J. Josˇ Bonner
Matter and mass
The terms, "matter" and "mass" are new to students at first. They are confusing, perhaps because they are synonyms for words they already know (like "stuff" and "weight"). Confusion may also relate to the perception that solids, liquids, and gases must be fundamentally different. If I perceive these to be different, how can I call them all "matter"? To make the concepts clear, and to see why we need these terms, we can follow the progression of reasoning outlined below, in The Strategy.
Young students also have a tendency to view "a large mass" as a large volume. For different amounts of the same substance (e.g. water or sand), this is valid. But it is usually not valid, particularly for materials of different densities, and those that contain air pockets. This, too, is addressed below.
What follows is written as a discussion, almost as a type of script you can use while presenting this to your students. Of course, the precise phrasing is not as important as conveying the concepts, and will diverge from what is written here as students discuss the material.
Begin by asking what things are made of, and continue the discussion with ordinary terms with which students are familiar:
What are things made of?
Everything is made of some kind of "stuff." You can feel this stuff. Put your hand on your desk and push. Yep. That's stuff, all right. You can't push your hand through it.
What about water? You can feel it, so it must be some kind of "stuff." But you can push your hand through it. How can you push your hand through it if it's actually "stuff"?
Here's an experiment. It provides a model of what water might look like if we could magnify it really really well.
Get some sand. Push your finger into it. Drag your finger around. Push your hand through the sand. Now, picture in your mind (and describe this picture) what the sand does as you move your hand through it.
-- the sand particles move out of the way. The individual pieces are not stuck together, so they can move as your hand goes past them.
Water must be the same way, except the individual pieces are way too tiny to see.
Hmmm...could your desk be made of tiny particles also? Could be. They'd just have to be stuck tightly together, so they can't move out of the way when you push on them.
Now...what about air? If you move your hand back an forth, you can feel air. You can feel wind. A strong wind can knock down trees! If you can feel it, it must be some kind of "stuff." Sure, you can see through it, but you can see through window glass, too, and that's obviously some kind of "stuff." Thinking of what we figured out about water, how would you explain air?
-- it must be made of tiny particles that can move out of the way. But there must not be as many of them, so they're easier to move through.
Maybe, if we could draw a picture, it would look like this:
Gosh...if everything is made of "stuff," whether it's solid (like the desk), liquid (like water), or a gas (like air), shouldn't we have a better name than "stuff"? After all, we use "stuff" to refer to ideas, thoughts, and stuff like that.
We use the word matter for this purpose.
First, show this NASA video of the moon landing: http://history.nasa.gov/40thann/mpeg/ap16_salute.mpg
Look at that odd jump! He doesn't come down very fast, does he? Why not?
-- the moon is smaller than earth, and has only about 1/6 as much gravity. He weighs less.
But, is he the same person as before he left, and after he got back? Yep. So...the matter of which he is made remains the same, whether on Earth or on the moon. But weight is not a good measure of how much matter he has. We need a new word to describe the amount of matter, independent of weight. That word is mass.
On earth, we can usually (but not always) measure mass by measuring weight. Sometimes, things seem to weigh less than normal, like a towel inside a swimming pool. It's easy to lift it from the bottom of the pool to the surface of the water (it seems to weigh rather little) but it's harder to lift it out of the pool (it seems to weigh much more). But its mass is the same.
Students often confuse mass and volume. "How much matter is there in this thing?" "It must be a lot because it's a big thing." Here's an investigation that may help clarify this:
Buy some Wonder Bread.
Have students weigh a slice. Have them measure its length, width, and height.
Then have them smash it into a small ball.
Have them weigh and measure it again.
Its volume is now smaller (length, width, height) but its mass is the same (mass approximated by weight.)
How do we explain what we've found?
-- bread is filled with air pockets, which make its volume large for the amount of mass that it has. When we smash the bread into a small ball, we squeeze out the air pockets.
Apparently, the mass of an object is not simply its volume.
The Rationale Behind the Strategy
"Science" is not simply learning facts; it is building understanding from the evidence. The evidence need not be complicated; here, we ask students to think about a number of observations that they have undoubtedly made many times before. This is evidence from which they can build their understanding. To work with evidence, it is necessary to build explanations that can explain the observations. It is these explanations that we call Scientific Knowledge.
