. Atoms are very small, and the very smallest part of an element that can exist. (The electrons, protons and neutrons that make up all atoms are of course much smaller but they cannot be distinguished. All electrons, protons and neutrons are the same whatever atom they are in.) Atoms of all elements remain essentially intact during all chemical reactions - it is only the outermost electrons that are affected during chemical reactions. When atoms approach each other it is these outer electrons that determine whether the atoms will repel each other or bond.
- Pupils get confused by the numbers we put in chemical formulae. This has led to the use of word equations at KS3, which only makes things worse - for there is little logic in word equations. What is needed is for the atoms to be drawn out or modelled. Only when pupils can see that the number of atoms is unchanged during a reaction, by actually counting them, should we offer them formulae as a short cut to drawing ‘blobs’ (of different colour) to represent the atoms in the substances that react. See Download 2b Molecular Models and equations
- The chemical symbols we use to represent the relative numbers of atoms involved in making up a pure substance and that are rearranged during chemical reactions need to become ‘commonplace’ and meaningful to pupils if they are to be useful. The use of symbols and formulae is implicit in the KS3 National Strategy in the Framework for teaching science - ‘particles’ (DfES 2002), but it seems unusual for sufficient emphasis to be placed on learning and using the basic formulae for many pupils to become fluent in the new language. However, rather than giving pupils basic rules about the ways in which atoms join together we could begin to show them that the combining power of an atom (valency) is linked to its position in the periodic table and can be explained in terms of electrons and atomic structure. This is part of the KS4 curriculum.
- It is remarkably difficult to relate, say, the white crystals of substance (common salt = sodium chloride) that we can see, with the conceptual picture of a huge three dimensional array of sub-microscopic charged atoms (ions) of sodium and chlorine in each crystal. These we can infer (from evidence provided over a long period of time) but cannot see (even through a microscope) and at the same time we have to connect these with the symbolic representation ‘NaCl’ (or, more accurately 'Na+Cl-'). Such interrelations make almost impossible cognitive demands on most of us until such time as these simple formulae and equations become unproblematic and begin to ‘mean’ real substances made up of the appropriate relative numbers of atoms. (Johnstone, 1999)
- Quantitative aspects of chemistry become accessible once we have chemical equations. Because this involves numbers, relative masses and (albeit simple) proportionality this is almost always perceived as very difficult by pupils and is avoided for all but the most able. There are a number of key ideas that need to be grasped and mastered, but the mathematics is not beyond an average student at KS2!
- The balanced equation shows exactly the numbers of atoms of each element involved in the reaction.
- The relative masses of all substances involved can simply be shown by calculating the relative formula mass of each substance and multiplying each by the appropriate number from the equation (that’s the big one in front of the formula that tells how many moles there are of that substance).
- If any of the substances is a gas under the conditions of the experiment then it is useful to remember that at room temperature and pressure the volume of 1 mole of ANY GAS is about 24 litres.
- The representation of elements and the formulae of some compounds can cause concern initially (and complications later). For example, the gaseous or volatile elements exist in the form of stable molecules that consist of pairs of atoms. It is these molecules that take part in chemical reactions and that we represent in chemical equations. These include Hydrogen (H2); Oxygen (O2); Nitrogen (N2); Fluorine (F2); Chlorine (Cl2); Bromine (Br2) and Iodine (I2). When the structure of elements becomes more complicated than this we usually ignore the bonding in the element and just use the symbol as its formula, thus implying to our students that the atoms are NOT bonded together. E.g. Fe; Zn; C. (Download k3.2_2.0c elements in chemical equations explores this a little further.)
