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Natural Science - Year I

Unit 8: Properties of Matter

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Science Lecture for Unit 8:
Properties of Matter

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Outline/Summary

Matter

The Beginnings of Chemistry

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.

States of Matter

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.

Solid
A solid holds its shape without a container, and is not compressible (you can't smoosh it into a smaller space).
Liquid
A liquid takes the shape of its container, but flows downhill to fill the bottom of it. It usually isn't compressible either.
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.
Gas
A gas expands to fill its container, and has no fixed shape. It can be compressed over a wide range of pressures.

Changes of State

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.

Properties of Matter

Modern chemists and physicists have defined a number of properties that belong to all forms of matter, regardless of their form or composition.

Mass
The amount of matter an object has is its mass. We sometimes measure this in terms of weight, which is the pull exerted by the earth's gravitational field on the object, but this can be misleading. The amount of matter doesn't change in an object if we simply move it from the earth to the moon, but is weight will change--because the moon's gravitational field is less than the earth's. Our object has the same mass in both places, but less weight on the moon.
In the scientific units of measurement used internationally, mass units are called grams.
Volume
The amount of space an object occupies is its volume. Space has three dimensions, which we often call height, width, and depth, and which we measure in units of length (feet or meters). We can directly measure these dimensions on the exterior of many simple shapes. Regular solid shapes like cubes, rectangular boxes, cylindars, sphere, and pyramids have volumes that can be calculated mathematically if we know their outside edges. For example:
Regular Solids volumes
Figuring out the volumes of irregular solids is more difficult. It must often be done directly, by submersing the solid in a liquid and see how much liquid is displaced:
Irregular Solid volumes
Because we usually determine the volume by calculation, we call it a derived quantity.
Density
The relationship of matter per unit of space is density, measured in grams/area3. Density is also derived quantity, because we don't measure it directly. We always determine the volume and the mass and then calculate the density as density = mass/volume. The density of an object is often compared to the density of water, which is 1 gram/cubic centimeter (sometimes you will see this as 1g/milliliter--a milliliter = a cubic centimeter). Things more dense than water sink in water; things less dense float. We'll come back to this, when we talk about Archimedes' discovery of the laws of bouyancy in a few weeks.
Plasticity, Malleability, Ductility
Plasticity is the ability of a material to change shape and hold the new shape without breaking. Malleability measures how easy it is to flatten the object into a sheet; ductility measures how easy it is to draw the object out into a thin wire. Both malleability and ductility are usually applied to metals, but you can apply them to synthetic materials such as plastics as well.
Strength and Hardness
The hardness of an object is a measure of how difficult it is to scratch the surface of the object. We measure the hardness of something by comparing it to the hardness of a set of specific substances, which together make up a scale. The scale starts with the hardness a diamond (which can be scratched by another diamond and few other special alloys), and work its way down through different metals to talc, which is very soft. Strength is the ability of a material to retain its shape when under pressure or when pulled apart (tensile strength).
Conductivity and Insulation
Another characteristic of matter is its ability to carry an electric current. A good conductor (most metals are good conductors) carries current easily. A really bad conductor (or very good insulator) such as rubber doesn't carry current. By putting good conductors inside good insulators, we make it posible to use electricity safely.
Solubility
Some materials can dissolve or come apart when placed in liquid forms of other materials. In general, the thing that dissolves is called the solute, the liquid in which it dissolves is called the solvent, and the resulting combination is called the solution. Some gases dissolve in liquids to form gas-liquid solutions: the carbon dioxide in soda pop is one example.
Odor, Taste
Many objects have some effect on our olafactory and taste systems, which are chemically sensitive. Whether or not these odors are characteristic depends on the substance. Some chemicals, such as alcohols, have odors which are easily recognized and can even be used for identifying the chemical. The odor of household natural gas is actually an additive, however, put there by the gas company so that you can recognize gas leaks and get out of the way.
Color
We often think of color as a non-essential or accidental characteristic of matter. But for some kinds of matter, especially matter in a gaseous state, color can be an important identifying characteristic. We'll learn how later when we talk about the nature of light and atomic theory.
This is not a definitive list--there are many other qualities you could observe and measure for some hunk of stuff. The problem both the Greeks then and we now face is deciding which qualities belong to matter as such, and which belong to this chunk of matter, and which belong to un-matter (space?, energy?).

Elements and Compounds, Pure Substances and Mixtures

Matter1

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.

Matter

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?

The modern definition of matter?

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.

Study/Discussion Questions

Further Study On your Own (Optional)