Chemistry
A brief diversion: the History of Chemistry
WebLecture
A Short History of Chemistry
Outline
Theories of Matter and the Interaction of Elements
There once was a really great chemistry site that I could use for you to learn something (briefly) about the history of chemistry....but it seems to have disappeared. So (especially for those of you who haven't already seen this information in Natural Science), here is a brief history of Chemistry Before Dalton.
The Ancient World
By the seventh century B.C., Asia Minor (what we now call Turkey) was a crossroads for trade between central Europe, the Mesopotamian valleys, and the Mediterranean kingdoms of the Phoenicians and the Egyptians. The tradesmen brought spices, precious metals, jewels, tools, and new ideas. From Babylon, and later Assyria, they also brought the records of a thousand years of astronomical events to minds eager to find a rational relationship in the patterns of the stars, and by analogy, patterns in just about everything else.
From about 600 B. C. until nearly 400 B. C., philosophers in the Greek city states of Ionia one the coast of Turkey tried to determine the whatness of matter, the nature of mathematics, and the ultimate source of all motion in the universe. The earliest philosopher of this group, called the Presocratics because they lived before Socrates of Athens, was Thales of Miletus, who reputedly predicted the eclipse of 585 B.C. While we know very few details about Thales' life or his sources, he seems to have been the first influential philosopher to generalize about the cause and effect in nature, and to try to come up with definitions or descriptions that would apply to all cases of an event, or all instances of a particular thing.
This practice makes an assumption, one of the most basic (and almost unmentioned, because we take it for granted) conceptions we have about the universe: Nature is ordered. We believe that natural events follow rules, and these rules can be discovered by thinking rationally about the limits and possibilities of a given situation. Notice that Thales' approach doesn't require experimentation or directed observation, however.
Thales made a distinction between the essential attributes of a thing, which make it a particular kind of thing, and the accidental characteristics, which make it unique. His theory is sometimes stated briefly as "all is water", that is, all matter shares certain basic characteristics similar to those of water (such as three states, solid, liquid, and matter). The philosophers who followed Thales disagreed with his conclusions, but adopted his approach: they all try to discover the essential thing that makes matter matter and not something else.
Many of the early Greek philosophers believed that there was one essential source for all matter., but disagreed on what the source was. For example, Anaximander believed that all matter came from a single infinite, unknowable source; Anaximenes believed that matter was the condensation of some airy substance. They are all classified as monists, from the word for one substance (we get monotonous, monatomic, etc. from the same root).
Another major group of philosophers were followers of Pythagoras. The Pythagoreans believed that the universe was not only ordered, but that many relationships between natural events followed arithmetic or geometric ratios. Philolaus proposed a concentric set of spheres to carry all the planets and the earth around a central holy fire (not the sun).
A later group of philosophers approached the problem of the nature of matter from a completely different premise: they were concerned with the concepts of indivisibility and continuity. Parmenides tried to apply prinicples of unity and multiplicity to matter, Zeno worried about the problem of continuous motion, and the philosphers Leucippus and Democritus proposed that matter came in discrete, individual units they called atoms. Their theories also give rise to the concepts of determinism and completely mechanistic explanations of the universe.
The First List of Elements
The transition from the monist theories of the Ionians and the classical philosophies of matter begins with Anaxagoras. Anaxagoras proposes the division of matter into parts or seeds of different types. Like the atoms of Democritus, the seeds have many types, but unlike atoms, the seeds can be divided into smaller parts forever. The stars and planets and sun were types of fire, but — and here Anaxagoras breaks with earlier philosophers — the moon as well as the earth is gross and solid matter. This important distinction leads to the realization that the moon makes no light of its own, so all the lunar phenomena, which include phases and eclipses, are the result of the moon's position and its ability to reflect sunlight back towards earth.
