UNIT 2 The Particulate Nature of Matter Take a moment to think about all of the different properties which can be exhibited by matter. The list seems endless. Matter can be found in a variety of forms, including everything from people to plants to rocks to water to air, and so on. The number of different properties associated with these forms of matter is even greater. The universe is filled with a seemingly unlimited amount of diversity. And yet, we have already found that it is possible to classify all of the different forms of matter which we normally observe into just three categories or phases: solids, liquids, and gases. In Unit 2 we shall go a step further. We shall consider whether or not there is one single scientific model which can both describe all three of the phases of matter, and which can also explain the variety of properties which are exhibited by these phases. Such a model, if it could be found, would be very powerful. Not surprisingly, the search for this model has intrigued scientists for thousands of years.
PARTICLES (A Model for Everything) Is there a single scientific model for matter which can explain the great diversity of matter which you see around you? You are probably somewhat familiar with one model which was proposed in ancient Greece. Aristotle (and others) argued that the properties of matter could be explained by assuming that all matter was made up of varying amounts of just five basic elements: earth, water, fire, wind, and quintessence. Earth and water were said to have the property of "gravity" because they naturally fall to the ground when you let go of them. Fire and wind were said to have "levity" because they tend to go up instead of down. The sun, moon, and stars neither went up nor fell down, but seemed to move endlessly in perfect circles. Thus, they were assumed to be made wholly from the fifth, perfect element, quintessence. Based on this model of matter, if something fell to the ground when it was released (as a rock does), then it was concluded that the object was made mostly out of earth and water. On the other hand, birds were thought to be made at least partly out of fire and wind, since they had the ability to fly. Paper was said to be made out of a combination of earth and fire, hence it was fairly light. When paper burned, the fire was released, and all that was left was the ashes (earth). Thus, as must be true for all scientific models, this model had the ability to explain, at least partly, the observed properties of matter. Another model for matter grew out of the religion and philosophy of Taoism. The adherents of this religion desired to explain everything in terms of creation and annihilation cycles. Based on this, they postulated that the five basic elements for all of matter were wood, dirt, fire, water, and metal. The creation cycle is shown below, and the explanation was as follows: Water creates wood, because as you water a tree it grows and produces wood. Wood creates fire when it burns. Fire creates dirt, because after wood is burned by fire the ashes (dirt) is what is left. Dirt must create metal, because metal is found by mining for it in the ground. And finally, metal can create water, because metal left outside overnight will be found to have condensation (water) on it in the morning. wood water metal fire dirt The annihilation cycle relied on equally imaginative reasoning and is shown below. metal wood dirt fire water Just for fun, you might want to try to come up with an explanation for this cycle. (A hint to get you started: They said that water annihilated fire because water extinguishes fire when it is poured onto it.) In terms of its ability to explain or predict the properties of matter, this model was about as good as the Greek model discussed earlier. There is yet another model of matter that comes to us from antiquity. Democritus of ancient Greece proposed that all matter consisted of tiny particles (called atoms). There were a finite number of different kinds of atoms, and the properties of a substance were said to depend on what kinds of atoms made up the substance. Unfortunately for Democritus, this model seemed ridiculous to almost everyone else at that time, including Aristotle. And no wonder! How could a solid object (like a table) hold together if it were made out of little particles? And certainly liquids, such as water, did not look as if they were made out of anything solid. And if air were made out of particles, wouldn't these particles just fall to the ground like sand? Alas, the atomic model of matter died a quick death. UNIT 2 PARTICLES II-1
Of course, today the atomic (particulate) model of matter is considered to be the best theory we have for matter. By making a few adjustments to Democritus' model, we obtain a theory which can give explanations for the properties of all the matter around us. This model has become an indispensable way for scientists to look at the world. Before a more complete description of the particulate nature of matter (i.e. the model that says everything is made out of little particles called atoms) is developed here, a few observations about elementary school students and their understanding of the model should be made. Elementary Students and Atoms In general, students today do not find the atomic model of matter any more reasonable to accept than did people in Democritus' time. This is so for several reasons. First, individual atoms are too small to be seen or felt. Yet, this concept is introduced while students are still concrete thinkers. Thus, students are asked to imagine things