If we look through the Science Standards, and ask what students typically study in the elementary grades, we find that much of it depends on a rudimentary knowledge of chemistry, especially the particulate nature of matter. We do not want to introduce terms like atom and molecule at this stage, but we can certainly talk about the tiny particles, or tiny pieces, of which things are made. In fact, to understand the differences between solids, liquids, and gases, students must have this rudimentary knowledge. Here, we begin the first discussion of "matter" by confronting and explaining solid (the desk), liquid (water), and gas (air). Of course, to be scientific, the teacher doesn't just tell the students the answers. It is very important for the teacher to ask the students how they might explain the evidence. If, at this young age, they have difficulty articulating their explanations, the teacher can guide them--preferably by thinking out loud rather than declaring "the answer."
The purpose of the sand activity is to build a model that represents what's going on. This should help students develop their explanations. It should also help them build a mental image of matter and its tiny particles. In science, this kind of mental imagery is tremendously important. Should the teacher find it necessary to think out loud while reasoning her way toward an explanation of water, the sand model is a very good tool for doing so.
In contrast to "normal" vocabulary words, the terms unique to science are very difficult to learn without the appropriate context. If a new term seems to be a synonym for some term students already know, they will wonder why we should bother with the new word. Without an appropriate context to "hang" the term on, they are likely to associate the term with something else that they know, but that is inappropriate. Therefore, we focus here on developing the concepts first, and then associating the new terminology with the concepts.
Note that it is tremendously important to ask students to construct their own understanding from the evidence (even if it requires some verbal thinking by the teacher). This is essential to students' long-term learning, and to their development of skills in problem-solving and scientific reasoning. (If we simply tell them the answers, they may be able to answer test questions in a few days, but they are unlikely to retain the learning for a significant length of time. Nor will they have the opportunity to practice the thinking skills that they will eventually need in the world they will enter as adults.) However, we do not leave their knowledge-construction to chance. We designed the learning experience to reach a goal; we need to ensure that it gets there.
The overall goal here is to understand the terminology, particularly matter and mass. But, quizzing students on word definitions is really boring, not to mention the fact that it's not particularly scientific. It might be more interesting to ask students to discuss, in small groups, things that they think are not matter, or things that have no mass, or things that might have mass but not be matter, or that might be matter but have no mass. The fundamental concept here is that all matter has mass, even if it doesn't weigh very much (like air). If students suggest things that might be matter without mass, for example, make a list of such "odd things," and then have a larger discussion aimed at the evidence to support, or refute, that idea.
Some students may ask what to call these "tiny pieces" of which stuff is made. If they do, tell them the term is "molecules." The term molecule comes from the Latin moles (meaning mass) and the suffix -cule (meaning tiny), so literally speaking, the term has no deep significance. It's just Latin for "tiny pieces" of which larger stuff is made.
Some students may ask whether everything is made of the same tiny pieces. The short answer, of course, is no. Each type of substance is made of molecules of that particular substance. The longer, and more fun answer, is to introduce the students to the formation of crystals, as described here.
The question may also come up (at least in the form of confusion) that "air can't be matter because it has no mass." We can't weigh air very easily, after all. To address this issue, we might turn to mountain climbing for help. Why, we might ask, does it become harder to breathe as we go higher and higher in the mountains? Why, when we see photographs of climbers on Mt. Everest, do they have masks and oxygen tanks? Perhaps some students may have heard that "the air is thinner at high altitude." If air is "thinner," and part of air is the oxygen we breathe, then there must be less oxygen at high altitudes. As high as Mt. Everest, it is necessary to carry tanks of oxygen in order to breathe. Why would air be "thinner" at higher altitudes? Simply because the farther one is from the earth (or the center-of-mass of the earth), the lower the gravity. The lower the gravity, the harder it is to pull very tiny molecules toward the earth. Phrased differently, gravity pulls air molecules down toward the ground. As a result, air molecules are more concentrated at low altitude, and less concentrated at high altitude. We see this in our own physiology by having a harder time breathing at high altitude. [Fortunately, our bodies can acclimate, so after a few days at moderate altitudes, we have no difficulty.]