Idea a: Substances may combine with other substances to form new substances with different characteristic properties.

There is a content match to this idea. The first cluster (particularly lessons 1 and 2) focus on how to distinguish whether a new substance has been formed when two or more substances are mixed. Students describe what happens when they mix a variety of substances (such as vinegar and baking soda) (p. 8s). The text introducing lesson 2: Is It A New Substance? relates what students have just done to what they are about to do:

In this lesson, students observe four reactions—iron rusting, baking soda and vinegar reacting, a butane lighter burning, and water decomposing (pp. 7–11s)—and, in each case, they describe the starting and ending substances to see if they find evidence of change. Near the end of the lesson, the text relates their observations to guidelines for determining whether a new substance has formed:

Periodically throughout the rest of the lessons, students are questioned about the differences between starting and ending substances. For example, at the end of lesson 6 (which examines a reaction in which a precipitate forms), students are asked: "How were the ending substances different from the starting substances?" (p. 27s); and at the end of lesson 10, students are asked to "Write a description of the product in your journal" (p. 40s).

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 is a content match to this idea. Lessons in Cluster 2 involve students in investigations of weight changes for both physical changes and chemical reactions. The early lessons in Cluster 2 ask students to consider what happens to the weight of substances as they undergo physical changes, such as pulling apart steel wool, dissolving sugar, melting ice, and boiling water (pp. 19s, 21–22s). In later lessons, students are asked to consider the weight of substances as they undergo chemical changes (such as a reaction between two liquids to form a solid, the formation of a gas when a liquid and a solid are mixed, and a reaction in which a gas is a reactant). The generalization is stated and related to the reaction involving the formation of a gas. In lesson 8: Do Gases Have Weight?, students compare weight changes associated with the Alka-Seltzer and water reaction in an open container versus in a closed container. After the demonstration, students read that:

Questions at the end of this lesson on weight changes and open and closed systems also engage students in thinking about this idea. Students are asked:

The expected answer is: "No, since all substances are included whether they escaped or not, the reactants and the products always weigh the same" (p. 33t). The follow-up question asks: "And now the big question, the one you’ve been trying to answer all along: How does the weight of the products compare to the weight of the reactants?" (p. 33s). The expected answer is: "The weight of the products is equal to the weight of the reactants" (p. 33t).

Another question at the end of this lesson has students "write a paragraph in your journal telling how this law [the Law of Conservation of Matter] can explain the result of the demonstration you observed today" (p. 35s). Part of the answer expected is that "In every case, the weight of the products formed was exactly equal to the weight of the reactants" (p. 35t).

Idea c: Different arrangements of atoms into groups compose all substances.

There is a content match to this idea. In the context of describing what makes one substance different from another, the text points out that "different substances are made up of different kinds of molecules" (p. 48s) and asks "How might a vinegar molecule be different from a water molecule? How might a salt molecule be different from a sugar molecule?" (p. 51s, question B). The lesson then asks students to use an alphabet analogy (that has just been presented) to answer the question: "How can all the different materials of the earth be made from only about 20 different building blocks?" (p. 53s, question 8); the idea is stated in the suggested response: "These building blocks (atoms) are put together with different numbers of pieces that are arranged differently. There are many, many possibilities, producing all the different substances on the earth" (p. 53t). Subsequent lessons (lessons 13 through 17) involve students in taking apart some models of molecules to form other molecules (in which the same atoms are joined in new combinations).

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.