of which they have no sensory experience. Second, there is no activity which you can do with students which will prove that atoms exist. At best, you can only infer that they might exist. (See the Brownian Motion Activity below.) And finally, the most obvious properties of many substances, such as liquids, appear to be most easily explained by not using a particle model. Despite all this, it is still being recommended that a simplified model of the particulate nature of matter be introduced and utilized in the later elementary grades due to its universal applicability in physical science. This is especially true as students learn about scientific models, for the atomic model of matter is the best way to explain a wide diversity of physical phenomena they will experience. The important thing to remember is that the atomic model should be introduced only in order to explain other phenomena. It is not worth studying if you simply want to memorize the names of the various parts of an atom and how they combine to form molecules. Brownian Motion Atoms and molecules are too small to be seen with optical microscopes. However, it is possible to infer the presence of atoms and molecules by observing their effects on larger objects. In this activity you will observe the small, jerky motions (Brownian Motion) of droplets of butterfat in milk. The jerky motions are caused by collisions between molecules in the milk and the butterfat droplets. As you make your observations, keep in mind that you will not see individual atoms or molecules moving. Instead, you must infer their presence based on your observations. PROCEDURE A) Use 1% milk, or dilute a small quantity of 2% milk by one-half. Put one drop of the 1% milk on a microscope slide, and cover it with a cover slip. B) View the liquid at a magnification of about 400X using maximum contrast. It might be advantageous to have the room lights off.) C) Look for the random "jiggling" of the butterfat droplets. Do the larger droplets move more, or less, than the smaller droplets? Is this as expected? The observation of Brownian motion is considered historically important in the affirmation of the existence of molecules and their motion. Most elementary students do not see such an obvious connection. Many students suspect that the observed motion results from the movement of live creatures. UNIT 2 PARTICULATE NATURE OF MATTER II-2
Basic Particle Model Content Overview Below is a summary of the main components of a particulate model for matter. As this unit continues, some of these ideas will be extended. The summary below is meant as a starting point for further investigation. 1. All matter is made up of tiny indestructible particles called atoms. There is nothing else contained within matter except for these atoms. (Atoms themselves are composed of particles called protons, neutrons, and electrons, but these are usually bound tightly together in the atoms. The elementary school teacher should probably know what these terms refer to and also what is meant by the term nucleus [the center of the atom containing the protons and neutrons], but these words should be of little or no concern to the lower elementary student) 2. Atoms are very small. They can not be seen, even if the most powerful optical microscope in the world is used. More than a trillion ( 1,000,000,000,000 ) atoms could fit on top of the period at the end of this sentence without any of them overlapping. If atoms were lined up end to end it would take over a million of them to reach a length of 1 mm. 3. There are about 100 different kinds of atoms. 4. Two or more atoms can be bound together in a semipermanent way to form a Molecule. Most substances we encounter in everyday life are made of molecules, although a few substances (helium, for example) are composed of individual atoms. There are millions of different molecules that can be formed from atoms. When the bonds between atoms are created or destroyed to make a new molecule, we say that a chemical reaction has taken place. 5. Atoms and molecules, whether they are making up a solid, liquid, or a gas, are always moving. 6. The atoms in solids and liquids are so close together that they are effectively touching each other. (See diagram below.) The positions where the atoms are not touching is empty space. The atoms making up a solid can not move around too far. They simply remain in one location and vibrate back and forth. The atoms or molecules in a liquid are free to move around and change positions. 7. The particles making up a gas are far apart, in general, but they are constantly colliding with each other as they move around. II-3
Here are simple models for solids, liquids, and gases that can involve students in a classroom: Solids: The students stand together (say, within one arm s length away from each other rather than actually touching) and imagine that one foot is glued to the floor. The students can wiggle and jiggle, but that one foot must not move. It is clear from this model why a solid retains its shape. Liquids: The individual students are now free to move about as well as wiggle and jiggle, provided they stay within one arm s length of other students. Note that the shape of the group may change as the students move about, just as the shape of a liquid can deform. (Although the particles collide with each other in a real liquid, some constraints obviously need to be placed on the students). Gases: In this phase the individual students are free to move about anywhere within the classroom. (They will quickly enough fill the entire container! ). II-4