Science Education Resources Project
CHEMISTRY THAT APPLIES
Conveying unit purpose (Rating = , Poor)
The student text does not state a purpose for the unit as a whole. The purpose of the unit, namely to "give students opportunities to explore various everyday chemical reactions, contrast them to physical changes, and construct explanations for them in terms of changes in the molecules that make up the substances" (p. viit), is presented in the Teacher's Guide only. As a result, students will not have a sense of what they are doing and why they are doing it when they begin the unit. Similarly, the first lesson cluster does not tell students what its purpose is. However, a purpose is given for clusters 2 and 3. At the end of lesson 2, the text informs students that in the next cluster, cluster 2, they will investigate how the weight of the starting substances compares to the weight of the ending substances, and in cluster 3, they will find out where the new substances come from (p. 12s). Because students have just witnessed several reactions involving everyday substances, these statements are likely to be comprehensible to them. Similarly, the experiences that they have with mass conservation in cluster 2 will help them to grasp the purpose of cluster 3: "In the next cluster, you will see how atoms and molecules can help you understand even more about the Law of Conservation of Matter" (p. 43s). Nevertheless, the purposes are not likely to be interesting or motivating to students, and they are not asked to think about them.
Conveying lesson/activity purpose (Rating = , Very Good)
Chemistry That Applies is likely to help students appreciate the purpose of its lessons and their relationship to other lessons. Most of the lessons begin with a section called Key Question (pp. 2s, 6s, 17s, 25s, 28s, 31s, 36s, 48s, 56s), which serves as "the 'objectives' for each cluster or lesson" (p. 2t). These questions are to be discussed by students in small groups (as indicated in the Teacher's Guide, p. 2t B and p. xi). In this way, each student is encouraged to think about the purpose of the lessons and activities.
In addition, statements at the end of lessons often provide a purpose for the upcoming lesson by summarizing what students have done and previewing what they will be doing next. For example, after struggling to reply to questions about whether or not new substances were formed in the reactions observed in lesson 1, the text states:
If you found it difficult to answer these questions, you are not alone. Scientists often have difficulty with these questions because sometimes, when two substances are mixed, they just move in and around each other, like mixing tea and lemon juice, or sugar and salt. At other times, when two substances are combined, a chemical reaction takes place where new substances are made that didn't exist before the mixing took place. So how can you tell which of these two things is happening? The first step in figuring this out is making good observations and writing good descriptions of what you see. In the next lesson you will get more practice at this. (p. 5s)
Chemistry That Applies typically ties lessons together like this (also see pp. 25s, 28s, 48s, 84s). However, only occasionally are students asked to think about what they have learned so far and what they need to do next. For instance, on one occasion the introduction to the lesson leads directly to the lesson's Key Question, which students are asked to think about.
Most of the lesson purposes (found in both the Key Question section and summarizing statements at the end of lessons) are likely to be comprehensible because they relate to students' recent activities, (e.g., pp. 5s, 20s).
However, since there is no unit purpose to provide an overall context for individual lessons, the relationship between each lesson and the unit purpose is not conveyed. Furthermore, the attempt to relate individual lessons to cluster purposes may be confusing to students. For example, the title of cluster 2, Weight Changes in Chemical Reactions, and its stated purpose likely will lead students to think that they are about to study chemical reactions:
As you probably just found out, it is sometimes very difficult, if not impossible, to tell if new substances were formed. And if new substances did form, where did they come from? Was anything lost or gained in the process? In cluster 2, you'll investigate how the weight of the starting substances compares to the weight of the ending substances and in cluster 3 you'll find out where new substances come from. (p. 12s)
Yet, the first lesson in cluster 2 (pp. 16-20s) involves students in examining various physical changes. While it may be obvious to the developers of this textbook that the simplicity of physical changes and students' greater familiarity with them make them good places to start, this is not communicated to students. Not until the third lesson in cluster 2 is this rationale implied: "Chemical reactions are a little more mysterious than physical changes...Unlike physical changes where the substances and the molecules that make them up do not change, chemical changes actually produce new substances with new molecules" (p. 25s). The Key Question sections in lessons 7 and 8 (pp. 28s, 31s) relate to the purpose of cluster 2, but Key Question sections for lessons 9 and 10 (pp. 36s, 39s) do not.
Justifying lesson/activity sequence (Rating = , Satisfactory)
Clusters are sequenced logically, as is evident from their description (pp. viii-ixt). Cluster 1 involves students in describing (and, hence, becoming familiar with) "various substances and changes that occur in those substances, with the purpose of eventually being able to recognize when chemical reactions have occurredthat is, when new substances are produced" (p. viiit). Cluster 2 has students consider the possibility of weight changes during physical and chemical reactions and leads students to discover the Law of Conservation of Matter. Cluster 3 helps students to construct explanations for both the formation of new substances and the Law of Conservation of Matter. Cluster 4 explores the energy changes that take place between reactants and products (pp. viii-ixt).
Although no rationale is provided for the sequence of the lessons within the clusters, the logic for the cluster sequences can be readily inferred. For example, the lessons in cluster 2 present physical changes (which are likely to be more familiar to students) before chemical changes (which are likely to be more abstract and mysterious to students). Likewise, the chemical reactions are sequenced from simple to complex. First, students investigate the formation of a precipitate, a reaction in which both the reactants and the products are visible. Next, students investigate a reaction in which a gas is produced (even though the gas is invisible, bubbles provide evidence of its existence). Then, students investigate a reaction in which a gas is a reactant (but, by now, students have had experience with finding evidence for gases from weight changes and with keeping systems closed).
