How Do You Know if a Physical Change Has Occurred

​​This focus idea is explored through:

• Contrasting student and scientific views
• Disquisitional teaching ideas
• Didactics activities

Contrasting student and scientific views

Student everyday experiences

Students have difficulty distinguishing between physical and chemical alter, despite formal didactics, and the distinction is somewhat arbitrary. However an understanding of the differences between purely concrete processes such every bit melting, evaporation and boiling and the changes that take place in chemic reactions, especially the idea that new substances are formed, is important to an understanding of chemical science 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 encounter whatsoever 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 oft utilise the term chemical change to describe changes in concrete state. Freezing and humid are considered to be examples of chemical reactions. This depends on their conception of substance. If students regard water ice as a different substance from liquid h2o they are likely to classify the melting of ice equally a chemical change. One written report found that lxxx% of students considered a divergence in color between the reactant and product show of chemical modify. Students can consider potassium permanganate (Condy's crystals) dissolving in water to be a chemic modify considering of the intense difference in colour. Melting and expansion on heating were besides considered to be bear witness 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 dissimilar 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: Commuter, Rushworth & Woods-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 chemic reaction is the charging and discharging of rechargeable batteries – including motorcar 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 chemic changes are caused by the mixing of substances/reactants or that oestrus (which is considered to exist some form of material) has to be added.

Scientific view

In a physical change the appearance or form of the matter changes just the kind of affair in the substance does non. 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 concrete or chemical. In fact this should be considered more of a continuum. For example salt dissolving in h2o is normally considered to be a physical change, notwithstanding the chemic 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 be a physical modify only in most cases dissolving is accompanied by an energy change and is probably better considered to be a chemical process even though information technology is possible to recover the original components past physical means. Many examples of materials dissolving (for instance, an Alka Seltzer in h2o, metal in acid and the effect of acid rain on marble and physical) involve both chemic and physical processes.

– Research: Fensham (1994)

Nigh chemical reactions are reversible although this tin exist difficult in practice. Many junior school scientific discipline texts land that chemical changes are irreversible while physical changes can be reversed. Cutting paper into tiny pieces or crushing a rock are obvious physical changes simply to restore the original slice 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 electric generator (alternator) on a motorcar recharges the machine battery constantly while the auto engine is running.

Critical teaching ideas

• In a physical change the nature of the substance, the particles of which it is equanimous 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 dissimilar and the number of particles tin change.
• While the distinction between physical and chemical change is a useful one it should be seen equally more of a continuum.
• Chemical reactions can exist reversed but this can exist difficult in practice.

Explore the relationships betwixt ideas about physical and chemic change in the Concept Development Maps - (States of Matter, Chemic Reactions)

When teaching most physical and chemical changes information technology is of import to permit students to see the nomenclature as a continuum. They should exist able to observe a number of changes and formulate their views on the kind of alter and issues with the classification process. Students should come to run across that chemical reactions produce new chemicals distinct from the starting materials only that chemical processes can exist reversed. Examining examples of reversible chemic reactions and considering why it is difficult to opposite many chemical changes tin be very useful.

Instruction activities

Promote reflection on and description of existing ideas
The following activities are intended to get students identifying and then refining their ideas virtually physical and chemical change. It is important for students to detect 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 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 meet: Using logbooks in year x electricity.

This tin 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 blackness powder that results, then weighing the reactant and product (there should be an increase). This could exist 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 common salt in h2o and comparing what happens to the electrical conductivity of the two solutions every bit the dissolving takes place. Recovering the salt past evaporation shows the common salt is still there but the conductivity is indicating something new is forming.
• Comparing boiling water with mixing vinegar and baking soda - both produce bubbling but what's the difference? The baking soda and vinegar tin be mixed in a Ziploc sandwich bag to testify the product of a new substance which blows the bag up.
• Precipitation reactions, especially ones that produce an obvious colour change, such as Epsom salts and ammonia solution. This can be compared with calculation potassium permanganate to water. What are the differences?
• Acid - base reactions can exist illustrated through the employ of indicators both natural (cabbage juice) and synthetic. The color changes help illustrate that new materials could be forming.

Do using and build the perceived usefulness of a scientific model or idea
It is of import that examples of changes are not confined to but 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 2 changes, one which is on the physical finish of the continuum and 1 on the chemical end and explicate to the class what the differences are. Communicating their ideas to others can help students clarify and consolidate new and existing ideas near changes.

Exercise using and build the perceived usefulness of a scientific model or idea
Scientific discipline is an surface 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 peculiarly the case with central ideas in the chemical sciences that cannot be 'discovered', 'proved' or even demonstrated by classroom experiments. Both elements and compounds (a primal 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 aid make sense of concrete versus chemical changes. These ideas may exist introduced hither, or referred dorsum to if they have been introduced before. Writing chemic equations in give-and-take and symbolic form can be introduced equally a useful fashion of describing some of the changes students have seen and also to bear witness the advantages of chemic symbols in keeping track of the elements (or atoms) in ways that words do not. If the verbal chemical formulae cannot be written (as is the example with most biochemicals) a desperate simplification can still be useful. For example forest is more often than not cellulose, a polymer of glucose, and a representation such every bit (C_viH_tenO₅)_n tin can be used to rail changes in processes similar combustion.

Models and diagrams can help here. For example, most schools accept molecular modelling kits which can exist adapted to show how molecules have contradistinct and atoms have rearranged as a outcome of changes. Poster size diagrams tin also be drawn by students to assist with their explanations.

Challenge some existing ideas
Although it is difficult to demonstrate the reversibility of chemic 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 enquiry project. A caveat here is that much of the available information can exist very technical.

Promote reflection on how students' ideas take inverse
Re-examining their original journal entries tin promote reflection on how students' views have changed. Students tin can then apply their new ideas to more examples of modify. Activities such as the following tin be used to promote word of the kinds of change taking place and of the difficulties of classifying some changes as concrete or chemic:

• Dissolving metals in acrid (magnesium and zinc) and testing the resultant gas.
• Investigating limestone and acrid 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 (h2o) and is reversible; the latter involves a chemical reaction (a thermosetting poly​merisation reaction) betwixt the two components and is not reversible.
  

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Source: https://www.education.vic.gov.au/school/teachers/teachingresources/discipline/science/continuum/Pages/physchem5.aspx

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