- Energy is usually given out during chemical changes that we commonly experience in the laboratory. This must be associated with the rearrangement of atoms. It always requires energy to break the bonds between atoms - and a rearrangement is not possible unless some bonds are first broken. If the new bonds that are formed are stronger than those in the reactants then these bonds have ‘lower energy’ [see Note * below] and some energy is given out - usually as heat so the ‘particles’ move about faster and the temperature rises. Such reactions are called ‘exothermic’. ‘Endothermic’ chemical reactions are also possible - but less easily demonstrated - a very important example is photosynthesis, the reaction between carbon dioxide and water that takes place in green plants where the energy is provided by sunlight. This energy input is needed because the bond in molecular oxygen (O2) is weak, so less energy is released as the oxygen is formed than was used to break apart the water and carbon dioxide. Glucose, the other product of photosynthesis, is a relative stable molecule, though, by convention, text books and food packets suggest it ‘contains’ energy. It is the dangerous and reactive gas oxygen that actually contains the weak bonds, and which ‘carries’ the energy potential that is available through respiration. This conception is reviewed in download 2.4 in the Energy unit of Subject Knowledge.
- Some students have difficulty in recognising reactions as exothermic - e.g the burning of a candle. Many exothermic reactions are not spontaneous and since energy has to be supplied to start them going (energy of activation) these reactions are perceived as endothermic!
[Note * Here lies the source of the misconception that bonds 'contain' energy. A 'lower energy bond' is a strong bond, one that requires a large energy input to break it. The lower its energy level, the stronger it is and the harder it is to break. Conversely a 'high energy bond' such as the third phosphate in ATP is actually a weak bond, somewhat 'unstable', and reactive, because it doesn't require much energy to break, but as the new bonds form, more energy is released than was used to break the original bond, causing the original unstable ATP to be labelled, somewhat confusingly, as a 'high energy' molecule]
Download K3.2_2.0a 'The Particle Concept'
Download K3.2_2.0b 'Molecular Models and Equations'
Download K3.2_2.0c 'Elements in Chemical Equations'
At this point it is convenient to link KEY IDEAS of science necessary for sustainability and environmental education. These have been argued to be the following:
- Matter cannot disappear - atoms are indestructible
- Although energy is indestructible (First Law of Thermodynamics) it spreads and becomes ‘degraded’ (Second Law of Thermodynamics)
- Matter also scatters unless some energy source is available to ‘tidy it up’ and keep it going in cycles (Second Law of Thermodynamics again)
- Thus, on this planet of ours, energy is constantly being degraded to enable matter to be re-cycled. This happens naturally in the great natural cycles: rocks, climate, and life*, but humans have recently failed to manage their resources in a similar way
- There is Value in Structure - organised matter*, such as the human brain, represents high quality
* Plants (strictly speaking we should say 'producers': algae, chemosynthetic bacteria etc. as well as green plants) begin this process of producing Structure and Order by using energy from the Sun, (allowing carbon dioxide and water to form glucose and oxygen). Subsequently, through energy provided by respiration (where oxygen re-joins with the glucose), Life on Earth keeps this high state of order, represented by the global ecosystem.
All of these are touched upon in this unit. Useful material dealing with this perspective can be gained from Download k3.2_3.0a Sustainability Links which relates to ‘forum-for-the-future’ and the international charitable organisation ‘The Natural Step’ (TNS).
It may thus be useful to keep in mind that
- the indestructibility of matter (and the limited supply of atoms on Earth),
- the dissipation of energy (Finite fossil fuel resources and problems of global warming),
- structure and photosynthesis
are basic requirements for discussion of the balance and continuation of (human?) life on Earth, and should therefore form an important part of a curriculum for scientific literacy in school.
All these local and global environmental issues: pollution, recycling, sustainable energy, food production, water supply, transport etc. cannot be solved by science alone nor can science answer questions as to what should be done - but hopefully an understanding of the science behind these issues will increasingly inform the human view of the problems and possibilities and will enable us to maintain a hope for the future.
N.B. As From Sept 2005 Sustainability Education is a required constituent of ITT.
Download K3.2_3.0a 'Sustainability Links'
There are a limited number of reaction types and all involve only the rearrangement of atoms and the associated energy changes. The various classifications of reactions are covered in almost all school science texts relating to chemistry - an interesting text is that by Ryan (1996). See also Ross et al (2004) - especially Chapter 12. “Difficult ideas in chemistry” a summary of which is in download k3.2_4.0c.
Some classifications that will be met:
- Combination and decomposition
- Double decomposition (Changing partners) - this includes precipitation reactions, acid base (often treated separately as ‘neutralisation.) and (probably) acid carbonate.
