Is Dissolving Of A Solid In Water A Chemical Or Physical Change?

This focus idea is explored through:

Contrasting student and scientific views
Disquisitional educational activity ideas
Education activities

Contrasting student and scientific views

Student everyday experiences

Students have difficulty distinguishing between concrete and chemical change, despite formal teaching, and the stardom is somewhat arbitrary. Yet an understanding of the differences betwixt purely physical processes such as melting, evaporation and boiling and the changes that accept place in chemic reactions, particularly the idea that new substances are formed, is important to an understanding of chemical science and students very frequently misfile the two. The following conversations are typical:

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

Pat (recording the group'southward notes well-nigh a prac): What happened?
Kim: Information technology went fizzy.
Pat: Did you see whatsoever new substances?
Kim: Nope.

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

Research: Loughran, Mulhall & Drupe (2002)

Enquiry 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 every bit 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 difference in colour between the reactant and product evidence of chemical change. Students tin can consider potassium permanganate (Condy's crystals) dissolving in water to be a chemical alter because of the intense departure in color. Melting and expansion on heating were also considered to be evidence of chemical change by some students.

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

Many students did not appreciate that a chemical modify 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 offering 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 modify or an apple tree ripening is a physical change.

Research: Tsaparlis (2003)

Students ordinarily believe that physical changes are reversible while chemic changes are not. Students also oftentimes 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 motorcar batteries; however students may believe that batteries are a container of stored electricity, rather than of chemicals that react in means that convert chemical energy into electrical energy.

Other commonly held views are that chemical changes are caused past the mixing of substances/reactants or that heat (which is considered to be some grade of material) has to be added.

Scientific view

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

The distinction between physical and chemical change is not articulate cutting. Often students are led to believe that a alter is either physical or chemic. In fact this should be considered more of a continuum. For example salt dissolving in water is usually considered to be a concrete change, however the chemic species in table salt solution (hydrated sodium and chlorine ions) are different from the species in solid table salt. Dissolving of instant coffee in water seems to be a physical modify but in nigh cases dissolving is accompanied by an energy modify and is probably ameliorate considered to exist a chemic procedure fifty-fifty though it is possible to recover the original components by concrete ways. Many examples of materials dissolving (for case, an Alka Seltzer in h2o, metal in acrid and the result of acrid rain on marble and concrete) involve both chemical and physical processes.

– Research: Fensham (1994)

Most chemic reactions are reversible although this can be difficult in exercise. Many junior school science texts state that chemical changes are irreversible while physical changes tin can exist reversed. Cutting newspaper into tiny pieces or crushing a rock are obvious physical changes but to restore the original piece of newspaper 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 automobile recharges the car battery constantly while the auto engine is running.

Critical teaching ideas

In a physical change the nature of the substance, the particles of which information technology is composed and the numbers of particles remain unchanged.
In a chemical modify the properties of the new substances are different from the original, the particles are different and the number of particles can change.
While the stardom between physical and chemical alter is a useful one information technology should exist seen equally more of a continuum.
Chemical reactions can be reversed simply this can be difficult in practise.

Explore the relationships between ideas about physical and chemical change in the Concept Evolution Maps - (States of Matter, Chemic Reactions)

When educational activity about physical and chemical changes information technology is important to allow students to see the classification equally a continuum. They should be able to detect a number of changes and codify their views on the kind of change and issues with the nomenclature process. Students should come to see that chemic reactions produce new chemicals distinct from the starting materials simply that chemical processes can be reversed. Examining examples of reversible chemical reactions and considering why information technology is difficult to reverse many chemical changes can be very useful.

Education activities

Promote reflection on and clarification of existing ideas
The post-obit activities are intended to go students identifying then refining their ideas about physical and chemic 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 almost and depict results and observations. They should be encouraged to formulate and tape hypotheses about what is happening with the cognition that their opinions volition non be assessed at this stage.

For an case of this technique run into: Using logbooks in year 10 electricity.

This can aid 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 pulverization that results, then weighing the reactant and product (there should be an increment). This could exist washed every bit a POE (Predict-Notice-Explain): students are asked to predict what will happen to the weight of the steel wool when information technology burns. What has been added to the steel wool during combustion?
Dissolving carbohydrate and salt in water and comparison what happens to the electrical conductivity of the ii solutions as the dissolving takes identify. Recovering the salt by evaporation shows the salt is nonetheless at that place just the electrical conductivity is indicating something new is forming.
Comparing boiling h2o with mixing vinegar and baking soda - both produce bubbles merely 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 color alter, such as Epsom salts and ammonia solution. This can be compared with adding potassium permanganate to water. What are the differences?
Acid - base reactions tin can be illustrated through the employ 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 encounter around them and allocate 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, ane which is on the physical end of the continuum and ane on the chemical terminate and explain to the course what the differences are. Communicating their ideas to others tin can help students analyze and consolidate new and existing ideas about changes.

Do using and build the perceived usefulness of a scientific model or thought
Science is an area where deeper significant for a number of key ideas is built gradually past using them in a range of situations and stressing how the aforementioned idea helps make sense of many situations. This is particularly the example with cardinal ideas in the chemical sciences that cannot be 'discovered', 'proved' or even demonstrated by classroom experiments. Both elements and compounds (a central thought at the macro level) and atoms and molecules (which involve the same thinking at the micro level) are examples of this, only their usefulness can be developed by showing (amid many other things) how they tin can help brand sense of concrete versus chemical changes. These ideas may be introduced here, or referred dorsum to if they accept been introduced earlier. Writing chemical equations in word and symbolic grade can be introduced equally 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 practise not. If the exact chemical formulae cannot exist written (as is the case with almost biochemicals) a drastic simplification tin can still be useful. For instance wood is more often than not cellulose, a polymer of glucose, and a representation such as (C₆H_xO₅)_n can be used to track changes in processes like combustion.

Models and diagrams tin assistance hither. For example, most schools have molecular modelling kits which can be adapted to bear witness how molecules accept contradistinct and atoms have rearranged as a result of changes. Poster size diagrams tin can as well be fatigued 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 demand 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 exist a useful inquiry projection. A caveat here is that much of the available information tin can exist very technical.

Promote reflection on how students' ideas have changed
Re-examining their original periodical entries can promote reflection on how students' views take inverse. Students can then apply their new ideas to more examples of alter. Activities such equally the following tin can exist 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 product of limestone caves - what sort of change is this?)
Adding zinc to copper sulfate solution and observing the colour changes that accept place.
Comparing the setting of h2o-based glues such equally Clag and Aquadhere with two pack adhesives such as Araldite. The erstwhile works past evaporation of the solvent (water) and is reversible; the latter involves a chemical reaction (a thermosetting polymerisation reaction) between the ii components and is not reversible.