Similarly, the sequence of the lessons in cluster 3 (in which students learn to account for mass conservation by tracking atoms using molecular models) moves from simple to complex. The first two reactions in cluster 3 involve simple molecules in simple combinations. Students use models to show how water decomposes (one molecule with two kinds of atoms splits into two molecules, each made of one kind of atom) then how iron rusts (similar to a previous reaction, but involving synthesis rather than decomposition, and the synthesized molecule is more complex than water). Next the students study the reaction of baking soda and vinegar (in which more complex molecules are involved).
While most of the lessons have been thoughtfully ordered, a few lessons interrupt the flow described above. The last lesson in each cluster guides students through an individual research project, in which students are to investigate a common substance, such as sugar, glass, and cholesterol (p. 13st). The purpose of the research lesson at the end of each cluster is not clear. Their "research" consists mainly of reading about these substances in books. Although these activities may relate to social perspectives or technology standards, they do little to support the key ideas associated with the conservation of matter and could derail students from focusing on them.
Furthermore, it is not clear why a cluster on energy is included. Although traditionally matter and energy are taught together, the cluster on energy seems only loosely connected to the first three clusters on matter in this unit. The Teacher's Guide states that it will be covered, but the rationale is vague: "To develop a broadly connected understanding of how new substances form, this unit applies ideas about atoms, molecules, chemical reactions, and energy changes to four relatively simple reactions" (p. viiit). Also, a rationale for the inclusion of energy can be inferred from a teacher note on student misconceptions: "A common misconception is that matter changes into heat and light energy, accounting for the decreased mass of a pile of ashes after a fire" (p. 84t). So the cluster on energy could be intended to show students that heat and light do not carry away the mass of starting materials (except, of course, the minuscule mass converted to photons).
Attending to prerequisite knowledge and skills (Rating = , Satisfactory)
For students to understand the ideas associated with the conservation of matter, they need to have an understanding of several prerequisite ideas: the states of matter (particularly that gases are matter), changes of state, the particulate nature of matter, and more simple notions of conservation (e.g., it is possible to keep track of things, see where they come from, and see where they go). For most prerequisites, Chemistry That Applies either alerts teachers to them or addresses them. In nearly all cases, the prerequisite ideas are addressed thoroughly and connected to the key ideas about conservation. However, the treatment of the particulate nature of matter is insufficient for students who do not understand this idea already, as noted below.
States of matter (particularly that gases are substances with mass). Of particular relevance to understanding that mass is conserved in a closed system is appreciating that gases must be considered as part of a system. If students do not realize that gases are substances (with detectable mass), then the idea that a part of the mass of a system can be due to the presence of a gas will not be grasped. Teachers are alerted to difficulties that students have in understanding that gases, although invisible, may still be present in a system and that gases have mass (p. 28t). However, the module does not state that the idea that gases are substances with mass is a prerequisite to the unit.
Nevertheless, the unit does address this prerequisite explicitly in several activities. First, students observe that when a reaction that produces bubbles proceeds in a corked container, the cork pops off (pp. 28-30s). Then, they see that although a reaction producing a gas loses mass in an open container, the mass does not change if the container is kept closed (pp. 31-35s). Next, they consider various phenomena involving air (e.g., air keeps water from rising in a funnel, a balloon expands when air is put in it, a plastic bag contracts when air is withdrawn) in order to recognize that air is a substance (pp. 37-38s). In each case, questions help students to reason that if air is a substance, then it can be "used" in a reaction (p. 38st, questions 1-5), thus enabling them to make the link between the prerequisite and the idea of mass conservation.
However, the approach could be confusing to students. In lessons 7 and 8, two difficult ideas are being taught at the same timethe idea that gases have weight and the idea that mass is conserved in reactions involving gases. Moreover, it appears that the focus of these lessons is to establish the idea that gases have mass, using the idea of conservation of mass, rather than the other way around. The argument made in the text runs something like this: In the reaction with Alka-Seltzer and water, a gas is formed. We find that if we do not trap the gas, the mass is not conserved. If we do trap the gas, the mass is conserved. Hence, the gas must have weight.
The sequence (and perhaps the choice of activities) in lesson 9 could be perplexing too. In particular, the funnel and beaker activity is quite complex (pp. 37-38). Students may have difficulty making the connection between the balloon contracting into the flask when iron rusts (pp. 36-37s) and the balloon inflating when the funnel is pushed down into the water. It probably would have been less bewildering to deal first with the idea that gases have mass (perhaps at the end of lesson 5, after students have observed that a beaker of boiling water loses mass).
Changes of state. The Teacher's Guide makes clear that students are expected to be familiar with changes of state and other physical changes before starting the unit (p. viiit), and it uses such changes to probe students' initial ideas about matter conservation. However, although the unit involves students in making predictions about the changes in mass that occur during physical changes (e.g., when water boils, when sugar dissolves in water), it never returns to connect their observations to the conservation of matter.