There is a content match to this idea. Several lessons illustrate the concept that the number of atoms in a reaction is conserved, no matter how the atoms rearrange. In lessons 13 through 17, students use molecular models to represent the rearrangement of atoms in several chemical reactions (e.g., iron rusting [pp. 64–65s], vinegar and baking soda reacting [pp. 69–71s], butane burning [p. 75s]), then write a balanced equation for the reactions. At the end of lesson 13, the text states the idea:

Although students have several experiences building models of reactants, disassembling them to form models of products, and tracking atoms through reactions, they are not asked to weigh the reaction systems to verify the weight constancy they observed in previous lessons (nor are they asked to weigh their models). However, in one instance, students are asked to think about what would happen if the reaction were carried out on a balance:

Chemistry That Applies does not provide an evidence-based argument for ideas about mass conservation or its explanation in terms of atoms. Although it gives students several first-hand opportunities to observe mass conservation in chemical and physical changes, it does not make the argument that links these observations to the conclusion. No statements help students to link their experiences to observations and reactions beyond those in this unit. It would have been helpful to explain that "observations like these, as well as many other similar observations, led to the law of conservation of matter." Furthermore, students are not asked to reflect on why they should have confidence in the law of conservation of matter. Although the evidence in this unit is likely to convince students that the law of conservation of matter essentially holds for all chemical reactions, nothing explains–or asks students to explain–why scientists and the students themselves can have confidence in this law. It would have been helpful to include a statement such as "in the reactions we saw, when the reactants and products were contained (even the invisible reactants and products like gases), the mass did not change and mass was conserved. Yet, when not all of the matter was contained (when some of the products, like gases, were allowed to escape), a mass change was observed. These reactions and probably all others show that the law of the conservation of matter is consistent."

Similarly, students are not asked to reflect on why the idea of atoms is credible. Although Chemistry That Applies does not attempt to prove or convince the students of the existence of atoms, students may misinterpret the use of molecular models as such. A statement could alert students to the difference between what should be interpreted as proof for the existence of atoms and what is simply consistent with the existence of atoms. This could be clarified in a statement such as "while molecular models of atoms combining and recombining does not prove the existence of atoms, the idea of atoms that combine and recombine but do not disappear allows us to explain all of the reactions that we observed. This gives us confidence in the idea of atoms." Furthermore, without this type of statement, students may think that the use of molecular models convinced scientists of the existence of atoms. However, understanding the initial evidence that convinced scientist of the existence of atoms (i.e., weight ratios in chemical reactions) is probably beyond the level of sophistication that can be expected for most students in grades 8 through 10, and therefore justifiably excluded from Chemistry That Applies.

Chemistry That Applies presents the key ideas associated with the topic of the conservation of matter by providing several physical and chemical changes for students to observe and then helping them see that these are instances of the law of conservation of matter. The instances of physical and chemical changes are presented as first-hand experiences for the students. Text explanations and questions following the investigations help the students to make connections between their experiences and the key ideas. For example, after students observe that mass is conserved when calcium chloride reacts with potassium carbonate, baking soda reacts with vinegar, iron rusts, and butane burns, students read:

Once students understand that matter cannot be gained or lost, they learn how atoms and molecules can explain mass conservation. Students revisit chemical reactions that were presented in earlier lessons, but now they use molecular models to track atoms through the chemical reactions. Using models, students observe that even though the reactant "molecules" come apart and the individual atoms reassemble to form new products, the total number of each kind of "atom" remains unchanged. The text relates students’ observations to the generalization: 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. For example, after students model the decomposition of water, they read:

Chemistry That Applies makes connections among the key ideas. For example, the introduction to lesson 13: Atoms In Equals Atoms Out: Decomposing Water relates the idea that substances may combine with other substances to form new substances with different characteristic properties to the concept that the idea of atoms explains the conservation of matter:

The text associates the idea that different arrangements of atoms into groups compose all substances with the idea that matter is conserved in this way:

Similarly, the text makes an explicit connection between the conservation of matter and the idea that atoms explain the conservation of atoms (pp. 61–62s).

