Science Matters

by Robert M. Hazen

  Solids

  Solids are materials that have a more or less fixed shape. Atoms in solids bind together with sufficient strength to stay in place. Crystals, glasses, and plastics, three of the most important varieties of solids, differ primarily in the regularity of their atomic structures.

  Metals, gemstones, bones, and computer chips consist of crystals. They contain regular three-dimensional arrays of atoms, repeated over and over again in a sort of Tinkertoy arrangement. You can imagine a crystal as something like a huge stack of boxes, with each box the same size and shape and all the boxes holding exactly the same atomic contents. The size of each “box” in a real crystal is usually no more than a ten millionth of an inch, and each box might contain up to a few dozen atoms. The structure of a crystal is orderly, with row upon row of exactly the same kinds of atoms; travel 100 boxes in any direction and you know exactly what you’ll find.

  Three different kinds of solids are distinguished by the regularity of their atomic structures. Crystals (A) possess row after row of identical atom boxes; trillions of boxes stack to form one tiny crystal. Glasses (B) have random structures; there is no way to predict what atom might reside a short distance from any starting point. Plastics (C) form from long chains of carbon atoms called polymers; atoms are ordered in one direction (along the chain) but are spaced randomly elsewhere.

  Plastics, invented in the twentieth century, are among the commonest solids we encounter in daily life. All plastics in common use are synthetic chemicals not found in nature, but built from long, chain-like molecules linked together by carbon atoms. Like any chain, they have a predictable structure in one direction; travel along any single chain and you’ll keep running into carbon atoms like beads on a string. The chains themselves, however, form a hopelessly tangled mass of interwoven strands that give plastics their rigidity and strength. When heated, the chains separate slightly and slide apart, softening the plastic and allowing it to be poured, rolled, or otherwise reshaped into new forms. The long list of common plastics includes nylon and polyester (in clothing and fabrics), Lucite (in sculptures), vinyl (floor tiles and furniture), and epoxy (in cements and glue).

  Glass structures differ considerably from both crystals and plastics. Atoms in a glass are jumbled together almost at random, but usually with some regularities in the atomic architecture. Common window glass, for example, contains mostly silicon and oxygen, with almost every silicon atom surrounded by four oxygen atoms and almost every oxygen atom next to two silicons. But there aren’t any “boxes” of structure neatly stacked in window glass. Travel just a few atoms in any direction from any point and there is no way to guess whether you will find an oxygen or a silicon atom. Beyond the nearest neighbor atoms, the structure is random.

  Changes of Phase

  If an ice cube falls on the floor it melts, leaving a puddle for you to clean up. If you boil water on the stove, you produce a cloud of steam. If you let your paintbrush dry without cleaning it, the brush becomes hard and useless. These are examples of processes in which matter changes phase—from solid to liquid in the first case, from liquid to gas in the second, and from liquid to solid in the third. The atoms and molecules are the same before and after the change of phase, but their relationship to one another is different.

  In the ice cube, for example, the molecules of water in the crystal lattice vibrate faster and faster as heat from the outside air moves in. Eventually they vibrate so fast that they tear loose and begin to move around freely. When this happens, the ice turns to water and we say that the cube has melted. In just the same way, molecules in boiling water move faster and faster until they lose contact with one another and enter the air as a gas. As paint hardens, on the other hand, atoms link up to form long chains where before there were separate molecules. Physicists know melting, boiling, and solidification as phase transitions.

  All phase transitions involve shifts of energy. Materials soak up heat while they melt or boil, but their temperature stays the same—all of the energy input goes into breaking up the old atomic arrangement. This is why you use ice cubes to keep your drink cold. When you take an ice cube from the refrigerator, it is at a temperature well below freezing. It starts to warm up as it absorbs heat from your drink, cooling the drink as it does so. When the ice gets to 32°F, however, it remains at that temperature until it has absorbed enough energy to break all the bonds between the water molecules in its crystal structure. Only after the cube has melted completely and all the water is in liquid form does the water temperature start to rise again.

  CHEMICAL REACTIONS

  Chemistry involves a lot more than bonding and changes of state. In fact, our world would be pretty boring if it weren’t for chemical reactions, the processes by which atoms and small molecules combine to form larger molecules, and larger molecules break up into smaller fragments. When we take a bite of food, light a match, wash our hands, or drive a car, we initiate chemical reactions. Every moment of every day, from the digesting of your food to the clotting of your blood, countless chemical reactions sustain life in every cell of your body.

  All chemical reactions arise from the rearrangement of atoms, as well as the shuffling of the atoms’ outer electrons to form new chemical bonds. So when a piece of soft silvery sodium metal comes into contact with the corrosive gas chlorine, the two elements react violently to form sodium chloride—table salt. When a piece of iron metal is exposed to oxygen in the atmosphere they react, albeit much more slowly, to form rust.