The particulate nature of matter. The material is unclear about its expectations of what students should know already about the particulate nature of matter. The unit lists 10 objectives for grades 8 through 10, as well as two objectives for grades 5 through 7. One of the middle grades objectives is to: "Describe matter as consisting of extremely small particles (atoms) that bond together to form molecules" (p. xvt). However, it is not clear whether the unit assumes students know that matter is particulate, intends to address the idea, or merely plans to make the link between this idea and the idea that mass conservation observed in various reactions can be accounted for by counting atoms. Furthermore, in cluster 2, the student text and the teacher notes continue to give mixed messages. The student text explains the loss of mass from the boiling water in terms of the loss of water molecules from the beaker: "Water molecules are going into the air, thus leaving fewer water molecules behind" (p. 23s). Then, students are asked: "What are all substances made up of? Use as much detail as you can" (p. 23s, question 1). The Teacher's Guide attempts to clarify what students should know:
Students have probably heard of molecules before so take a few minutes to see how their thinking about molecules relates to these substances. If students are not comfortable with the concept of molecules, take some time to discuss the concept. It is sufficient here if students understand that all substances are made up of particles and that these particles are called molecules. The concept of atoms and molecules are [sic] developed much more in cluster 3 so there is no need to go into any detail here-especially, do NOT discuss atomic structure or formulas. (p. 23t)
However, this treatment of the particulate nature of matter will not be a sufficient clarification for students who do not understand it. If students do not need to know about molecules yet, it is unclear why they are asked to use the term "molecules" in their explanation.
Confusion also could result from an explanation given in lesson 8 for gases having weight: "Are the gases produced in this reaction matter or energy? If they are matter, then they are made of molecules. If they are composed of molecules, then they have weight" (p. 34s).
The unit does well in connecting the prerequisite to the idea that counting atoms can account for mass conservation. First, the unit relates the idea that all substances are composed of molecules to the idea that different substances are composed of different kinds of molecules (pp. 48-53s). Then, it explains that just as different words are made from a small number of letters, different molecules are made from a small number of kinds of atoms (pp. 51-53s). Lastly, it uses the idea that different molecules are made of different combinations of atoms to explain mass conservation in the various reactions that students have observed (lessons 13-16, pp. 56-77s).
Several of the above problems would not be serious if students had already studied Matter and Molecules (Berkheimer G. D., Anderson, C. W., Blakeslee, T. D., Lee, O., Eichinger D., & Sands, K., 1988), a research-based unit that devotes considerable time to having students work with particles, but no mention is made of this unit.
Conservation. An understanding of the idea that matter is conserved (or, more simply, that nothing is ever lost or gained because it just changes into different forms) is required for students to appreciate the need to account for seemingly "lost" mass when gaseous products are formed (e.g., when Alka-Seltzer reacts with water or when vinegar reacts with baking soda [pp. 28-30s]) or for seemingly "gained" mass when a gaseous reactant is consumed (e.g., iron rusting [pp. 36-38s]). Although this concept is not identified as a prerequisite, connections are made between it (nothing is ever lost or gained) and its more sophisticated version (mass is not lost or gained). After observing that the Alka-Seltzer and water reaction "loses" weight in an open container but does not lose weight in a closed container, the text explains: "You don't see anything leave the container, do you? But the evidence from your experiment is a weight loss. So something must be leaving, and whatever it is must have weight" (p. 33s), then states:
But with the cap on the bottle, the gases could not get out of the bottle, so no weight left the container. Nothing could leave the closed system. The weight did not change. But when the cap was released (open system), the gases flew out into the air. (p. 34s)
Similarly, teacher notes guide teachers to relate the rust reaction to less sophisticated notions of conservation: "So the gases do not disappearthey change into a different substance which is no longer a gas" (p. 40t).
Alerting teacher to commonly held student ideas (Rating = , Satisfactory)
Research indicates that, typically, students have several difficulties with the ideas that serve as the basis of the Chemistry That Applies analysis:
The module alerts teachers to several of these common student beliefs, typically in the descriptions of the clusters (pp. viii-ixt) or in the Teacher's Guide in brief sidebars near the student text (pp. 16t, 23t, 26t, 27t, 36t, 39t, 40t, 41t, 49t, 76-77t). For example, cluster 2 warns teachers about confusions that students have with density: "Many students confuse density with weight so they think that compacted steel wool weighs more" (p. 16t) and "A common misconception is that the weight increases because a solid [is] formed, and solids weigh more than liquids. Many students will be surprised to find that there is no change and will find it hard to accept this fact even after they have done the experiment" (p. 26t). And teachers are cautioned about students' common misconceptions regarding the particulate nature of matter in cluster 3, as the recombination of atoms that make up molecules is needed to account for the conservation of mass:
One typical misconception is the idea that molecules are in substances, rather than making [sic] up the substance-e.g., that there are molecules inside an ice cube or perhaps between the water, or that the molecules in liquid water are similar to germs, floating around in the water. Underlying this misconception is the general notion that substances are continuous, not made up of discrete particles-that water is a continuous liquid, or that solid aluminum is a continuous substance, not actually composed of discrete, individual particles. (p. 49t)
In most cases, students' widely held beliefs are explained in terms of their likely responses to questions or tasks in the unit, rather than in the abstract. For example, at the beginning of the lesson in which students predict (and then observe) what happens to weight when Alka-Seltzer reacts with water or baking soda reacts with vinegar, the Teacher's Guide states:
Because the gases produced in these activities are invisible, many students are not even aware that they are being formed and given off. Most students do not know what bubbles are and they certainly do not think of them as pockets of gases that were formed as the reaction occurred. Many students think the bubbles contain air. Another common misconception is that these gases have no weight and are not matter, so they take no account of them in their predictions. Students are frequently very surprised when the cork pops off the vinegar and baking soda reaction. Some students think something is being formed and given off but may think it is energy. (p. 28t)
These descriptions may not be adequate to give teachers a real sense of what students' difficulties are and to appreciate the significance of these misconceptions. It would have been helpful to give teachers a more coherent perspective on students' difficulties at the beginning of each cluster, so that teachers could take account of them in their planning. Then, the brief notes in sidebars could have served as helpful reminders during the lessons.
