In a physical change the appearance or form of the matter changes but the kind of matter in the substance does not. 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.
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. Students also frequently believe that the original substance in a chemical reaction vanishes completely and forever. Students commonly believe that physical changes are reversible while chemical changes are not. Students may believe that beer frothing is an example of a chemical change or an apple ripening is a physical change. 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) Melting and expansion on heating were also considered to be evidence of chemical change by some students. Students can consider potassium permanganate (Condy's crystals) dissolving in water to be a chemical change because of the intense difference in colour. One study found that 80% of students considered a difference in colour between the reactant and product evidence of chemical change. If students regard ice as a different substance from liquid water they are likely to classify the melting of ice as a chemical change. This depends on their conception of substance. Freezing and boiling are considered to be examples of chemical reactions. Research shows that students frequently use the term chemical change to describe changes in physical state. Research: Loughran, Mulhall & Berry (2002) Pat (recording the group's notes about a prac): What happened? 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. Students have difficulty distinguishing between physical and chemical change, despite formal teaching, and the distinction is somewhat arbitrary.
Contrasting student and scientific views.
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