How Are Physical And Chemical Changes Similar?

This focus thought is explored through:

Contrasting student and scientific views
Disquisitional teaching ideas
Education activities

Contrasting student and scientific views

Educatee everyday experiences

Students have difficulty distinguishing between physical and chemical alter, despite formal teaching, and the distinction is somewhat arbitrary. All the same an understanding of the differences between purely physical processes such every bit melting, evaporation and boiling and the changes that have place in chemical reactions, particularly the idea that new substances are formed, is important to an agreement of chemical science and students very oft confuse the two. The following conversations are typical:

Student adding universal indicator to various solutions to rack of 6 test tubes.

Pat (recording the group's notes about a prac): What happened?
Kim: It went fizzy.
Pat: Did you lot see any new substances?
Kim: Nope.

Sam: What shall I write downwards was formed?
Chris: A bluish colour.

Research: Loughran, Mulhall & Berry (2002)

Enquiry shows that students frequently utilize the term chemical change to describe changes in concrete country. Freezing and humid are considered to exist 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 equally a chemical change. 1 study found that 80% of students considered a deviation in colour between the reactant and production evidence of chemical change. Students can consider potassium permanganate (Condy'south crystals) dissolving in water to exist a chemical change considering of the intense difference in colour. Melting and expansion on heating were also considered to be bear witness of chemical alter by some students.

Research: Commuter, Rushworth & Forest-Robinson (1994)

Many students did not appreciate that a chemical alter is characterised by the formation of a substance having unlike 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 chemic change or an apple ripening is a physical change.

Research: Tsaparlis (2003)

Students commonly believe that physical changes are reversible while chemical changes are non. Students also frequently believe that the original substance in a chemic 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 unremarkably held views are that chemical changes are caused by the mixing of substances/reactants or that estrus (which is considered to be some form of material) has to be added.

Scientific view

In a physical modify the appearance or form of the matter changes just the kind of matter in the substance does not. However in a chemical change, the kind of matter changes and at least i new substance with new properties is formed.

The distinction between physical and chemical change is non clear cut. Oft students are led to believe that a change is either physical or chemical. In fact this should exist considered more of a continuum. For example salt dissolving in h2o is commonly considered to exist 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 java in water seems to exist a physical change but in most cases dissolving is accompanied by an free 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, metallic in acid and the effect of acid rain on marble and concrete) involve both chemic and concrete processes.

– Enquiry: Fensham (1994)

Well-nigh chemical reactions are reversible although this can be difficult in practice. Many inferior school science texts state that chemic changes are irreversible while concrete changes tin be reversed. Cutting paper into tiny pieces or crushing a rock are obvious physical changes but to restore the original slice of paper or rock is difficult. Rechargeable batteries utilise i chemical reaction when discharging and recharging involves driving that reaction backwards, turning the products dorsum into the original reactants. The electric generator (alternator) on a car recharges the automobile battery constantly while the automobile engine is running.

Critical teaching ideas

In a physical modify 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 unlike from the original, the particles are different and the number of particles can change.
While the stardom betwixt physical and chemical change is a useful i it should be seen as more of a continuum.
Chemical reactions can be reversed merely this tin can be hard in practice.

Explore the relationships between ideas about physical and chemical alter in the Concept Development Maps - (States of Matter, Chemical Reactions)

When teaching most physical and chemical changes information technology is important to permit students to come across the nomenclature as a continuum. They should be able to observe a number of changes and formulate their views on the kind of change and bug with the classification process. Students should come to see that chemical reactions produce new chemicals distinct from the starting materials but that chemical processes tin be reversed. Examining examples of reversible chemic reactions and considering why it is hard 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 concrete and chemic change. It is of import for students to find a number of changes and to record their opinions on what is happening. They could record this in a booklet where they write almost and describe results and observations. They should exist encouraged to formulate and record hypotheses about what is happening with the knowledge that their opinions will not be assessed at this phase.

For an example of this technique see: Using logbooks in year 10 electricity.

This can help bring out their existing ideas and help them claiming 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 exist an increase). This could exist done as a POE (Predict-Observe-Explain): students are asked to predict what volition happen to the weight of the steel wool when it burns. What has been added to the steel wool during combustion?
Dissolving sugar and table salt in h2o and comparison what happens to the conductivity of the two solutions as the dissolving takes place. Recovering the common salt past evaporation shows the common salt is still there simply the electrical conductivity is indicating something new is forming.
Comparing boiling h2o with mixing vinegar and blistering soda - both produce bubbles but what's the divergence? The baking soda and vinegar can be mixed in a Ziploc sandwich handbag to bear witness the product of a new substance which blows the bag up.
Precipitation reactions, particularly ones that produce an obvious colour alter, such equally Epsom salts and ammonia solution. This can be compared with calculation potassium permanganate to h2o. What are the differences?
Acrid - base reactions tin be illustrated through the use of indicators both natural (cabbage juice) and synthetic. The colour changes aid illustrate that new materials could be forming.

Practice using and build the perceived usefulness of a scientific model or idea
Information technology is important that examples of changes are not bars to merely the materials and chemicals students are exposed to in the classroom. Every bit a homework activity students could exist asked to collect examples of changes they see around them and classify these on their calibration of concrete and chemic 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 exist asked to choose two changes, 1 which is on the concrete terminate of the continuum and one on the chemic stop and explicate to the class what the differences are. Communicating their ideas to others can help students clarify and consolidate new and existing ideas well-nigh changes.

Do using and build the perceived usefulness of a scientific model or thought
Science is an expanse where deeper meaning for a number of key ideas is congenital gradually by using them in a range of situations and stressing how the same idea helps make sense of many situations. This is peculiarly the case with key ideas in the chemical sciences that cannot be 'discovered', 'proved' or fifty-fifty 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, just their usefulness tin be developed by showing (amongst many other things) how they tin help make sense of physical versus chemical changes. These ideas may exist introduced here, or referred dorsum to if they take been introduced before. Writing chemical equations in word and symbolic grade can be introduced as a useful way of describing some of the changes students accept seen and too to show the advantages of chemical symbols in keeping rails of the elements (or atoms) in means that words do not. If the exact chemical formulae cannot exist written (as is the example with nigh biochemicals) a desperate simplification can still be useful. For example woods is mostly cellulose, a polymer of glucose, and a representation such every bit (C_half-dozenH₁₀O₅)_n can be used to track changes in processes like combustion.

Models and diagrams tin help hither. For instance, nigh schools have molecular modelling kits which can exist adapted to show how molecules have contradistinct and atoms have rearranged equally a effect of changes. Poster size diagrams tin also be drawn by students to assist with their explanations.

Claiming 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 exist just discussed, although investigating the chemical reactions that power these devices could be a useful inquiry project. A caveat hither is that much of the available information tin be very technical.

Promote reflection on how students' ideas take changed
Re-examining their original journal entries can promote reflection on how students' views have inverse. Students tin can and so apply their new ideas to more than examples of change. Activities such equally 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 chemic:

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 identify.
Comparison the setting of h2o-based glues such as Clag and Aquadhere with 2 pack adhesives such as Araldite. The quondam works by evaporation of the solvent (water) and is reversible; the latter involves a chemical reaction (a thermosetting polymerisation reaction) betwixt the two components and is non reversible.