Assisting the teacher in identifying his or her own students' ideas (Rating = , Satisfactory)
The module provides numerous activities to assist teachers in identifying their own students' ideas. For example, the following questions and tasks help teachers check for:
Teacher notes make clear that the purpose of these questions and tasks is to probe students' ideas, and they instruct teachers not to provide answers at this point (e.g., pp. 2-4t, 12t, 16t, 28t, 36t).
Students are asked to make and justify their predictions (e.g., pp. 20s, 26s, 30s), and/or design an experiment to test them (e.g., pp. 26s, 31t). For example, after students make predictions about the weights of compact versus teased-apart steel wool, unmelted versus melted ice cubes, undissolved versus dissolved sugar, and water before and after boiling for 10 minutes, they are asked to (a) share and debate their predictions in their team; (b) make any changes in their predictions or reasons; (c) share their predictions and reasons with the class, noting differences of opinion and reasons; and (d) revise their predictions or reasons if they find others more compelling (p. 20s).
However, no suggestions are offered on how teachers can probe beyond their students' initial responses, nor are they told that they should do so. While students may reveal more about their ideas when they justify their predictions, teachers are not asked to listen to, or read, their responses.
Addressing commonly held ideas (Rating = , Satisfactory)
An attempt is made to address most of the important commonly held student ideas. Sometimes, misconceptions are addressed by challenging students to compare predictions based on a widely held idea to what they observe. For example, after asking them to predict how the weights of teased-apart versus compacted steel wool compare, teams of students carry out the experiments and consider whether their results helped to prove or disprove their predictions. Then, the whole class is to answer the question: "What must happen during any change for the weight of the materials to increase or decrease?" If they are having trouble answering, the following questions are provided to help: "In which of the experiments did the weight decrease significantly? What was different about that experiment from the others where there was no weight change?" (p. 22s).
This same strategy is used in other lessonsfor example, to address confusions about weight versus density (pp. 26-27s) and misconceptions about gases (pp. 33s, 40-43s).
However, the strategy is not used very effectively to address student misconceptions. Throughout the text, sidebars convey the notion that if students are given time to explain their predictions and compare them to what happens, then the issues will be clarified (e.g., pp. 26t, 28t, 41t). No specific guidance is given to teachers about how to structure discussions that will help to bridge student ideas with scientific ideas. For example, in the reaction between calcium chloride and potassium carbonate, students may predict that the weight will increase because solids weigh more than liquids. However, after they test the prediction, they are never asked explicitly whether they still believe the reasons they gave initially. The sidebar on page 26t suggests that students should become aware of any discrepancies in their thinking. It does not indicate what the discrepancies are likely to be, suggest what teachers should do after students become aware of them, or give teachers suggestions for how they can help students to resolve them.
In addition, students' commonly held ideas are addressed by prompting them to contrast their ideas with the scientifically correct ideas and to resolve the differences between them. Students respond to questions like: "Why might your little brother think that matter was created in this experiment [steel wool weighs more after rusting than before]?" (p. 41s) and "Why might your little sister think that butane just burned up and disappeared in this reaction?" (p. 43s), and they are asked to write a letter to the editor refuting a newspaper article that claims that a chemist has discovered a new substance that loses weight continually (p. 35s).
The widely held misconception that chemical changes result from the disappearance of the original substances and the appearance of new substances (which is why students do not expect weight to always be conserved) is addressed in lessons 12 through 16. Students are given the model of combining and recombining atoms to account for the conservation of mass observed in situations where the system was closed. Students work with molecular models and then with chemical formulas and equations to account for mass conservation by counting atoms for each of the chemical reactions encountered previouslywater decomposing, baking soda and vinegar reacting, iron rusting, and butane burning (pp. 56-77s). However, students are never asked to explain how the idea of atoms combining and recombining can explain mass conservation, even when substances seem to "disappear." For example, they are never asked to use models to show why mass is not conserved in those situations where the system is open.
The idea that oxygen is an "enabler" of burning (probably a result of the often-used expression "oxygen supports combustion") is not addressed. For instance, students are not asked to explain how an "enabler" role for oxygen is not consistent with their observations about mass changes in open systems and mass conservation in closed systems (i.e., why should the weight change in one case but not the other?), nor are they asked how they might respond to a sibling who held this view.
Providing a variety of phenomena (Rating = , Excellent)
A sufficient number and variety of phenomena are used to support each of the key ideas. For the idea that mass is conserved in a closed system, several phenomena are provided and are linked explicitly to the idea (the Alka-Seltzer in water reaction, the reaction between baking soda and vinegar, iron rusting, and butane burning). For example, students observe that when Alka-Seltzer is mixed with water or when baking soda reacts with vinegar in an open container, the mass decreases (lesson 7, activities 1 and 2, p. 29s); but when Alka-Seltzer is mixed with water or when baking soda reacts with vinegar in a closed container, the mass does not change (lesson 7, pp. 29-30s). In each case, the link is made to the Law of Conservation of Matter (Alka-Seltzer and water and baking soda and vinegar are linked on p. 34s, question 5; rusting iron is linked on pp. 40-41s, questions 5-7; burning butane is linked explicitly in the text, p. 43s). In a practice task, students are asked what happens to the mass of a house when it burns to the ground, leaving behind only a pile of burned wood and ashes (p. 86s), and the text then provides a chart that accounts for all the matter involved (p. 87s). These same reactions and others (e.g., water decomposition, pp. 10-11s; precipitate formation, pp. 26-27s; copper chloride and aluminum reaction, pp. 78-79s) are used to support the idea that substances may combine with other substances to form new substances with different characteristic properties. The four main phenomena (water decomposing, iron rusting, butane burning, and baking soda and vinegar reacting) are repeated again in cluster 3. This time they are linked to the idea that in chemical reactions, atoms combine and recombine in various combinations but are not created or destroyed and to the idea that the notion of atoms explains the conservation of matter (in other words, if the number of atoms stays the same no matter how they are rearranged, then their total mass stays the same).
