Changes in matter that do not change the composition of the substance
This focus idea is explored through: Show
Contrasting student and scientific viewsStudent 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? Sam: What shall I write down was formed? 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
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 activitiesPromote reflection on and clarification of existing ideas 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:
Practise using and build the perceived usefulness of a scientific model or idea Clarify and consolidate ideas for/by communication to and with
others Practise using and build the perceived usefulness of a scientific model or idea 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 Promote reflection on how students’ ideas have changed
What changes do not change composition of a substance?A physical change is a change in the state or properties of matter without any accompanying change in its chemical composition (the identities of the substances contained in the matter). We observe a physical change when wax melts, when sugar dissolves in coffee, and when steam condenses into liquid water (Figure 1.4.
What is the change in form of a matter but not its composition?Physical changes are changes affecting the form of a chemical substance, but not its chemical composition. Physical changes are used to separate mixtures into their component compounds, but can not usually be used to separate compounds into chemical elements or simpler compounds.
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