Science Web Assignment for Unit 8
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The Greeks and modern chemists both concern themselves with the essence of matter. An essential characteristic is one which belongs to an object because of its being that kind of object; such a characteristic is shared by all objects of that class. For example, we might say the essence of a chair is in its shape. A "true chair" has a shape suitable for humans to sit on it, three to four legs, a flat seat, and a back. Anything with such a shape is "chair", regardless of the peculiarities of its shape or its composition (what it is made of); anything without such a shape is a "not chair", even if you can sit on it.
A given chair may have many accidental characteristics which are unique to that particular chair or some group of chairs. These would include color, material composition, and fuzzy coverings; they might even include the scratches you made when you climbed on the chair in your skates long ago. We recognize them as accidents which may be changed or removed without changing the essential chairness of the piece of furniture--but that is only because experience has taught us what chairness is.
Characteristics are sometimes measurable quantities, like length or mass. Any characteristic that can be directly measured is called a base quantity. There are only a few base quantities (length, time, mass, electrical charge, light intensity, and temperature); most other quantities are derived from recognizing relationships between these base quantities. Density (see below) is a derived quantity.
Each time we are confronted with something completely new, we have to figure out what characteristics are important and essential, and which are accidental. This was the problem that faced the Greeks when they set out to define matter in general and different kinds of matter in particular.
The Greeks recognized that matter—whatever it was—could exist in multiple states: solid, liquid, and gas. In modern times, we have added a fourth state, plasma, to account for the peculiar form matter should take (theoretically) under the extreme pressures and heat at the center of stars, and we are in the process of trying to determine whether a fifth state of matter exists. Our modern explanation for states of matter depends on the atomic model, but for now, we will concentrate on the characteristics of the three states which are commonly observed.
- True, there are peculiar properties of helium at near-absolute zero which defy this rule, but the Greeks didn't have the ability to make liquid helium, and you probably don't have the equipment lying around the house, either.
A given type of matter, such as water, moves from solid to liquid to gas and back by absorbing or losing heat. This means that the temperature at which the matter boils will be higher than the temperature at which it melts. The temperature at which a phase change takes place is characteristic of the type of matter, and can sometimes be used to identify it. In most cases, the solid form of a substance is more dense than the liquid form: a solid block of metal will sink in a liquid bath of the metal. The major exception to this is water: ice is less dense than liquid water and floats. This has important ramifications for all life on earth, since if ice sank, most of the ice that forms in winter would never melt.
There are two other states of matter discovered in the twentieth century. Plasma consists of base particles of atoms (electrons, protons, neutrons, and so on) in free association--that is, without atomic structure. The conditions required to pull atoms apart exist only at the core of stars, which are both very hot and under so much pressure that atomic structure collapses. Bose-Einstein condensate matter is a form of matter observed at very-near absolute zero temperatures (within a few millionths of a degree of "as cold as it can get"); it is not yet clear whether this is a new state of matter. Neither type of matter comes into our everyday experience.
Modern chemists and physicists have defined a number of properties that belong to all forms of matter, regardless of their form or composition.
The Greeks considered elements to be the basic form of a particular type of matter that had some distinct set of the above characteristics. A substance made of a single type of element cannot be broken down further into other elements with different essential characteristics. For many years, the list of true elements was short: earth, fire, water, air, and the "fifth element" from which the stars were made. Our modern list of 92 natural elements and a few man made ones comes from the work of chemists in the nineteenth century. We now identify elements with specific types of atoms.
Compounds are combinations of different elements. The study of how compounds form and recombine is the basis of modern chemistry.
A pure substance can be either a sample of a single element or a single compound; it has unique chemical characteristics. It cannot be broken down unless it undergoes a chemical reaction (and elements cannot be broken down unless their nucleur structure is destroyed).
A mixture is a combination of pure substances which remain physically distinct, so that they can be separated without chemical reactions. A cup full of sand, salt, and iron filings is a mixture.
The Greeks, as we have seen, didn't necessarily make the same kinds of distinctions that we do now about matter and motion. Empedocles and Aristotle, in particular, attributed different kinds of motion to different types of matter--fire naturally moves up and away from the center of the earth, water moves toward the center of the earth, and so on. This is still a problem for us in the twentieth century. As we consider the differences between inanimate matter and living matter, we need to ask: is there something intrinsic to living matter which causes it to behave differently than "dead" matter, or is it just a very complex form of matter in which peculiar reactions can occur?
In order to accomodate some of the subatomic particles predicted by quantum mechanics, modern physicists use the following definition: matter is anything that has mass and can be detected. Note that matter doesn't have to take up space--extension is not a requirement for an item to be matter.
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