Providing vivid experiences (Rating = , Excellent)
All of the phenomena are firsthand. Students observe directly the phenomena involving mass conservation in a closed system and lack of mass conservation in an open system.
Introducing terms meaningfully (Rating = , Excellent)
The unit links technical terms to relevant experiences and limits the terms used to those needed to facilitate thinking and promote effective communication. For example:
Open and closed systems. The text introduces the terms "open system" and "closed system" as students are about to carry out experiments with them (p. 32s). Both are defined in relation to the phenomena that students are experiencing: An open system is a reaction occurring in which "the cap is left off the bottles," and a closed system is defined as "when there's a top on the jar, so that nothing can get out." Students use their understanding of the terms in comparing their observations and considering whether the reactants and the products are the same in an open system versus a closed system (p. 33s, questions 1, 2, 4). The text then uses the terms to explain why students observe a weight loss in an open system but not in a closed system (pp. 34s, 43s).
The Law of Conservation of Matter. After students have observed that weight is lost when Alka-Seltzer reacts with water or when baking soda reacts with vinegar in an open system but not in a closed system, the text relates their observations to the Law of Conservation of Matter (pp. 33-35s). Students then explain the burning butane reaction (p. 43s, question 12) and the recycling of metals (p. 80, question 7) with respect to matter conservation.
Reactant and product. The terms "reactant" and "product" are used after students have had experiences comparing starting substances with ending substances (p. 28s). Students then respond to questions that use these terms (p. 33s, question 4; 35s, question 7).
Physical change and chemical reaction. Before using the terms "physical change" and "chemical reaction," students are given experiences with changes where "only the size, shape, space it occupied or temperature [of the material] changed" or where "the material seemed to lose its character, and something with entirely new properties appeared" (p. 16s). However, the definitions used here and on page 25s are confusing. Teacher notes indicate that the purpose of these terms is to provide students with language to use in communicating about their observations, and the notes make it clear that the distinction is not to be emphasized (pp. 11-12t). But, given the difficulty in distinguishing chemical from physical changes, it is not clear why this distinction is made.
Molecules. The term "molecules" is used (p. 23s) before it has been defined in relation to atoms (lesson 12, pp. 48-55s), but its earlier use is to give students a name for the particles they will be talking about. As the unit was designed for grades 8 through 10 and the term "molecules" is included in Michigan's grades 5 through 7 objectives, familiarity with the term may be assumed legitimately.
Atoms. The term "atoms" is introduced as the name for identifying the building pieces or building blocks of substances. The text provides the analogy to letters making up words: just as "the 26 letters of the alphabet are used to build hundreds of thousands of different words" (p. 52s), so nature uses atoms as pieces to build hundreds of thousands of different substances. Then, students are asked to come up with their own model for atoms and molecules (pp. 53-54s).
No other technical terms are used in the unit.
Representing ideas effectively (Rating = , Satisfactory)
Representations are used to make abstract ideas intelligible. For example, to illustrate the idea that atoms combine to form molecules, an analogy is drawn between bricks making up structures and atoms making up molecules, then students are asked to consider how the analogy is apt and how it is not (p. 54s). The text uses several models to represent the ideas that atoms can combine and recombine to form different molecules and that counting the atoms involved accounts for the mass conservation. To wit, for each of the chemical reactions dealt with in the unit (water decomposing, pp. 57-61s; iron rusting, pp. 64-65s; butane burning, pp. 74-75s; vinegar reacting with baking soda, pp. 69-71s; aluminum reacting with copper chloride, p. 79s), students use gumdrops and toothpicks to make models of the starting substances and disassemble them to make models of the products, then use chemical formulas and equations to show that atoms are conserved. The text also discusses the assembly and disassembly of Lego structures to show that all of the pieces are used (p. 60s). (It would have been helpful if the material had extended the Lego analogy to show how accounting for atoms can explain observations of mass conservation in closed systems; e.g., by showing students that if a Lego structure is disassembled and then reassembled to make other structures, the weight of the starting structure is the same as the weight of the ending structures. This same analogy could be extended further to represent why mass is not conserved in an open system; e.g., a few of the Lego pieces fall off the balance.)
However, one representation is likely to be confusing. To represent the loss of weight observed when water boils or when a chemical reaction produces a gaseous product, the text draws the analogy between these observations and the weight loss of a child who throws quarters or stones out of his pocket (pp. 23s, 33s, 40t). This could be bewildering to students who think of the quarters as representing molecules because the quarters are not really a part of the person standing on the bridge. It would have been helpful to ask students to consider how the analogy is like and different from the real thing or how the analogy could be improved (e.g., by having the child spit off the bridge instead).