  Oxidation and Reduction

  Countless millions of chemical reactions take place in the world around us. Some occur naturally and others occur as the result of human design or intervention. In your everyday world, however, you are likely to see a few types of chemical reactions over and over again. Perhaps the most distinctive chemical feature of our planet’s atmosphere is the abundance of the highly reactive gas oxygen, which drives many of the most familiar chemical reactions in our lives. Oxidation includes any chemical reaction in which oxygen accepts electrons while combining with other elements. Rusting is a common gradual oxidation reaction in which iron metal combines with oxygen to form the familiar reddish iron oxide, while burning is a much more rapid oxidation in which oxygen and carbon-rich fuel combines to produce carbon dioxide.

  Reduction is the opposite of oxidation—that is, it involves the addition of electrons to an atom. Thousands of years before scientists discovered the element oxygen, primitive metalworkers had learned how to reduce metal ores by smelting. In the iron-smelting process, ironmasters heat a mixture of iron oxide ore and lime (calcium oxide) in an extremely hot charcoal fire. The lime lowers the melting temperature of the entire mixture, which then reacts to produce iron metal and carbon dioxide.

  Oxidation and reduction reactions are essential to life, and they define the principal difference between plants and animals (see Chapter 15). Animals eat carbon-rich food and obtain energy through oxidation in their cells, releasing carbon dioxide as a by-product. Plants take in carbon dioxide and use the energy in sunlight to reduce it, releasing oxygen as a by-product.

  Acid-Base Reactions

  Acids are common corrosive chemicals that, when put into water, produce positively charged hydrogen ions in the solution. Lemon juice, orange juice, and vinegar are examples of common weak acids, while sulfuric acid (used in car batteries) and hydrochloric acid (used in industrial cleaning) are strong acids. Bases are another class of corrosive chemicals that, when put into water, produce negatively charged OH ions. Milk of magnesia and other antacids are weak bases, while ammonia cleaning fluids and most drain cleaners are stronger bases.

  Acid-base chemical reactions occur when an acid and a base are brought together. When you swallow an antacid, your stomach acid (with excess H) and the antacid (with excess OH) mix together. When the H and OH ions react to form water (H2O), we say that the stomach acid has been neutralized.

  Polymerization and Depolymerization

  The molecular building blocks of
most common biological structures, including simple sugars and amino acids (see Chapter 15), consist at most of a few dozen atoms. Yet proteins, DNA, and other essential biological molecules are huge, with millions of atoms in a single unit. How can small building blocks yield the large structures characteristic of living things? The answer lies in the process of polymerization.

  A polymer is a large molecule formed by linking together many smaller molecules. In spiderwebs, clotted blood, muscle fibers, and a thousand other substances, living systems have mastered the art of combining small molecules into long chains through polymerization reactions. Synthetic polymers from plastics to paints usually begin in liquid form, with small molecules that move freely past their neighbors. Polymers form from the liquid when the ends of these molecules begin to link up.

  The longevity of some polymers presents a growing problem in an age of diminishing landfills. Nevertheless, over time most polymers decompose into short segments through the process of depolymerization. Some of the most familiar depolymerization reactions occur in your kitchen. Polymers cause the toughness of uncooked meat and the stringiness of many raw vegetables, so we marinate and cook our food to break down these polymers.

  As museum curators are painfully aware, not all depolymerization reactions are desirable. The breakdown process can affect leather, paper, textiles, and other historic artifacts made of organic materials. Storage in a cool, dry environment may slow the depolymerization process, but there is no known way to repolymerize old brittle objects.

  PHYSICAL PROPERTIES

  When you go to the grocery store you test the food you’re about to buy. Are the tomatoes firm? Does the meat have good color? Is the lettuce crisp, the bread soft? As you evaluate your potential purchases you are determining physical properties of the materials you buy.

  A physical property is any aspect of a material that you can measure. Every measurement has three essential components: a sample, a source of energy, and a detector. The sample is the thing you want to measure. Samples vary widely in size and character—you can study single subatomic particles, a piece of fruit, the whole Earth, entire galaxies, and anything in between.

  All measurements depend on an interaction of the sample with some form of energy. Color is the interaction of matter with light. Electrical conductivity is the interaction of matter with an electric field. Brittleness can be measured by the interaction of matter with the force of a moving hammer. Without energy, you can’t measure these properties.

  The detector measures the interaction between the sample and the energy. The human senses provide marvelously sensitive and portable detectors such as the eyes and ears, and scientists have developed many detectors that extend our senses, such as photographic film, speedometers, thermometers, and radios. All of these detectors (and many others) record the interactions of matter and energy.

  We are surrounded by countless materials, each with properties ideally suited to its function. You are reading a book with thin, white, flexible paper pages, fast-drying black ink, and strong glues for binding. You are probably sitting in a comfortable chair constructed from some combination of laminated woods, resilient plastics, light metal alloys, and synthetic fabrics. We wrote this book on PCs made of semiconductor integrated circuits, ultrathin LCD (liquid crystal display) screens, flexible copper wires, and colorful plastic insulation.

  All of these adjectives—flexible, strong, light, colorful—refer to physical properties that you can measure. Scientists organize the dozens of different physical characteristics into a few basic categories, including mechanical, magnetic, and optical properties. All of these properties are determined by the atoms and how they fit together.