Several important prerequisite ideas are presented clearly and are related to the key ideas that serve as the basis for this analysis. For example, the material proposes the prerequisite idea that air is a substance and connects it to the idea that no matter how substances within a closed system interact with one another, or how they combine or break apart, the total mass of the system remains the same. Students first observe that if a system is not closed during a reaction in which a gas is produced, then the weight changes, but if the system is kept closed, the weight does not change (p. 32s). Then they observe that when iron rusts in a container covered with a balloon, the balloon deflates (pp. 36–37s). Then the prerequisite idea is presented to account for the changes observed. Students are given the opportunity first to note that air has properties consistent with it being a substance (e.g., a balloon over an inverted funnel inflates when the funnel is pressed down into water) before the prerequisite idea is introduced:

In the next lesson, the prerequisite idea that air is a substance is related to the fact that air (or one of the gases in air) can be a reactant:

A few connections are made to ideas outside the focus of the conservation of matter. For example, one important connection is made to the use of models in science. The text states:

Chemistry That Applies also makes connections between the idea of matter conservation and real-world events. For instance, students are asked to speculate about matter conservation on a much larger scale:

Furthermore, questions at the end of lesson 17 also emphasize worldly issues of matter conservation. For example, students are asked:

This unit takes particular care not to include topics that are beyond those needed for science literacy, as recommended by Benchmarks for Science Literacy (American Association for the Advancement of Science, 1993) and the National Science Education Standards (National Research Council, 1996). Even many common topics typically presented alongside mass conservation, such as the details of atomic structure and the mole concept, are noticeably absent in this unit.

In its attempt to help students focus on the key ideas, Chemistry That Applies occasionally oversimplifies details, presents inaccurate statements, and contains faulty logic. For example, the text uses general numbers, such as "billions," rather than a more accurate number for the number of molecules in a few grams of steel wool, and indicates incorrectly that steel wool is made of "iron molecules" (p. 66s). The text also oversimplifies its presentation of the Law of Conservation of Matter by ignoring the possibility that matter can be converted to energy (pp. 34–35s). And, in its comments about our inability to see atoms and molecules "even under the most powerful microscope," it ignores the fact that tunneling and atomic force microscopes do enable molecules to be seen (p. 53s). It should be noticed that these two instances are accompanied by notes in the Teacher’s Guide that provide the correct information (pp. 35t, 53t). However, for examples such as these, it is recognized that simplifying details that either require a more sophisticated explanation (such as how some matter can be converted to energy, and why iron is the only component of steel that reacts with oxygen) or may derail students from learning the generalization of conservation (such as what the other components of steel are, or the differences between types of microscopes) is a useful technique. Yet it probably would be more helpful to students if either they were alerted to such oversimplifications or the text corrected them at an appropriate time.

On other occasions, the Teacher’s Guide makes statements that are inaccurate. For example, in attempting to alert teachers to the common student misconception that the macroscopic properties of a substance can be attributed to its molecules (e.g., the molecules of a solid are hard, whereas the molecules of a gas can be compressed), the Teacher’s Guide uses smell as an example (p. 51t). In fact, smell is a molecular phenomenon. It takes very few molecules of vinegar to be detected by scent. Although this misleading statement is not likely to interfere with students learning about matter conservation, it could interfere with their general understanding of molecules.

In a few instances, Chemistry That Applies is illogical. For example, the text asks students if certain changes are chemical or physical (p. 24s, question 5) when there is not enough information for them to make such a decision. In another instance, the text provides a false premise by explaining that studying weight changes will help students be able to determine the difference between physical and chemical changes: "[T]o get a deeper understanding of chemical reactions and the making of new substances, you are going to consider another question—how the weight of materials changes as they change in all these different ways" (p. 16s). In one case, the text relies on a greater understanding of chemistry than is typical for middle grades and early high school students. In lesson 10, students are expected to appreciate that burning steel wool is analogous to rusting (p. 40s). Lastly, this material relies on students’ ability to weigh reactants and products with high precision on a single pan in order to observe evidence for matter conservation. However, instead of attributing minor weight differences to scale inaccuracies, their own lack of skill with laboratory equipment, or other variations in the conditions, students are likely to interpret differences as counter-evidence for matter conservation. An alternative would have been to discuss precision of measurement briefly with students or to use a double pan balance, where only the differences between the weights of open versus closed systems would be noted.