Demonstrating the use of knowledge (Rating = , Very Good)
The unit models consistently the use of the key ideas to explain phenomena. Demonstrations are step-by-step and are identified clearly as demonstrations. For example, after students observe that burning steel wool increases its mass, the text demonstrates how to explain this by using the idea that substances combine with other substances to form new substances with different properties (which also reflects the idea in a related benchmark that "an especially important kind of reaction between substances involves combination of oxygen with something elseas in burning or rusting" [AAAS, 1993, pp. 78-79). Chemistry That Applies expresses this concept thus:
How would scientists explain the steel wool rusting and burning reactions? You may remember that steel wool is a form of iron. When steel wool burns, a chemical reaction occurs that is very similar to the chemical reaction of rusting. Just like a candle or a match or a piece of paper needs oxygen from the air to burn, steel wool needs oxygen from the air to rust or to burn. Candles, matches and paper wouldn't burn on the moon, and neither would steel wool rust or burn on the moon. In both cases, steel wool combines with oxygen from the air to form new substances, both of which are called iron oxide. The reddish, flaky rust is actually a new substance and the jet black, powdery substance left after burning the steel wool is also a new substance. (p. 41s)
The text also uses this idea and the idea of mass conservation to model an explanation of the burning butane reaction (pp. 42-43s). Then, lessons 13 and 14 use the ideas of mass conservation and that atoms can combine and recombine to "discover" that mass conservation can be accounted for by counting atoms (pp. 56-68s). Students are led step-by-step through the process of making models of the reactant molecules, deciding what products can be formed, disassembling the models of the reactants, using them to make models of the products, and seeing that no atoms are added or left unused. Teacher notes describe the role of the teacher in modeling this reaction step-by-step, coaching students as they work through subsequent reactions, and reducing feedback until students are able to work independently (p. 56t). Finally, the text models the use of the ideas that atoms combine and recombine but are not lost or gained to consider three questions:
Could it be true that the atoms that make up my body may have once been a part of a dinosaur?
Could it be possible that some atoms that are a part of me could someday be a part of a spaceship that travels to distant galaxies?
And would it be true that the materials we throw in landfills just don't rot into nothingness? (p. 78s)
However, no criteria are given for judging the quality of explanations nor is running commentary included that points out particularly good aspects of an explanation.
Providing practice (Rating = 5, Excellent)
There are some tasks for students to practice using all of the key ideas, and, in all cases, novel tasks are included. For example, students use
Just as the key ideas move from summaries of observations to abstract explanations, the tasks within the unit increase in complexity. For example, students initially explain four chemical reactions in terms of (observable) mass conservation, then in terms of atoms. Students are coached through the atomic explanation of the first and second reactions (water decomposing and iron rusting); expected to work more independently on the third and fourth reactions (baking soda and vinegar reacting and butane burning); and expected to work without help on the aluminum and copper chloride reaction (as indicated in the Teacher's Guide, p. 56t).
Encouraging students to explain their ideas (Rating = , Excellent)
Students are asked consistently to clarify, justify, and/or represent their thinking about the key ideas. For the many physical changes and chemical reactions that they observe, they make predictions about whether or not new substances will be formed (e.g., pp. 4s, 11-12st) and what happens to the mass (pp. 22-24st). Then, they observe what happens and attempt to explain it. Such tasks ask students to use the ideas that substances may combine with other substances to form new substances with different characteristic properties and that when substances interact with one another in a closed system, mass is conserved. Students are asked repeatedly to explain their ideas about atoms and their relationship to mass conservation in end-of-lesson questions. For example:
What's the difference between boiling and decomposing water? Talk about the different gases that are formed in your explanation.
Could chlorine gas, Cl2, be a product in this reaction? What about carbon dioxide, CO2? Explain why you think this. (p. 62s)
Students are asked to answer similar questions about the other chemical reactions observed, such as, rusting iron (p. 67s), baking soda reacting with vinegar (p. 73s), and burning butane (pp. 76-77s). They also are asked to make drawings of what is going on with the atoms and molecules in each of these reactions and to use them to "account" for the mass.
In most cases, each student is expected to respond to questions and tasks. Teacher notes explain the role of group work and the need for students to write individual answers to questions "so that those [students] who do not contribute strongly or learn well from group discussions have opportunities to articulate their own ideas in writing" (p. xit).
Suggested responses to questions give guidance to teachers on how to provide feedback to students. The answers in the teacher notes contain the correct answer (always), suggest limits to the expected answer (when appropriate), indicate how students might answer the question (occasionally), and provide questions to probe the student's answer further (only once). An example that does all but the last of these follows:
Question 1: What are all substances made up of? Use as much detail as you can. (p. 23s, Think and Write, question 1).
Suggested response: All substances are made of molecules. Students have probably heard of molecules before so take a few minutes to see how their thinking about molecules relates to these substances. If students are not comfortable with the concept of molecules, take some time to discuss the concept. It is sufficient here if students understand that all substances are made up of particles and these particles are called molecules. The concept of atoms and molecules are [sic] developed much more in cluster 3 so there is no need to go into any detail hereespecially, do NOT discuss atomic structure or formulas. (p. 23t)
Task 6a: Predict what would happen to the weight of a cold glass of water on a humid summer day. Explain your prediction. (p. 24s, question 6a)
Suggested response: It becomes very moist on the outside of the glass as it picks up water vapor from the air. It has added more matter so it will weigh more. Students have difficulty with this question. Some think that the water seeps through the glass. That doesn't make much sense to them, but they do not have a better explanation (p. 24t).
Task 6b: Predict what would happen to the weight of a car that gets very rusty over several years. Explain your prediction. (p. 24s, item 6b)
Suggested response: There are several plausible answers for thisthe car gets lighter because metal rusts and falls off; the car gets heavier because the iron combines with oxygen from the air and forms a heavier molecule; there is no change in weight since rust is just the same iron discolored. Accept any reasonable explanation. Do not give answers here as this reaction is explored in lessons that follow. (p. 24t)
The text also provides feedback to students, typically by demonstrating how to explain a phenomenon that they have just considered. For example, after observing that the reaction of Alka-Seltzer and water loses weight and attempting to explain their observations, the text provides an explanation: "Here's how scientists think about this reaction. . ." (p. 33s).
