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Middle Grades Science Textbooks: A Benchmarks-Based Evaluation

Matter and Molecules (stand-alone unit). Michigan State University, 1988
Earth Science Life Science Physical Science

1.
 About this Evaluation Report
2.
 Content Analysis
3.
 Instructional Analysis
[Explanation] This section examines whether the curriculum material's content aligns with the specific key ideas that have been selected for use in the analysis.
[Explanation] This section examines whether the curriculum material develops an evidence-based argument in support of the key ideas, including whether the case presented is valid, comprehensible, and convincing.
[Explanation] This section examines whether the curriculum material makes connections (1) among the key ideas, (2) between the key ideas and their prerequisites, and (3) between the key ideas and other, related ideas.
[Explanation] This section notes whether the curriculum material presents any information that is more advanced than the set of key ideas, looking particularly at whether the “beyond literacy” information interrupts the presentation of the grade-appropriate information.
[Explanation] This section notes whether the curriculum material presents any information that contains errors, misleading statements, or statements that may reinforce commonly held student misconceptions.

Alignment

Idea a: All matter is made up of particles called atoms and molecules (as opposed to being continuous or just including particles).
There is a content match. The idea that matter is made up of particles is an explicit learning goal of Matter and Molecules (Science Book, Goal Conceptions, pp. T-14, T-27, T-40; Lesson Objectives, pp. T-11, T-26) and is presented in different contexts and in several locations in the student and teacher materials. After students describe ice, liquid water, and water vapor as states of the same substance, molecules are introduced as the tiniest parts of water (Science Book, pp. 2-7s). Students learn that all three states of water are the same in that they are made of water molecules that are moving constantly. A representation of molecules viewed through magic eyeglasses is shown to give students a concrete picture of what the molecules might look like if they could be seen (Science Book, p. 5s). Next, students are introduced to the idea that other substances (such as lead, sugar, alcohol, and air) are made of molecules (Science Book, pp. 13-20s). The student text explains that different substances are made of different kinds of molecules (Science Book, pp. 13–16s) and describes pure substances as being made of a single kind of molecule, with mixtures consisting of two or more different kinds of molecules (Science Book, pp. 17–18s). The generalization that all matter is made of molecules is advanced and contrasted to things that are not matter, and therefore are not made of molecules (such as forms of energy) (Science Book, p. 15s). Magic eyeglasses are used to reinforce the molecular composition of all substances (Science Book, pp. 15s, T-30, T-32, T-35). The remaining lesson clusters make use of the idea of molecular composition, along with the ideas of molecular arrangement, motion, and interaction, to explain a variety of physical phenomena, such as the compression of gases, dissolving, thermal expansion, and changes of state.

In most cases, the small, seemingly invisible pieces of matter are labeled as molecules." In Lesson Cluster 2, atoms as components of molecules are introduced briefly to convey the idea that different arrangements of atoms into molecules compose different substances (Science Book, p. 14s). Unfortunately, no statements contrast the idea that all matter is made of particles with the naive idea that matter is continuous. However, the idea that matter is made of molecules is contrasted with the naive idea that matter just includes molecules, and the fact that there is empty space between a substance’s molecules is stressed repeatedly.

Idea b: These particles are extremely small—far too small to see directly through a microscope.

There is a content match. This idea is a specific learning goal of Matter and Molecules (Science Book, Goal Conceptions, pp. T-14, T-27, T-40). The extremely small size of molecules is discussed in the student text. Furthermore, this idea is seen in the context of questions posed to students in the Activity Book and in the notes and suggestions in the Teacher’s Guide.

In Lesson Cluster 1, the incredibly small size of molecules is explained in statements such as "every drop of liquid water—and every sliver of ice—is made of trillions of water molecules…" and "In fact, a typical cell in your body might be made of 100 trillion (100,000,000,000,000) molecules" (Science Book, p. 6s). Although the text explains that molecules are much smaller than things such as specks of dust and things that can be seen only with a microscope, such as cells and germs, the student text does not state explicitly that molecules cannot be seen with a microscope.

