AAAS Project 2061 Middle Grades Science Textbooks Evaluation

Helping Students Learn the Kinetic Molecular Theory:
What Should Textbooks Do?

The following examples illustrate some of the features that textbooks should include to help students learn important ideas about the kinetic molecular theory. We have distilled these from five of the criteria used in the Project 2061 Curriculum-Materials Evaluation Procedure. Although each feature was examined separately, they work together to tell a story about the textbook. Like any story, this one is effective only if all its elements are in place.

Textbooks should alert teachers to commonly held student ideas.

Of the materials examined, only Matter and Molecules was rated excellent on this criterion. This stand-alone unit lists all relevant misconceptions that are reported in student learning research and explains each one. For example, the teacher's guide provides the following information about what many students believe about the motion of molecules:

Students also have difficulty in understanding that molecules are constantly moving. Molecules are always moving, even in substances such as ice where no motion of the substance is visible. Many students think that molecules are moving in liquid water because liquid water is flowing, but molecules are not moving in ice because ice is not moving. The constant motion of molecules is difficult for students to believe, both because it seems to contradict the evidence of their senses and because they have never encountered objects that, like molecules, are so tiny that they are unaffected by friction and thus never come to a stop.

The paragraph states the misconception, explains it, explains why students believe it, and describes what has to change for them to find the scientific idea that molecules are perpetually in motion credible.

In contrast, while a few textbooks have notes in the teacher's guide labeled "Misconceptions" or "Prior Knowledge," the statements included are not very helpful. The following were the only instances in which the textbooks attempted to alert teachers to commonly held student ideas about the kinetic molecular theory:

Students may confuse the terms atoms and elements.
Macmillan McGraw Hill Science, Unit 32, p. 46a

Students may identify matter as only things that they can see.
Science Insights: Exploring Matter and Energy, p. 135

These statements provide little information to teachers about their students' ideas about the kinetic molecular theory. The first statement dwells on the meaning of terms and the second, merely states the misconception without explaining it. And none of the other published student misconceptions (such as the idea that matter is continuous or the idea that molecules are in substances) are even mentioned. If teachers (or perhaps even the textbook developers themselves) are unaware of common student misconceptions, then it will be hard to plan effective instruction.

Textbooks should provide a variety of phenomena.

A phenomenon is an event that can be scientifically described. Much of the point of science is explaining phenomena in terms of a small number of principles or ideas. For students to appreciate this explanatory power, they need to have a sense of the range of phenomena that science can explain.Matter and Molecules and three of the textbooks evaluated provide a satisfactory variety of physical science phenomena related to the kinetic molecular theory. For example, to make the idea plausible that "atoms and molecules are perpetually in motion," Matter and Molecules and the three textbooks show students that:

  • When air is compressed in a syringe, it pushes back on the plunger
  • Gases (like perfume) spread out evenly in a room or container
  • Liquids (like food coloring or tea) spread out evenly in a glass, rather than falling to the bottom
Unfortunately, none of the materials provides phenomena to help make plausible the (less believable) idea that the particles of a solid are in continuous motion. For example, while it takes considerably longer to observe, solids like gold and lead can mix a bit.

But experiences with phenomena are not enough! Students need help to understand and appreciate how the phenomena relate to the scientific ideas.

Textbooks should guide student interpretation and reasoning.

Of the materials examined, only Matter and Molecules provides excellent guidance to help students interpret and reason about the phenomena.

For example, after students see that air can be compressed in a plastic syringe and read about the large amount of air that can be compressed in a small scuba tank, questions guide student thinking about the phenomena. The question below probes the common student misconception that molecules are not perpetually in motion:

My friend says there is more air near the valve of the bike tire where the air was pumped in. Do you agree with him? Explain why or why not.

In another lesson, students observe that even though sugar grains appear (with magnifying glass) too big to get through a presweetened tea bag, the sweet taste does indeed "get out." They try to explain how the sugar might have gotten out and then read the text explanation:

Did you say that you could see wavy lines under the tea bag and taste the sugar in the water? That is true. We cannot see the tiny molecules of sugar or the tiny molecules of water; but we can taste the sugar in the water. The sugar did not disappear forever, but the sugar grains broke into separate, tiny molecules, so that we could no longer see the sugar. Just because we cannot see the sugar does not mean it is not there. The water tastes sweet, so it must still be there.

