The Solid Phase Help (page 2)
A sample of matter in the solid phase will retain its shape unless subjected to violent impact, placed under stress, or subjected to high temperatures. Examples of solids are rock, steel at room temperature, water ice, salt, wood, and plastic at room temperature.
The Electrical Force
What makes a solid behave as it does? Why, if you place a concrete block on a concrete floor, does the block not gradually sink into the floor or meld with the floor so that you can’t pick it up again later? Why, if you strike a brick wall with your fist, are you likely to hurt yourself rather than having your fist go into the bricks? Internally, atoms are mostly empty space; this is true even in the most dense solids we see on Earth. Why can’t solid objects pass through one another the way galaxies sometimes do in outer space or the way dust clouds do in the atmosphere? They’re mostly empty space too, and they can pass through each other easily.
The answer to this question lies in the nature of the electrical forces within and around atoms. Every atomic nucleus is surrounded by “shells” of electrons, all of which are negatively charged. Objects with electrical charges of the same polarity (negative-negative or positive-positive) always repel. The closer together two objects with like charge come to each other, the more forcefully they repel. Thus, even when an atom has an equal number of electrons and protons so that it is electrically neutral as a whole, the charges are concentrated in different places. The positive charge is contained in the nucleus, and the negative charge surrounds the nucleus in one or more concentric spheres.
Suppose that you could shrink down to submicroscopic size and stand on the surface of a sheet of, say, elemental aluminum. What would you see? Below you, the surface would appear something like a huge field full of basketballs (Fig. 10-1). You would find it difficult to walk on this surface because it would be irregular. However, you would find the balls quite resistant to penetration by other balls. All the balls would be negatively charged, so they would all repel each other. This would keep them from passing through each other and also would keep the surface in a stable, fixed state. The balls would be mostly empty space inside, but there wouldn’t be much space in between them. They would be just about as tightly packed as spheres can be.
The foregoing is an oversimplification, but it should give you an idea of the reason why solids normally don’t pass through each other and in fact why many solids resist penetration even by liquids such as water or gases such as air.
Brittleness, Malleability, And Ductility Of Solids
The atoms of elemental solids can “stack up” in various ways. This is evident in the shapes of the crystals we observe in many different solid substances. Salt, for example, has a characteristic cubical crystalline shape. The same is true of sugar. Ice crystals, however, can appear in a fantastic variety of shapes, but they always have six sides, axes, or facets. Some substances, such as iron, don’t seem to form crystals under normal circumstances. Some materials, such as glass, break away along smooth but curved boundaries. Some solids can be ground up into a fine powder, whereas others defy all attempts to pulverize them.
Crystalline solids are brittle. If a sample of such a material is subjected to a blow with enough force, it will crack or shatter. These types of solids cannot be stretched or squashed or bent out of shape very much without breaking. Glass is an example, although you may have noticed that glass has a little bit of “give.” You can observe the flexibility of glass if you watch the reflections from large window panes on a windy day. However, you cannot bend a straight glass rod into a donut shape.
Soft copper wire, in contrast to glass, is malleable (it can be pounded flat) and ductile (it can be stretched and bent). The same is true to some extent of iron. Gold is one of the most malleable known metals. It is expensive but can be pounded into sheets so thin that towers of buildings can be gold-plated without breaking the government budget. Aluminum is more ductile and malleable than glass, but not to the extent of soft copper or gold. Wood can be bent to a variable extent, depending on its water content, but can’t be pounded into thin sheets or stretched into wire.
The brittleness, ductility, and malleability of some solids depend on the temperature. Glass, copper, and gold can be made more malleable and ductile by heating. The professional glass blower takes advantage of this phenomenon, as does the coin minter and the wire manufacturer. A person who works with wood has no such luck. If you heat wood, it gets drier and less flexible. Ultimately, if you heat glass, copper, or gold enough, it will turn into a liquid. As wood is heated, it will remain solid; then at a certain temperature it will undergo combustion , a rapid form of oxidation . That is, it will catch on fire.
Hardness Of Solids
Some solids are literally “more solid” than others. A quantitative means of expressing hardness, known as the Mohs scale , classifies solids from 1 to 10. The lower numbers represent softer solids, and the higher numbers represent harder ones. The standard substances used in the Mohs scale, along with their hardness numbers, are shown in Table 10-1. The test of hardness is simple and twofold: (1) a substance always scratches something less hard than itself, and (2) a substance never scratches anything harder than itself.
An example of a soft solid is talc, which can be crumbled in the hand. Chalk is another soft solid. Wood is somewhat harder than either of these. Limestone is harder still. Then, in increasing order of hardness, there are glass, quartz, and diamond. The hardness of a solid always can be determined according to which samples scratch other samples.
Many substances have hardness numbers that change with temperature. In general, colder temperatures harden these materials. Ice is a good example. It is a fairly soft solid on a skating rink, but on the surface of Charon, the bitterly cold moon of the planet Pluto, water ice is as hard as granite.
Hardness is measured by maintaining laboratory samples of each of the 10 substances noted in Table 10-1. A scratch must be a permanent mark, not just a set of particles transferred from one substance to the other. Substances commonly have hardness values that fall between two whole numbers on the scale. The Mohs hardness scale is not especially precise, and many scientists prefer more elaborate methods of defining and measuring hardness.
Density Of Solids
The density of a solid is measured in terms of the number of kilograms contained in a cubic meter. That is, density is equal to mass divided by volume. The kilogram per meter cubed (kg/m 3 or kg · m −3 ) is the measure of density in the International System (SI). It’s a rather awkward unit in most real-life situations. Imagine trying to determine the density of sandstone by taking a cubical chunk of the stuff measuring 1 m on an edge and placing it on a laboratory scale! You’d need a construction crane to lift the boulder, and it would smash the scale.
Because of the impracticality of measuring density directly in standard international units, the centimeter-gram-second (cgs) unit is sometimes used instead. This is the number of grams contained in 1 cubic centimeter (cm 3 ) of the material in question. Technically, it is called the gram per centimeter cubed (g/cm 3 or g · cm −3 ). To convert from grams per centimeter cubed to kilograms per meter cubed, multiply by 1,000. Conversely, multiply by 0.001.
You certainly can think of solids that are extremely dense, such as lead. Iron is quite dense too. Aluminum is not so dense. Rocks are less dense than most common metals. Glass is about the same density as silicate rock, from which it is made. Wood and most plastics are not very dense.
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