Guiding student interpretation and reasoning (Rating = , Excellent)
Chemistry That Applies contains questions to guide students' interpretation of and reasoning about investigations and readings. For example, after observing whether or not the weight changes during four physical changes (pulling apart steel wool, melting an ice cube, dissolving sugar in water, and boiling water), the text assists students in thinking about their results and relating them to the idea of open and closed systems, an understanding which is needed in order to appreciate when mass will be conserved:
What must happen during any change for the weight of the materials to increase or decrease?
If you're having trouble coming to a good answer to this question, think about these questions:
In which of the experiments did the weight decrease significantly?
What was different about that experiment from the others where there was no weight change? (p. 22s)
Later in the unit, lesson 13 is used to help students see how the idea of atoms explains the mass conservation that they have observed previously. Teacher notes make clear that teachers should guide their students through the process of using physical models to represent the conservation of matter and then move their students from physical models to pictures of models, to formulas that account for atoms, and to the balanced equation (p. 56t). Lesson 13 uses questions and answers to help students interpret the decomposition of water at the molecular level. First, the questions focus students on the problem they are trying to answer:
In an earlier lesson, you used a battery to make bubbles appear under water. The water level went down as the bubbles were formed.
How can atoms and molecules be used to explain the formation of bubbles from water? (p. 56s)
Next, students carry out the experiment and write the common name of the reactant and its chemical formula, H2O (p. 57s). Then the text guides them to relate their observations to the idea that atoms can combine and recombine but do not disappear:
Now think about the products that were formed. What could they be? They were bubbles, of course, but what was in the bubbles? Since the water level went down, we might assume that the water changed into the bubbles. We know, though, that the water wasn't boiling, because it never got hot. So the bubbles couldn't have been water vapor. What else could they be?
Here's a hint. Look at the types of atoms that make up a water molecule. Since water molecules are made up of only hydrogen and oxygen atoms, the substances formed inside the bubbles can only contain hydrogen and oxygen. Would it be possible to have carbon dioxide (CO2) as a product of this reaction? Why?
So what substances are inside the bubbles? Did someone say "Maybe there's oxygen gas inside some of the bubbles, and hydrogen gas in the other bubbles?" Yes! The water molecule is coming apart and making hydrogen and oxygen molecules. Hydrogen gas is in the bubbles coming off one of the pencil leads, and oxygen gas is in the bubbles coming off the other lead. You can prove this by collecting the gases and conducting tests on them. (p. 58s)
After this, students are led through an exercise in which they make two water molecules, drawing and labeling them; then they disassemble and reassemble them to make two hydrogen molecules and one oxygen molecule (pp. 58-60s).
Next, they are asked to think about how many water molecules they used in all, how many hydrogen molecules were formed, and how many oxygen molecules were formed. The text asks: "Are you beginning to see how atoms rearrange themselves to make new substances?" (p. 61s).
Finally, students are helped to relate atoms and molecules to the conservation of matter:
Now let's see how atoms and molecules can be used to explain conservation of matter. Remember that conservation of matter in chemical reactions means that the beginning weight of all the reactants is exactly the same as the ending weight of all the products. Can you speculate about why this might be?
How many atoms of oxygen are there in the molecules of the reactantthe starting substance? How many atoms of hydrogen are there in the molecules of the reactant? Record this information on your data sheet under ACCOUNTING FOR ATOMS.
How many oxygen atoms are there in the product moleculesthe ending substances? How many hydrogen atoms are there in the product molecules?
WHAT DO YOU NOTICE ABOUT THE NUMBERS OF ATOMS IN THE STARTING SUBSTANCES AND THE ENDING SUBSTANCES? They are the same! The atoms don't disappear or appear out of nowhere...they just rearrange themselves into new molecules.
And if each atom has a certain weight (which it does), then how does the weight of the reactant compare with the weight of the products?
This is the Law of Conservation of Matter, or the Law of Conservation of Mass. No weight is lost or gained in chemical reactants. No mass is lost or gained. No matter is lost or gained. Why? Because no atoms are lost or gained during chemical reactions. (pp. 61-62s)
In the above and other examples, the questions are arranged in an increasingly complex sequence in order to help students move from phenomena and their own ideas to the scientific ideas (e.g., pp. 27s, 33-35s, 40-43s, 50-55s).
Encouraging students to think about what they have learned (Rating = , Poor)
Chemistry That Applies gives students numerous opportunities to revise their ideas. In all of the mass conservation experiments in cluster 2, students make and explain predictions about whether or not the weight will change, then revise their ideas after weighing the products (e.g., pp. 20s, 22s, 23-24s, 27s, 32-33s, 40s). However, they are not asked to consider how their ideas have changed.
A unit assessment is included as an appendix. While its purpose is to assess the unit, rather than individual students, the questions could be used for the latter purpose. These items were examined for the criteria "Aligning assessment to goals" and "Testing for understanding."
Aligning assessment to goals (Rating = ,Very Good)
There are enough assessment items for ideas b and c but not enough for ideas a and d.
For idea b ("No matter how substances within a closed system interact with one another, or how they combine or break apart, the total weight of the system remains the same"), there are several helpful items:
You just carried a log into your house to put into the fireplace. Before you burned it, you decided to weigh it. It weighed exactly 10 pounds. After it had burned up and cooled, you found that the ashes from the log weighed only one-half of a pound.
What happened to the other 9.5 pounds of log? Choose as many as apply.
If you burn magnesium metal (formula is Mg) in air, it combines with air to form magnesium oxide (chemical formula MgO). Magnesium oxide is a white powder. Would the white magnesium oxide powder weigh more, less or the same as the magnesium metal before the reaction?