How did the sugar get out of the tea bag? The holes in the tea bag are much smaller than a grain of sugar, but much larger than a molecule of sugar. As the water molecules hit the solid sugar, the molecules of sugar break away rapidly and mix with the water molecules. The tiny molecules easily pass through the holes in the tea bag.

Finally, students are asked a question that anticipates a common misconception (that molecules are not perpetually in motion but only move if the substance appears to move):

If you let this cup stand overnight, would the sugar rise to the top, settle to the bottom, or spread evenly throughout the water? Talk about molecules to explain your answer.

None of the other materials have questions with these characteristics and hence were not rated very good or excellent on this criterion.

Once an idea begins to take hold, students need many and varied opportunities to apply it.

Textbooks should provide practice in using scientific ideas.

Matter and Molecules scores higher than all textbooks examined in providing a sufficient number and variety of practice tasks for most of the physical science ideas examined. These include novel tasks that ask students to develop descriptions and explanations of phenomena they see all around them. Since explaining real world phenomena often requires using more than a single idea, the phenomena students are asked to explain increase in complexity. For example, the following questions (taken from several places in the unit) have students move from using mainly the idea that "molecules are in perpetual motion" to using this and related ideas:

  • Draw pictures to show how water molecules are moving.
  • Can water molecules in ice slow down and stop?
  • If you want something to dissolve fast, should you mix it with hot water or cold water? Why?
  • Explain how you can smell an open bottle of vinegar even though you are across the room. What is actually reaching your nose? How did the vinegar molecules get into the air? How did the vinegar molecules reach your nose?
  • When food covered with plastic wrap in the refrigerator (or when soup is warming on the stove, but not boiling, with a lid on the pot), water evaporates and then condenses. Where does the water evaporate from? Where does the water condense? How do the water molecules get from the place where water evaporates to the place where water condenses?

In contrast, other textbooks include few, if any, relevant practice tasks. Those that are included are usually like the one below.

As the temperature of a material increases, the average _______ of its particles increases. (a) kinetic energy (b) potential energy (c) specific heat (d) mass
Glencoe Physical Science, p. 148

Since a similar statement appeared in the text (p. 134-135), filling in the blank requires little more than copying from the text. The only textbook to include several decent practice tasks did not do so consistently across the set of ideas used as the basis for the physical science analysis.

But what good is practice if students don't understand what they are to practice? (Imagine being asked to practice the breast stroke in swimming if you've never been shown what it looks like done well)! So, rather than expecting students to figure out what a good explanation should be like, the Project 2061 procedure includes a criterion to probe whether a material demonstrates for students how to use scientific ideas.

Textbooks should demonstrate the use of knowledge.

Only Matter and Molecules receives an excellent rating on this criterion. The unit first shows students how to explain phenomena and then coaches them through a few explanations before turning them loose to explain how the world works. It (a) announces it is beginning the demonstration, (b) provides a step-by-step demonstration, and (c) provides criteria for judging its quality.

How does evaporation happen? Let's try explaining it in terms of molecules. You know that the molecules in liquid water are constantly moving. In a liquid, though, the attractive forces between molecules keep them close together. What you might not know is that the molecules in a liquid move at different speeds. Some molecules are moving very fast, while other molecules are moving more slowly.

What do you think would happen if a fast-moving molecule reached the surface of a drop of water? Yes, it would escape! It would break away from the strong attraction of the other water molecules and become a molecule of water vapor in the air. If all the water molecules escape in this way, we say that something has "dried out." The liquid water has turned into water vapor in the air, and the water vapor makes the air more humid.

Earlier in the unit, students read about the two characteristics of a good explanation (and the teacher posts them as criteria to use when judging their own explanations and those of others):

You have also learned something about scientific explanations. To make a good explanation, you often need to talk about molecules. You need to talk about the way molecules move and the way they are arranged in solids, liquids, and gases. You also need to know what kind of molecules you are talking about. You need to identify the substance that is changing and tell how it is changing.

In other words, a good explanation answers at least two questions:
A question about substances: What substance is changing and how is it changing?
A question about molecules: What is happening to the molecules of the substance?

When students first begin to practice explaining phenomena, they are reminded of the criteria and asked whether their explanations meet them. As students proceed through the unit, they are reminded less often until they are explaining on their own. No other material was rated even satisfactory on this criterion.


Continued: Ratings of Instructional Quality