- Oxidation / Reduction
- Exothermic endothermic
- Catalysed reactions - especially important are reactions in life (biochemistry) where reactions are controlled by enzymes.
A common example of a chemical reaction is that of a fuel burning. Indeed, fire is a phenomenon that has interested human beings for many thousands of years. It is, however, only since the 1770s, based upon the work of Priestly, Scheele and Lavoisier that we have come to believe (know) that fire is the visible region where energy is being given out (as light and heat) when ‘fuels’ react with oxygen from the air.
There is a most important parallel to be drawn between burning fuels and aerobic respiration in living things on Earth. Animals, plants and almost all living things use enzymes to break large molecules of fuel (such as starch) into simple sugars. The small sugar molecules are now able to react with oxygen within the living cells. This reaction with oxygen, called aerobic respiration, releases energy, which is needed to drive endothermic processes such as growth (e.g. protein synthesis) and movement.
The expression 'burning up calories' is a reference to this joining of oxygen and the fuel in our bodies. Of course it is not the calories that are burnt, but the fuel. The result is we breathe out carbon dioxide and water, and the energy transferred is available for our bodies to 'use'. See the biology units for further discussion on living things. Further perspectives on Fire and fire safety are given in Download k3.2_4.0a Fire Safety (Teachers notes) and some supporting Power-point slides are in Download k3.2_4.0b Fire!
Download k3.2_4.0c Difficult ideas in chemistry summarises some of the problems pupils might encounter as they try to come to grips with some of the harder parts of chemistry to do with reaction kinetics and energetics. It is taken from chapter 12 of Ross et al (2004). It is discussed more fully in Materials unit 3 Patterns of Behaviour
Download K3.2_4.0c 'Difficult Ideas in Chemistry'
Through their time in school children will hopefully come to an understanding of their global environment. It is an understanding based on the idea that matter (at least at an atomic level) cannot be created or destroyed, therefore materials are constantly being recycled in all processes on Earth. We see this in the simplicity of the water cycle through to the most complex cycling of materials by the processes of life itself.
At primary level these ideas are best approached by getting children to think about where materials come from, for example in
- a cup of milk (grass - cow - dairy - shops/milkman)
- a wooden table (carbon dioxide and water - growing tree - saw mill - factory/carpenter - shops)
- a china plate (granite - erosion - river sediment - clay - pottery works - shops)
- a metal spoon (metal ore - mining/quarrying - metal extraction (eg iron and steel works) - factory/smith - shops)
This process of tracing things back shows that materials don't come from nowhere - in the same way they don't disappear when they are eaten or thrown away. For a full discussion of the progression from reception to KS4 see download k3.2_5.0a:
Download K3.2_5.0a 'Changing Materials'
- DfES (2002) ‘Key Stage 3 National Strategy - framework for teaching science’
- Johnstone A (1999) The paper is available on the internet at: <http://www.rsc.org/Membership/Networking/InterestGroups/ChemicalEducationResearch/Lectures.asp> Accessed March 2006. This is Download 1a of the Materials 1: classification Unit. (NOT this unit)
- McDuell B (2000) “Teaching Secondary Chemistry.” London, John Murray.
- Ross K, Lakin L & Callaghan P (2004) “Teaching Secondary Science - Constructing Meaning and Developing Understanding.” Second edition London, David Fulton Publishers.
- Ross, K.A., Lakin, L., Littledyke, M. and Burch, G (2005) "The Science of Environmental Issues" CD-rom. Cheltenham: University of Gloucestershire (available from: www.glos.ac.uk/science-issues)
- Ryan L (1996) ‘Chemistry for you.’ Stanley Thornes Ltd., Cheltenham.
- Taber K (2002) ‘Chemical misconceptions - prevention, diagnosis and cure. Volume 1: Theoretical Background. Volume 2: Classroom Resources. http://www.uoi.gr/cerp/2001_February/07.html (full text available as PDF)
- 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: http://www.chemistryteachers.org/ (Accessed 23/11/09)
Section Developed by: Alan Goodwin, MMUand Keith Ross, University of Gloucestershire
Published: 09 May 2006, Last Updated: 13 Sep 2008