Empedocles (remember Empedocles? From the first weblecture?) proposed that all matter consists of four elements with opposing characteristics of dryness and heat: earth, fire, water, and air. Now, in the ancient Greek view of the world, none of these elements is found in a pure state, but only as mixtures that are in constant turmoil because of the motion of the planets. In the diagram below, which is similar to a number of medieval diagrams, the elements earth, fire, water, and air are drawn together by shared qualities of dryness, coldness, heat, and wetness. So earth is attracted to fire by shared dryness, and to water by shared cold. Empedocles' proposal further fueled debates about the nature of matter: later philosophers asked whether the materials of the elements were real, and exhibited the qualities, or whether the qualities were real, and "manifested" themselves as elements.
Living matter was made of forms of these elements called humors. Earth took the form of the humor black bile, fire blood, water was phlegm, and air yellow bile. Changes in the amounts of each humor cause growth, corruption, and eventually death. A person with too much black bile was dour, heavy, and slow, able to hold anger for a long time. We still use these images when we talk of someone in a "black humor". Imbalances of humors led to disease; both mental and physical health were the result of the presence of the humors proper proportions.
During the second half of the fifth century, the philosopher Socrates challenged the citizens of Athens to search themselves and the universe for meaning and true understanding. He believed that sense perception was often misleading and that only logic could distinguish between appearance and reality. Socrates questioned the established views about many things, including the nature of the gods. Unfortunately for him, the Athenians had just undergone the disastrous Peloponnesian War with Sparta and had suffered through a major plague (which we think was an incident of bubonic plague like the Black Death which struck Europe in 1348), and they were looking for someone to blame for their troubles. Socrates was accused of impiety and offered the choice of exile or death; he chose death, since he felt that to leave Athens was a kind of living death anyway.
Socrates' ideas were preserved in the works of his student Plato, who established in Athens a school called the Academy that lasted for nearly 1000 years. According to the many dialogues of Socrates, which Plato claims to have recorded (but for which we have no other documentation, so Plato could have made some of them up), the enduring truth of a thing rests in the Idea of that thing. For example, a real chair is the Idea of the Chair which has all the appropriate essential attributes of chairness. If you have the Idea of the Chair firmly in mind, you can make many and varied chairs, but without the idea, you could not make even one. A physical chair (on which you might actually sit) is only manifestation or example of this Idea, but is an imitation at best. It can be manipulated, damaged, or even destroyed. A painting of a physical chair is as much (or as little) like the physical chair as the physical chair is like the Ideal chair.
Of course, Socrates (and Plato) were more interested in the reality of Beauty, Goodness, Justice, and in Matter itself, than in Chairness. Behind any solid material object, were the ideas of solidity and matter and shape. The simplest and most fundamental shapes were the five geometric regular solids. With three edges and three vertices (corner) to each face (making a triangle), and four faces, the simplest shape is the tetrahedron. Each edge is the same length, each vertice is the same angle. The cube, octahedron, dodecahedron, and icosohedron also share these characteristics.
When Socrates did get around to thinking about the nature of the universe, his position (recorded by Plato in The Timaeus) was that the Eudoxean universe of four elements and twirling spheres was "a likely story", and that any such story was as good as any other, as long as it "saved the appearances", or we would say, accounted for all the observations by relating them to something real. In the universe of The Timaeus all of nature is spherical and all the elements -- earth, fire, water, and air, have their counterparts in geometrical shapes. This relationship between element and shape was added to the already existing relationships detailed by Empedocles and Hippocrates between the elements and seasons, body parts, and disposition:
| Element | Earth | Water | Air | Fire | Ether |
| Shape | Cube | Icosohedron | Octahedron | Tetrahedron | Dodecahedron |
| Body fluid | Black bile | Phlegm | Yellow Bile | Blood | -- |
| Body part | Spleen | Brain | Heart | Liver | -- |
| Season | Autumn | Winter | Spring | Summer | -- |
This assertion preserved the concept that natural phenomena could be described mathematically throughout late antiquity and the medieval period, when a more strict division between physical and abstract sciences was dominant.
A second kind of characteristic of matter is that it moves. Empedocles' followers identified particular motions with each element. In his work The Physics, Aristotle explains that it is part of the nature of the elements earth and water to move downward, where down is always toward the center of the earth, while fire and air move upward, away from the center. Since earth's downward tendency is stronger than water's downward tendency, earth sinks through water. Only vertical motion to or away from the center of the earth is natural to these elements; any horizontal or sideways motion is the result of forces acting on the object. This theory tied motion and matter and human disposition so tightly together that it wasn't seriously questioned for nearly 1500 years.
