This focus idea is explored through:
Contrasting student and scientific views
Student everyday experiences
Students have difficulty distinguishing between physical and chemical change, despite formal teaching, and the distinction is somewhat arbitrary. However an understanding of the differences between purely physical processes such as melting, evaporation and boiling and the changes that take place in chemical reactions, particularly the idea that new substances are formed, is important to an understanding of chemistry and students very frequently confuse the two. The following conversations are typical:
Pat (recording the group's notes about a prac): What happened?
Kim: It went fizzy.
Pat: Did you see any new substances?
Kim: Nope.
Sam: What shall I write down was formed?
Chris: A blue colour.
Research: Loughran, Mulhall & Berry (2002)
Research shows that students frequently use the term chemical change to describe changes in physical state. Freezing and boiling are considered to be examples of chemical reactions. This depends on their conception of substance. If students regard ice as a different substance from liquid water they are likely to classify the melting of ice as a chemical change. One study found that 80% of students considered a difference in colour between the reactant and product evidence of chemical change. Students can consider potassium permanganate (Condy's crystals) dissolving in water to be a chemical change because of the intense difference in colour. Melting and expansion on heating were also considered to be evidence of chemical change by some students.
Research: Driver, Rushworth & Wood-Robinson (1994)
Many students did not appreciate that a chemical change is characterised by the formation of a substance having different properties from the original substance and a considerable proportion of students who did were unable to offer suitable reasons for distinguishing a ‘new’ substance.
Research: Driver, Rushworth & Wood-Robinson (1994)
Students may believe that beer frothing is an example of a chemical change or an apple ripening is a physical change.
Research: Tsaparlis (2003)
Students commonly believe that physical changes are reversible while chemical changes are not. Students also frequently believe that the original substance in a chemical reaction vanishes completely and forever. A common everyday application of a reversible chemical reaction is the charging and discharging of rechargeable batteries – including car batteries; however students may believe that batteries are a container of stored electricity, rather than of chemicals that react in ways that convert chemical energy into electrical energy.
Other commonly held views are that chemical changes are caused by the mixing of substances/reactants or that heat (which is considered to be some form of material) has to be added.
Scientific view
In a physical change the appearance or form of the matter changes but the kind of matter in the substance does not. However in a chemical change, the kind of matter changes and at least one new substance with new properties is formed.
The distinction between physical and chemical change is not clear cut. Frequently students are led to believe that a change is either physical or chemical. In fact this should be considered more of a continuum. For example salt dissolving in water is usually considered to be a physical change, however the chemical species in salt solution (hydrated sodium and chlorine ions) are different from the species in solid salt. Dissolving of instant coffee in water seems to be a physical change but in most cases dissolving is accompanied by an energy change and is probably better considered to be a chemical process even though it is possible to recover the original components by physical means. Many examples of materials dissolving (for example, an Alka Seltzer in water, metal in acid and the effect of acid rain on marble and concrete) involve both chemical and physical processes.
– Research: Fensham (1994)
Most chemical reactions are reversible although this can be difficult in practice. Many junior school science texts state that chemical changes are irreversible while physical changes can be reversed. Cutting paper into tiny pieces or crushing a rock are obvious physical changes but to restore the original piece of paper or rock is difficult. Rechargeable batteries use one chemical reaction when discharging and recharging involves driving that reaction backwards, turning the products back into the original reactants. The electrical generator (alternator) on a car recharges the car battery constantly while the car engine is running.
Critical teaching ideas
- In a physical change the nature of the substance, the particles of which it is composed and the numbers of particles remain unchanged.
- In a chemical change the properties of the new substances are different from the original, the particles are different and the number of particles can change.
- While the distinction between physical and chemical change is a useful one it should be seen as more of a continuum.
- Chemical reactions can be reversed but this can be difficult in practice.
Explore the relationships between ideas about physical and chemical change in the Concept Development Maps - (States of Matter, Chemical Reactions)
When teaching about physical and chemical changes it is important to allow students to see the classification as a continuum. They should be able to observe a number of changes and formulate their views on the kind of change and problems with the classification process. Students should come to see that chemical reactions produce new chemicals distinct from the starting materials but that chemical processes can be reversed. Examining examples of reversible chemical reactions and considering why it is difficult to reverse many chemical changes can be very useful.
