This idea of focus is explored through:
- Contrasting views of students and scientists
- Critical Teaching Ideas
- teaching activities
Contrasting views of students and scientists
everyday experiences of the students
Despite formal instruction, students have difficulty distinguishing between physical and chemical changes, and the distinction is somewhat arbitrary. However, understanding the differences between purely physical processes such as melting, evaporating, and boiling and the changes that take place in chemical reactions, particularly the notion that new substances are formed, is important to understanding chemistry, and students often confuse the two . . The following conversations are typical:
Pat (taking the group's notes on an exercise): What happened?
Kim: It got bubbly.
Pat: Did you see any new substance?
Kim: No.
Sam: What should I write was formed?
Chris: A blue color.
Research: Loughran, Mulhall & Berry (2002)
Research shows that students often use the term chemical change to describe changes in physical state. Freezing and boiling are examples of chemical reactions. This depends on your idea of the substance. If students think of ice as a substance other than liquid water, they will likely categorize the melting of the ice as a chemical change. One study found that 80% of students took a color difference between the reactant and the product as evidence of a chemical change. Students can view the dissolving of potassium permanganate (Condy crystals) in water as a chemical change due to the intense color difference. Some students also considered melting and expansion by heating as evidence of a chemical change.
Research: Driver, Rushworth and Wood-Robinson (1994)
Many students failed to recognize that a chemical change is characterized by the formation of a substance with properties different from the original substance, and a significant proportion of the students who did did not adequately justify the distinction of a 'new' substance.
Research: Driver, Rushworth and Wood-Robinson (1994)
Students may believe that beer froth is an example of a chemical change or that the ripening of an apple is a physical change.
Research: Tsaparlis (2003)
Students generally believe that physical changes are reversible while chemical changes are not. Students also often believe that the original substance disappears completely and forever in a chemical reaction. 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 for stored electricity and not chemicals that react in a way that converts chemical energy into electrical energy.
Other common beliefs are that the chemical changes are caused by the mixing of substances/reactants or that heat needs to be added (considered a type of material).
scientific point of view
A physical change changes the appearance or shape of matter, but not the nature of matter in the substance. However, with a chemical change, the nature of matter changes and at least one new substance with new properties is created.
The distinction between physical and chemical change is not clear. Students are often misled into assuming that a change is physical or chemical. In fact, this should be viewed more as a continuum. For example, the dissolution of salt in water is generally considered a physical change, however, the chemical species in the brine (hydrated sodium and chlorine ions) differ from the species in solid salt. The dissolution of instant coffee in water appears to be a physical change, but in most cases the dissolution is accompanied by an energy change and is probably best viewed as a chemical process, although it is possible to recover the original constituents through physical means. Many examples of materials that dissolve (e.g. 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 in practice this can be difficult. Many high school science texts state that chemical changes are irreversible, while physical changes can be reversed. Cutting paper into tiny pieces or crushing rocks are obvious physical changes, but restoring the original rock or paper is difficult. Rechargeable batteries use a chemical reaction where, as they are discharged and recharged, that reaction is driven backwards and the products are converted back to the original reactants. A car's electrical generator (alternator) constantly charges the car's battery while the car's engine is running.
Critical Teaching Ideas
- With a physical change, the nature of the substance, its components, and the number of particles remain unchanged.
- With a chemical change, the properties of the new substances differ from the original ones, the particles are different, and the number of particles can change.
- The distinction between physical and chemical change, while useful, should be viewed more as a continuum.
- Chemical reactions can be reversed, but this can be difficult in practice.
Examine the relationships between ideas about physical and chemical changes in theconcept development cards- (States of matter, chemical reactions)
When teaching about physical and chemical changes, it is important to allow students to view the classification as a continuum. They must be able to observe a range of changes and articulate their views on the rate of change and the problems with the classification process. Students should recognize that chemical reactions create new chemicals alongside the starting materials, but that chemical processes can be reversed. It can be very helpful to look at examples of reversible chemical reactions and consider why many chemical changes are difficult to reverse.
teaching activities
Encourage reflection and clarification of existing ideas.
