The Scientist’s Essay for 1. Water, a Liquid

What are materials made of?

if you keep breaking down any uniform material — like water, or aluminum — into smaller and smaller pieces, you will eventually get to a smallest unit, or fundamental particle of that material

When you start paying attention to the materials around you, the most stunning thing is their extraordinary diversity — from gases so insubstantial we almost forget they exist, to lead bricks; from clear water to thick, smelly tar, from stretchy rubber to brittle glass, from rocks to living tissue. Yet science has shown that all of this vast array of substances, without exception, is comprised of a small assortment of building blocks, something like the way a modest set of Lego shapes can be used to construct an immense variety of structures.

There are actually two pieces to this assertion about the fundamental structure of matter. The first is that if you keep breaking down any uniform material — like water, or aluminum — into smaller and smaller pieces, you will eventually get to a smallest unit, or fundamental particle of that material. These particles, unimaginably tiny and numerous, can combine like minuscule Lego blocks to make the water drop or aluminum picture frame that we can see and handle. We can call this the particulate model of matter, and it is the focus of our present curriculum.

There's a further step, which is that those fundamental particles — molecules — can be further broken down into yet smaller entities — atoms, of which there are only about 100 different kinds. Just a handful of them (carbon and hydrogen and a few others) can be combined into so many different kinds of molecule that just studying them is an entire field of chemistry — as well as the basis for all of biology. We will not get to the atomic model in the present curriculum, but we are laying the foundations for it.

How do we know? These ideas about the fundamental structure of matter at the tiniest of scales arose from detailed study of what changes and what doesn't when matter is transformed from one form to another, processes like melting, freezing, evaporating, condensing and dissolving. (There is another set of processes described by words like burning, or corroding, that we are leaving to future chemistry classes.) To the casual observer, it seems like just about everything changes in these transformations. When water freezes it becomes hard and brittle, its color changes, its density changes — it seems to have become an entirely different material. When water evaporates it seems to simply disappear, and the same is true of salt or sugar when we dissolve it in water. Yet on closer examination, we find evidence that the changes are not as complete as they appear. For one thing, it is possible to recover the original material. If we warm up the ice, the water reappears. If we place a chilled piece of glass over the water as it evaporates, we collect drops of liquid water. If we allow the salt solution to sit until the liquid evaporates, we find the salt left behind. (Chemical transformations, like burning a piece of paper, are not so easily reversed, but in principle it's always possible.)

Moreover, if we are careful, we can discover something else that remains constant: the total amount of matter, as indicated by its weight. The weight of the ice is the same as the weight of the water that froze. The weight of the salt solution is equal to the weight of the water plus the weight of the salt before it was dissolved. And if we're very careful we can even show that the water vapor is not weightless, but in fact has exactly the weight of the water that evaporated to produce it. Why should that be so?

The particulate model provides a very natural way to explain these observations. If water, ice and water vapor are all different ways of organizing the same bunch of tiny “water particles” — like different structures made from the same set of Legos — then it makes sense that the total weight doesn't change, and that you can always get back to the state you started with. These observations by themselves do not prove that the particulate model is true, but they are the beginnings of a vast web of phenomena that all point in that direction.

It took a century of careful experiments and intricate reasoning for the particulate model to become widely accepted by the scientific community, so we don't expect that these few experiences will enable students to understand all its implications, let alone convince them of its validity. We do hope to give them a taste of the “What if?” game that is in many ways at the heart of science. “What if” there are tiny “water particles”, far too small to see, but that still have weight and take up space? Would that help explain the way water, ice and water vapor behave? What other observations or tests can I think of that would either support or contradict that model?

* Strictly speaking, what remains constant is its mass. Scientists distinguish between mass, which is a measure of the amount of matter, and weight, which is how hard Earth's gravity pulls on the matter. The distinction is sometimes important, but for our present purposes, the two quantities are interchangeable.

—Roger Tobin