The Child's Ideas for 1. Water, a Liquid

What makes the particulate model of matter so challenging for students?

At first blush, the claims made about the wild activity going on at a nanoscopic scale might seem like an unbelievable “fairy tale.”

The particulate model of matter is one of the central ideas in modern science, as well as in middle- and high-school curriculum. Thinking about materials in particulate (as well as atomic-molecular) terms is critically important in all science disciplines—not only physics and chemistry, but also Earth science and astronomy, genetics and molecular biology, and neuroscience. Students cannot achieve science literacy without understanding this profound idea.

Yet, research has shown that students not only have great difficulty believing this idea to be true—they find it counter—intuitive that all matter is made of discrete, spaced particles in constant motion; they also have great difficulty even understanding many of its key aspects. For example:

  • Students often do not appreciate the tiny size and scale of the particles, instead confusing them with dust particles or with germs or cells that can be seen with a simple microscope.
  • Students often do not recognize that the discrete particles are the matter itself; instead, when learning about particles of water (or some other material), they may think they are little pieces of stuff floating in the water, rather than the water (or other material) itself.
  • Students often do not think of particles of specific materials (e.g., water and iron) as pre-existing units that have a fixed size, shape, and weight even across physical changes; instead they may think these particles are created when something is broken into little bits. Therefore their sizes, weights, and shapes can be quite variable and changeable. Further, they may think these tiny particles, when small enough, no longer take up space or have weight at all.
  • Students often do not realize that properties at the particulate level can dramatically differ from those they observe in everyday objects; instead, they expect that the particles to be like what they observe. Thus, if something is solid or liquid, red or green, hard or soft, then it must be made of particles that are themselves solid or liquid, red or green, and hard or soft.

Why do students have all these difficulties? One reason is that the nature of materials on an atomic scale is so profoundly different from how materials appear macroscopically. Tables appear to be made of continuous, unmoving stuff. How can they be made of constantly moving particles separated by empty space? Ice, water, and water vapor are each different. How can they all be composed of the same kind of particles? At first blush, the claims made about the wild activity going on at a nanoscopic scale might seem like an unbelievable “fairy tale.”

Another reason is that children harbor ideas based on interactions with large-scale objects that are fundamentally incompatible with the particulate theory. They trust their senses to reveal what things are like. If they see it, it is there; if it feels heavy, it has weight. Material objects you see, feel, and touch are fundamentally different from “ethereal” things like air, heat, light, or thoughts. Indeed, they often group “air” with “heat and light” as examples of “nonstuff.”

Imagine then the puzzlement children experience when told of the existence of particles of a material that are too small to see, feel, or touch. Certainly these can't be the material itself, but rather some ethereal thing or impurity “in” the stuff. Children think that even small pieces of clay are weightless because they have no “felt weight,” so how can they possibly believe that much tinier particles have any weight? Thus, coming to accept these tiny invisible particles as the constituents of matter calls for profound changes in their concepts of matter and weight.

Still another (related) reason for children's difficulties is that the ways scientists learn how the world works are quite different from children's everyday sense-making. For example, in trying to figure out what materials are like, scientists carefully measure what changes and stays the same when things melt and dissolve. They use their imaginations to explain what they see and then evaluate how well their ideas account for results and make accurate predictions.

Although children certainly can be astute observers with wonderful imaginations, they typically do not apply their imaginations and observations for scientific model building and testing. Thus, understanding the particulate model of matter calls for more sophisticated sense-making in children. Further, it calls for a shift from thinking of models as little replicas that should look like the thing modeled, to thinking of models as conjectures of what something may be like at a scale too small to see that can explain observed results.

Other reasons for children's difficulties relate to how the particulate model is taught in schools, including: a) introducing the idea too soon before foundational understandings are in place; b) introducing too many details at once rather than letting students gradually add complexity to the idea; c) presenting the ideas as “facts” rather than as explanatory conjectures; and d) failing to discuss obvious objections to these ideas by helping students answer fundamental questions like:

  • How can something that is invisible be matter?
  • How can the same stuff be hard, runny, or invisible?
  • How can something be a good model if it doesn't look like what you see?

Prior research shows that the best way to help students develop new ideas is to be respectful of them as thinkers and learners, build from their initial ideas, and appeal to their desire to make sense of how things work. We need to encourage them to make drawings of their ideas about unseen events so they can grasp the implications of their ideas, as well as discuss and contrast their ideas with those of others. We need to give them reason to believe new ideas by introducing them to observations that are puzzling given their initial ideas — such as why the weight of an ice cube remains the same when melted — but that can be explained in terms of the particulate model. We need to help them imagine the atomic scale when thinking about the smallest particle of a material and to allow them to reflect and discuss initial objections to the model. Finally we need to acknowledge that even when we have given them reasons to believe, we haven't “proved” the theory nor have students fully understood it. Their understanding is still a work in progress.

In doing all these things, however, we will not only have helped students to begin to understand elements of the particulate model of matter, but also (equally importantly) helped them begin to understand the power of the "what if" game that is at the heart of all of science.

—Carol L. Smith