Explain your reasons for your choice. (Unit Assessments, p. 8, question 11a)
This same chemical reaction is used in flashcubes [sic] for cameras. If you burn the same magnesium metal in a sealed flashcube [sic] with oxygen inside, it makes a big flash and forms the same white powder. Would the sealed flashcube [sic] weigh more, less or the same as it did before it flashed?
Explain your reasons for your choice. (Unit Assessments, p. 8, question 11b)
If you put a piece of the same magnesium metal in a small amount of acid in an open beaker, much bubbling occurs and eventually the metal is no longer seen. [Do] the beaker and its contents at the end of the reaction weigh more, less or the same as the beaker, acid and metal weighed before the reaction?
Explain your reasons for your choice. (Unit Assessments, p. 9, question 11c)
Do the products that were formed weigh more, less or the same as the reactants (the metal and the acid)?
Explain your reasons for your choice. (Unit Assessments, p. 9, question 11d)
When you repeated the acid and magnesium experiment and felt the container, you noticed that it was very hot. Does this fact change your answer to part c or d?
Explain your reasons for your choice. (Unit Assessments, p. 9, question 11e)
For idea c ("In chemical reactions, atoms combine and recombine in various combinations but are not created or destroyed"), there are four items:
Alcohol has a chemical formula of C2H6O. It is sometimes produced by the fermentation of grains or fruits. At other times, scientists can make it from substances in their labs. Which combination of substances COULD be the ones that alcohol is produced from? Check ALL the possibilities. Assume that there are no other reactants but there could be other products.
1) carbon dioxide (CO2) and water (H2O)
2) carbon dioxide (CO2) and oxygen (O2)
3) hydrogen gas (H2) and carbon tetrachloride (CCl4)
4) carbon dioxide (CO2) and hydrogen gas (H2)
5) pentane (C5H12) and oxygen (O2)
Explain how you made your choice(s). (Unit Assessments, p. 2, question 4)
When candle wax (C30H62) burns, it reacts with the oxygen in the air. As it burns, some of it drips down into its holder, but some of it changes into other substances. Which of the following could NOT be [one of] these other substances? Circle all the possibilities.
1) hydrogen gas, H2
2) carbon monoxide, CO
3) alcohol, CH3OH
4) hydrogen sulfide, H2S
5) water, H2O
6) sulfur dioxide, SO2 (Unit Assessments, p. 2, question 5a)
Explain how you decided which ones could NOT form candle wax? (Unit Assessments, p. 3, question 5b)
You hear a lot these days about the importance of wearing sunscreen because the ozone layer in the atmosphere is disappearing. It is no longer protecting us from the ultraviolet radiation from the sun. Can anyone (or any substance) really disappear into nothingness? How would you explain to a friend why the use of the word "disappearing" is misleading? Explain what is really meant. (Unit Assessments, pp. 14-15, question 14e)
Question 13c (see Unit Assessments, p. 12) is also relevant, but students must complete questions 13a and b first, which require additional information.
However, for idea a ("Substances may combine with other substances to form new substances with different characteristic properties"), there are only two items:
Draw a picture2 of what happens to [iron and gas] molecules when they form rust with the chemical formula Fe203. Explain your picture. (Unit Assessments, p. 5, question 7c)
Ordinary table salt has a chemical formula of NaCl. Na is the chemical symbol for sodium which is a very reactive metal. It is so soft that it can be cut with a knife and so reactive with oxygen and water that it must be stored in kerosene. Cl2 is the symbol for chlorine gas, which is frequently used in swimming pools to kill bacteria. It is poisonous and if breathed it can cause severe headaches, nausea and even death. When you put a piece of sodium metal into a flask filled with chlorine gas, you noticed the white salt crystals on the bottom of the flask.
Do you think that the products formed are new substances? Use atoms and molecules to explain why you think this. (Unit Assessments, pp. 11-12, question 13d)
Other items, such as question 6c (see p. 3a), test knowledge of specific examples, rather than the generalization.
For idea d there is one item only: "How do your picture and/or your equation [relating to the formation of NaCl] show that mass is conserved?" (Unit Assessments, p. 12, question 13c).
Testing for understanding (Rating = , Very Good)
Several assessment items focus on understanding the key ideas. Most of the tasks are novel, involving reactions not studied in the unit. Since only one assessment item focuses on idea d (the idea of atoms explains the conservation of matter: If the number of atoms stays the same no matter how they are rearranged, then their total mass stays the same), more would be needed to test for understanding. For example, students could have been asked to identify situations where atoms were conserved, predict what might happen in familiar situations if atoms were not conserved, or apply atom conservation to situations like:
If your body might contain atoms that were once in a dinosaur, where might the rest of the dinosaur be?
Using assessment to inform instruction (Rating = , Poor)
Although student responses are solicited throughout the unit, the stated purpose of these questions is either to give teachers the opportunity to probe their students' initial ideas or to give students the opportunity to express their ideas, hold them up to scrutiny, and revise them as seems appropriate. No mention is made of using these questions to help teachers make decisions about instruction.
American Association for the Advancement of Science. (1993). Benchmarks for science literacy. New York: Oxford University Press.
Berkheimer, G.D., Anderson, C.W., Blakeslee, T. D., Lee, O., Eichinger D., & Sands, K. (1988). Matter and molecules. East Lansing, MI: Michigan State University, The Institute for Research on Teaching.
Driver, R., Squires, A., Rushworth, P., & Wood-Robinson, V. (1994) Making sense of secondary science. Research into children's ideas. London: Routledge.