Teaching activities
Promote reflection on and clarification of existing ideas
The following activities are intended to get students identifying and then refining their ideas about physical and chemical change. It is important for students to observe a number of changes and to record their opinions on what is happening. They could record this in a booklet where they write about and draw results and observations. They should be encouraged to formulate and record hypotheses about what is happening with the knowledge that their opinions will not be assessed at this stage.
For an example of this technique see: Using logbooks in year 10 electricity.
This can help bring out their existing ideas and help them challenge and extend their existing beliefs.
Students could investigate:
- Heating steel wool in air and collecting the black powder that results, then weighing the reactant and product (there should be an increase). This could be done as a POE (Predict-Observe-Explain): students are asked to predict what will happen to the weight of the steel wool when it burns. What has been added to the steel wool during combustion?
- Dissolving sugar and salt in water and comparing what happens to the electrical conductivity of the two solutions as the dissolving takes place. Recovering the salt by evaporation shows the salt is still there but the conductivity is indicating something new is forming.
- Comparing boiling water with mixing vinegar and baking soda - both produce bubbles but what's the difference? The baking soda and vinegar can be mixed in a Ziploc sandwich bag to show the production of a new substance which blows the bag up.
- Precipitation reactions, particularly ones that produce an obvious colour change, such as Epsom salts and ammonia solution. This can be compared with adding potassium permanganate to water. What are the differences?
- Acid - base reactions can be illustrated through the use of indicators both natural (cabbage juice) and synthetic. The colour changes help illustrate that new materials could be forming.
Practise using and build the perceived usefulness of a scientific model or idea
It is important that examples of changes are not confined to only the materials and chemicals students are exposed to in the classroom. As a homework activity students could be asked to collect examples of changes they see around them and classify these on their scale of physical and chemical changes. Some examples they may collect are combustion of fuels, cooking and processes such as digestion, respiration and photosynthesis.
Clarify and consolidate ideas for/by communication to and with others
To consolidate their views students in groups could be asked to choose two changes, one which is on the physical end of the continuum and one on the chemical end and explain to the class what the differences are. Communicating their ideas to others can help students clarify and consolidate new and existing ideas about changes.
Practise using and build the perceived usefulness of a scientific model or idea
Science is an area where deeper meaning for a number of key ideas is built gradually by using them in a range of situations and stressing how the same idea helps make sense of many situations. This is particularly the case with key ideas in the chemical sciences that cannot be ‘discovered’, ‘proved’ or even demonstrated by classroom experiments. Both elements and compounds (a key idea at the macro level) and atoms and molecules (which involve the same thinking at the micro level) are examples of this, but their usefulness can be developed by showing (among many other things) how they can help make sense of physical versus chemical changes. These ideas may be introduced here, or referred back to if they have been introduced earlier. Writing chemical equations in word and symbolic form can be introduced as a useful way of describing some of the changes students have seen and also to show the advantages of chemical symbols in keeping track of the elements (or atoms) in ways that words do not. If the exact chemical formulae cannot be written (as is the case with most biochemicals) a drastic simplification can still be useful. For example wood is mostly cellulose, a polymer of glucose, and a representation such as (C6H10O5)n can be used to track changes in processes like combustion.
Models and diagrams can help here. For example, most schools have molecular modelling kits which can be adapted to show how molecules have altered and atoms have rearranged as a result of changes. Poster size diagrams can also be drawn by students to assist with their explanations.
Challenge some existing ideas
Although it is difficult to demonstrate the reversibility of chemical changes, students are very familiar with the need to recharge their mobile phones, cameras and other rechargeable devices. This could be simply discussed, although investigating the chemical reactions that power these devices could be a useful research project. A caveat here is that much of the available information can be very technical.
Promote reflection on how students’ ideas have changed
Re-examining their original journal entries can promote reflection on how students' views have changed. Students can then apply their new ideas to more examples of change. Activities such as the following can be used to promote discussion of the kinds of change taking place and of the difficulties of classifying some changes as physical or chemical:
- Dissolving metals in acid (magnesium and zinc) and testing the resultant gas.
- Investigating limestone and acid reactions (the production of limestone caves - what sort of change is this?)
- Adding zinc to copper sulfate solution and observing the colour changes that take place.
- Comparing the setting of water-based glues such as Clag and Aquadhere with two pack adhesives such as Araldite. The former works by evaporation of the solvent (water) and is reversible; the latter involves a chemical reaction (a thermosetting polymerisation reaction) between the two components and is not reversible.