  MECHANICAL PROPERTIES

  Mechanical properties include a host of characteristics that de scribe how a material withstands stress and strain. Does it scratch easily? Does it break when you squeeze, twist, or stretch it? Mechanical properties are critical to thousands of day-to-day applications. You want your floor to be strong, your bed soft, and your toothbrush bristles flexible. Thousands of materials scientists spend their professional lives looking for “new and improved” products with useful mechanical properties.

  Elasticity and Strength

  Some materials are strong and others weak; some boughs bend while others break. These profound differences in material properties reflect equally profound differences in atomic structures. When two objects, like your feet and the floor, come into contact, two forces are involved. The force of gravity pushes you down, but an equal and opposite electrostatic force acts as electrons in feet and floor try to occupy the same space. Any time two chunks of matter are forced together, both objects must experience some deformation, even if only a subtle one. (Your feet become a little flatter, for example, and the floor bends just a bit.) Under mild stress most solids like feet and floors deform elastically; when stress is released they spring right back to their original shape. If too much force is applied, however, materials reach their elastic limit and a permanent deformation occurs: metals bend, paper rips, glass shatters, fruit squishes.

  Elasticity and strength have their origins in atomic structure. We already have part of the story in the relative strength of different chemical bonds. Clearly a material can’t be strong if all its bonds are weak. But even compounds with the strongest bonds can be weak if the atoms aren’t properly arranged. The way bonds are assembled into a three-dimensional structure makes all the difference. To grasp this point, consider again the two forms of pure carbon, diamond and graphite. Diamond is the hardest, strongest material known. In a diamond crystal, strong covalent bonds link each carbon atom to four neighbors. The entire carbon array forms a rigid three-dimensional framework.

  Diamond and graphite are both crystals formed entirely from carbon, but their atomic structures are strikingly different. In diamond, each carbon atom is surrounded by four neighbors in a rigid three-dimensional framework. Graphite, on the other hand, is a layer structure with each carbon atom near only three others.

  Carbon-carbon bonds also dominate the graphite structure, but each atom is tightly bound to only three neighbors in layers of atoms. Graphite layers are held together by van der Waals forces so weak that graphite is one of the softest known minerals. Every time you use a “lead” pencil, black layers of graphite shear off the end and transfer to paper because the small force you exert on the pencil is enough to break the van der Waals force. Graphite illustrates the common adage that a chain is only as strong as its weakest link. The distribution of strong and weak bonds, rather than the type of atom, makes the critical difference in mechanical properties.

  Carbon-carbon bonds hold together the strongest known fibers. Long chains of carbon atoms are essential structural elements of spiderwebs, nylon fibers, wood, and a host of plastics. These chain molecules, or polymers, can also make the best elastic bands when carbon-carbon chains adopt a folded or zigzag form, straightening only under tension.

  Ductility and Brittleness

  Grab a hammer and strike a piece of fine china with all your might. The ceramic shatters into a thousand pieces, each retaining the shape of a fragment of the original dish. Do the same thing to a lump of lead. The lead deforms, absorbing the blow by changing shape. These differences reflect the diversity of atomic bonding.

  China has ionic bonds, with alternating plus and minus ions that cause inflexible links in fixed directions. Attractive forces line up exactly along atomic pairs. Ionic crystals are like models constructed of rigid balls and sticks. Once a few sticks come loose, the results are often catastrophic. If bent too far, the sticks break and are not easily re-formed.

  Bonds in metals, with their freely roving electrons, are much less directional. Metal atoms are something like marbles in a bowl of molasses. Each round marble fits neatly into the array, surrounded by its neighbors. Molasses, like the metal’s electron sea, keeps everything stuck together. Tilt the bowl, however, and the marbles slowly slide past one another, adopting a new arrangement.
The efficient packing of marbles is retained, but individual marbles shift. The same sort of thing happens when a metal is hammered or bent. Layers of atoms slide against each other, yielding a new shape to the metal object, but the bonding is preserved.

  Rubber balls adopt a different strategy in response to a blow. Individual covalent carbon-carbon bonds in rubber are strong, but they have a great deal of directional leeway. The stress of a bat or foot deforms the ball, forcing the atoms closer together momentarily. This process parallels what happens when you squeeze a spring: you do work, adding energy to the system, which is stored as elastic potential energy in the spring. But though the bonds may bend, they do not break. The stored energy reconverts to kinetic energy as the bonds straighten out and the ball snaps back into shape and zooms off. Most sports would be a lot less interesting if we had only metal or ceramic balls!

  Composite Materials

  If you’ve performed any sort of do-it-yourself work around your home you’ve probably run into composite materials, one of the hallmarks of modern materials science. Composites overcome the shortcomings of one substance by combining it with others. Shatterproof car windshields, integrated circuits, and steel-reinforced concrete are all examples of the new technology.

  Plywood is the classic composite material. It consists of thin layers of wood glued and pressed together, with the grain direction alternating layer by layer. The resulting lumber has no weak direction due to the grain, so plywood is stronger than traditional boarding of the same thickness. Furthermore, it can be fabricated in large sheets from relatively small trees, because the wood veneers can be sliced off a rotating tree like paper towels off a roll.