The following activities are designed to help students identify and then refine their ideas about physical and chemical changes. It is important that students observe a range of changes and record their opinions of what is happening. They could record this in a notebook, writing down and drawing results and observations. They should be encouraged to formulate and record hypotheses about what is happening, knowing that their opinions will not be evaluated at this stage.
For an example of this technique see:Use of logbooks in the 10th electricity year.
This can help bring their existing ideas to light and help them question and expand on their existing beliefs.
Students could explore:
- Heat steel wool in air and collect the resulting black powder. Then weigh the reagent and the product (there should be an increase). This could be 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 was added to the steel wool during combustion?
- Dissolve sugar and salt in water and compare what happens to the electrical conductivity of the two solutions when dissolved. The recovery of the salt by evaporation shows that the salt is still there, but the conductivity indicates that something new is being formed.
- If you compare boiling water to the vinegar and baking soda mixture, both produce bubbles, but what's the difference? Baking soda and vinegar can be mixed in a Ziploc sandwich bag to show the production of a new substance that will burst the bag.
- Precipitation reactions, especially those that produce a noticeable color change, such as Epsom salts and ammonia solution. This can be compared to adding potassium permanganate to water. What are the differences?
- Acid-base reactions can be illustrated with both natural (cabbage juice) and synthetic indicators. The color changes help illustrate that new materials could be forming.
Practice using and constructing the perceived usefulness of a scientific model or idea
It is important that examples of change are not limited to the materials and chemicals that students are exposed to in the classroom. As homework, students could be asked to collect examples of changes they see around them and place them on their physical and chemical change scale. Some examples they can collect are burning fuels, cooking, and processes like digestion, respiration, and photosynthesis.
Clarify and solidify ideas for/through communication with and to others
To solidify their views, students can be asked in groups to choose two changes, one on the physical end of the continuum and one on the chemical end, and explain the differences to the class. Sharing their ideas with others can help students clarify and consolidate new and existing ideas about change.
Practice using and constructing the perceived usefulness of a scientific model or idea
Science is a field in which a set of key ideas are gradually given a deeper meaning by using them in a variety of situations and emphasizing how the same idea helps make sense in many situations. This is especially true for key ideas in the chemical sciences that cannot be “discovered,” “proved,” or even demonstrated through 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, but their usefulness can be developed further by showing (among many other things) how they do it can help to understand physical and chemical changes. These ideas may be presented here or referenced if they have been presented previously. Writing chemical equations in verbal and symbolic form can serve as a useful way to describe some of the changes students have seen and also to show the benefits of chemical symbols by tracing the elements (or atoms) in ways the words can't. When precise chemical formulas cannot be written (as is the case for most biochemicals), a drastic simplification can still be useful. For example, wood is primarily cellulose, a polymer of glucose, and a representation like (C6H10Ö5)NorteIt can be used to track changes in processes such as combustion.
Models and diagrams can help here. For example, most schools have molecular modeling kits that can be modified to show how molecules have changed and how atoms have been rearranged as a result of the changes. Students can also draw poster-sized diagrams to support their explanations.
Challenge some existing ideas
Although proving the reversibility of chemical changes is difficult, students are very familiar with the need to charge their cell phones, cameras, and other rechargeable devices. This could easily be argued, although studying the chemical reactions that power these devices could be a useful research project. A caveat here is that much of the information available can be very technical.
Encourage reflection on how students' ideas have changed
Reviewing your original journal entries can encourage reflection on how students' views have changed. Students can then apply their new ideas to other examples of change. Activities such as the following can be used to stimulate discussion about the types of changes that take place and the difficulties in classifying some changes as physical or chemical:
- Dissolving metals in acid (magnesium and zinc) and examining the resulting gas.
- Investigate the reactions of limestone and acid (Limestone cave making: what is that change?)
- Add zinc to a copper sulfate solution and observe the color changes that occur.
- If you compare the setting of water-based glues such as e.gHashtagjAquadherewith two-component adhesives such asHeraldit. The first works by evaporation of the solvent (water) and is reversible; The latter involves a chemical reaction (a thermosetting polymerization reaction) between the two components and is not reversible.