The small size of molecules is mentioned periodically throughout the rest of the unit in both the student text and in the Teacher’s Guide. For example, Lesson Cluster 2 explains that "Even the largest molecules, though, are far too small to see" (Science Book, p. 14s). When students are asked whether one can tell a pure substance from a mixture by looking at the molecules with a magnifier, the suggested response is: "You could tell the difference between pure substances and mixtures if you could see molecules with a magnifier. But you cannot see molecules with a magnifier" (Activity Book, p. T-9, question 2). Likewise, when students are asked about seeing "dust molecules in the air" the suggested response is that "…a speck of dust is huge compared to a molecule… A molecule is so tiny you cannot see it even with a microscope" (Activity Book, p.11s, T-11). Also, in the context of sugar dissolving in water, students are asked whether the holes in a tea bag are big enough for molecules of sugar to get through them. The teacher notes suggest: "You might point out here that molecules are much smaller than grains. Grains of sugar are made of trillions of molecules" (Activity Book, p. T-25).

Idea c: Atoms and molecules are perpetually in motion.

There is a content match. The idea that all molecules are moving constantly is one of the learning goals of Matter and Molecules (Science Book, Goal Conceptions, pp. T-14, T-27, T-40, T-48, T-61, T-69; Lesson Objectives, p. T-47). In Lesson Cluster 1, the idea is introduced that water molecules in solid, liquid, and gaseous states are always moving and that they never stop (Science Book, pp. 7–8s). In Lesson Cluster 2, the different kinds of motions of molecules in the three phases (solid, liquid, and gas) of all substances are presented. In Lesson Clusters 3 through 5, students use the idea of molecular motion to explain a variety of phenomena, such as the dissolving of sugar in water, the diffusion of gases (such as perfume, ammonia, and other smells), and the tendency of gases to occupy all of the space available to them (e.g., when the valve in a scuba tank is opened, the air rushes out; one can hear it making a rushing noise). Furthermore, in several instances, the Teacher’s Guide suggests that teachers emphasize the idea that molecules are moving constantly. For example, when the motion of molecules in the three states of water is introduced in the student text, teachers are reminded to explain that “molecules are constantly moving and never stop, even in a solid" (Science Book, p. T-21).

Idea d: Increased temperature means greater molecular motion, so most substances expand when heated.

There is a content match. The ideas that "molecules of hot substances move faster" and that "increased motion moves molecules farther apart" are explicit learning goals of Matter and Molecules (Science Book, Goal Conceptions, pp. T-69, T-80, T-100). Lesson Cluster 6 explains and represents the idea that "heating any substance makes the molecules of that substance move faster" (Science Book, p. 39s). In the first activity of lesson 6.1, students are to predict, observe, and then explain (in terms of molecules) why hard candy dissolves faster in hot water than in cold water. Then, the student text proposes the idea that when molecules move faster, they move farther apart and has the students explain various solid, liquid, and gas expansion phenomena in terms of molecules. In Lesson Clusters 7 through 9, the text uses the idea that "molecules of a substance move faster when the substance is heated, and slower when it is cooled" (Science Book, Key Elements of a Good Explanation, item 1, p. T-78) to explain changes of state. The student text does not relate temperature to the greater average energy of motion.

Idea e: There are differences in the arrangement and motion of atoms and molecules in solids, liquids, and gases. In solids, particles (1) are packed closely, (2) are (often) arranged regularly, (3) vibrate in all directions, (4) attract and “stick to” one another. In liquids, particles (1) are packed closely, (2) are not arranged regularly, (3) can slide past one another, (4) attract and are connected loosely to one another. In gases, particles (1) are far apart, (2) are arranged randomly, (3) spread evenly through the spaces they occupy, (4) move in all directions, (5) are free of one another, except during collisions.

There is a partial content match. The following presentation of Idea e show which parts of the idea are treated (in bold) and what alternative vocabulary, if any, is used (in brackets): There are differences in the arrangement and motion of atoms and molecules in solids, liquids, and gases. In solids, particles (1) are packed closely, (2) are (often) arranged regularly, (3) vibrate in all directions, (4) attract and "stick to" one another. In liquids, particles (1) are packed closely, (2) are not arranged regularly, (3) can slide past one another, (4) attract and are connected loosely to one another. In gases, particles (1) are far apart, (2) are arranged randomly, (3) spread evenly through the spaces they occupy, (4) move in all directions, (5) are free of one another, except during collisions.

The different arrangement and motion of molecules in the solid, liquid, and gaseous states are explicit learning goals of Matter and Molecules (Science Book, Goal Conceptions, pp. T-14, T-27; Lesson Objectives, pp. T-11, T-26). The idea that "molecules of one substance attract each other" is also a learning goal (Science Book, Goal Conceptions, pp. T-80, T-100), but how the attraction differs in the three states is not a learning goal.

Lesson Cluster 1 describes and represents with diagrams the different arrangement and motion of water molecules in the solid, liquid, and gaseous states. Lesson Cluster 2 generalizes this difference in the solid, liquid, and gaseous states to all other substances. Although many diagrams show the molecules of solids, liquids, and gases moving in all directions, no statement in the text explains this point. Furthermore, the text says that the different arrangement and motion of molecules can explain the different properties of the three states, but it does not explain any specific properties.

In Lesson Cluster 4, several activities target the idea that air molecules are far apart. For example, students observe differences in compressibility between air and water and explain these differences in terms of the differences in the proximity of their molecules. This cluster also introduces the idea that air molecules (e.g., in a syringe or a bicycle tire) spread evenly through the spaces they occupy (rather than "bunch up together in one place more than another place" [Science Book, p. 33s]). The student text describes the concept of forces of attraction between water molecules to explain how liquid water freezes, and, later, how liquid water evaporates and water vapor condenses. However, the student text does not state precisely that there are forces of attraction among the molecules of all substances (even though students are expected to use this idea as they explain how other substances solidify and evaporate). Also it is never said specifically that molecules of gases are free of one another, except during collisions. Although, occasionally, gas molecules are described as moving "freely in space," this phrase is not explained.

Idea f: Changes of state—melting, freezing, evaporating, condensing—can be explained in terms of changes in the arrangement, interaction, and motion of atoms and molecules.

There is a content match. Explanations of changes of state—melting, freezing, evaporating, and condensing—in terms of changes in the arrangement, interaction, and motion of molecules are explicit learning goals of Matter and Molecules (Science Book, Goal Conceptions, pp. T-61, T-80, T-90, T-100; Lesson Objectives, pp. T-59, T-78, T-87, T-97). The student text includes several step-by-step explanations of the melting, freezing, evaporation, boiling, and condensation of water in terms of changes in the arrangement, interaction, and motion of its molecules. In addition, students are asked to explain a large number and variety of phenomena involving changes of state, such as the melting and solidifying of various kitchen substances, the evaporation of alcohol, solar stills, and the water cycle.

Building a Case

Matter and Molecules does not provide a historic account of the development of the kinetic molecular theory, nor does it involve students in experiences with evidence in order to decide whether to accept the key ideas. Although, there are many phenomena (the compressibility of gases; the dissolution of sugar in water; the expansion and contraction of solids, liquids, and gases; and changes of state) that can be explained by the key ideas, students are not alerted to the fruitfulness of the kinetic molecular theory in explaining them.


Coherence

There is a logical sequence of encounters with the key ideas in Matter and Molecules. First, the ideas are introduced that all matter is made of small, constantly moving molecules and that the different states of a substance are due to the different arrangement and motion of its molecules. Then these ideas are used to explain increasingly complex phenomena, such as the compressibility of gases, dissolving, thermal expansion and contraction, and changes of state. Additional ideas are brought in as needed to explain the more complex phenomena. For example, the idea that increased temperature means increased molecular motion is presented to explain the thermal expansion and contraction of substances; the idea that there are forces of attraction between molecules is introduced to explain changes of state. Later in the unit, practice tasks involve students in using several ideas (the different molecular arrangement and motion in the three states, the relation between temperature and molecular motion, and the idea of forces between molecules) which could be used to illustrate how the ideas fit together to explain increasingly complex phenomena.

Matter and Molecules concentrates on explaining physical phenomena with the kinetic molecular theory. Particularly, it explicitly develops the connection between the kinetic molecular theory and the water cycle. In the last paragraph of the unit, connections that can be made to ideas and phenomena in other areas are alluded to, but such connections are not made:

Even if you have learned a lot about molecules, there is still much more to learn. We can use ideas about molecules to explain what happens inside our bodies when we breathe, for example, or how we grow, or what happens when things burn or decay. We cannot explain those things in this unit, but we hope that this unit will prepare you to learn and understand much more in the future. (Science Book, p. 69s)

Beyond Literacy

All of the readings in the Science Book, the activities in the Activity Book, and the notes to teachers are relevant to learning goals appropriate for middle grades students. The unit does not include material that is not required for science literacy (American Association for the Advancement of Science, 1993; National Research Council, 1996).

Accuracy

The content of the unit is accurate, with one minor exception. The text accounts for solid expansion by the statement: "Heating the ball made the molecules of the metal vibrate faster, so they pushed each other farther apart" (Science Book, p. 42s). Although this is not technically correct, the correct explanation is too sophisticated for middle grades students. (It is the lack of symmetry of the potential energy function that accounts for the thermal expansion of solids.)

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Copyright 2002 by the American Association for the Advancement of Science. All rights reserved.