Early Modern Chemistry
For much of the medieval and early Renaissance periods, the classification of matter by the European scientists remained firmly rooted in the Aristotelian tradition of earth, fire, air and water. Throughout the middle ages, alchemists tried to identify which combinations of these four elements made up different kinds of matter like wood, iron, gold, and silver. They were especially interested in producing precious metals.
The same spirit of scientific revolution that resulted in Copernicus' heliocentric theory and Vesalius' new ideas about human anatomy also created challenges to the traditional division of matter into four elements. During the Renaissance, most of the new theories still rested on the assumption that only a few elements were necessary to account for all the different kinds of matter. But Robert Hooke's investigations in the seventeenth century and the search for the element of fire in the eighteenth eventually led to the realization that many elements were necessary to account for all the combinations of matter.
The Renaissance Chemists
In the early sixteenth century, a German alchemist by the name of Philippus Aureolus Theophrastus Bombastus von Hohenheim (1493-1541) condemned the approach of Galen and Aristotle toward both matter and medicine. Writing under the name of Paracelsus, he proposed that inorganic chemical compounds should replace many of the organic and herbal remedies used for treating disease. He tried to create a new systematic division of matter using salt, mercury, and sulfur as the basic elements in place of earth, fire, water, and air, but he still mapped their properties to the ideas of heat and cold, wet and dry which had defined the Aristotelian elements.
Most of Paracelsus' works were published after his death, and his followers were often considered crackpots by their contemporaries and by the medical doctors, who preferred their herbal treatments to Paracelsus' new medicine. But the idea of iatrochemistry (chemical studies to produce medicines) seemed like a good idea to a number of people.
Among those who disagreed with Paracelsus' conclusions but thought alchemy or chemical studies should produce knowledge in the service of medicine was the Flemish scientist Joan-Baptista van Helmont. Like Paracelsus, van Helmont wanted to limit elements: he was radical enough to propose that all matter was a form of water and, in the case of living matter, some kind of organic force. But van Helmont was also a good observer, and he performed systematic experiments to test all of his theories. When he became convinced that he could not create air from water, he accepted it as a separate element. But he insisted that living matter was imbued throughout with some kind of spiritual or organic force, in contrast to theories of his contemporaries, Descartes and Newton, who claimed that the human body, except for the soul, could be explained in completely mechanical terms.
Another approach to the division of matter was proposed by a follower of Paracelsus, Franciscus Sylvius, who taught at the university of Leyden in the last half of the seventeenth century. Sylvius though matter could be divided into two opposing types, acids and alkalis. When combined, the two types of matter fought for awhile, then formed the neutral compound, water. He used as an example the common acid-base reaction between vinegar and baking soda. When the two are combined, the resulting solution fizzes violently for a short while, then settles down. Sylvius' theory was very popular, but could not account for a number of compounds which could not be clearly identified as acidic or basic.
The Skeptical Chemist
During the seventeenth century, a number of philosophers resurrected the Epicurean theory of atoms or corpuscles (small particles of different shapes and sizes) to replace the idea of Aristotelian elements, but most of their arguments were based on logical considerations, not on experimentation. In 1661, the English scientist Robert Boyle published a work called The Skeptical Chemist. Like Galileo's Dialogue Concerning Two World Systems, The Skeptical Chemist is presented as an argument between different positions: the sceptic, an Aristotelian, an Paracelsian, and a neutral observer. It reads like the script for a stage play. Unlike Galileo's work, however, Boyle's is rambling and difficult to follow, and so was not as popular as Galileo's work.
Boyle's work did, however, carefully point out the shortcomings of Paracelsus' three-element theory, van Helmont's water-only theory, and the acid-alkali theory. In place of the earlier limited-element theories, Boyle proposed that mater was made up of corpuscles which differed in shape, size, and natural motion. These differences accounted for the interaction of the different compounds and could be explained in purely mechanical terms without any reference to spiritual motivation or incorporeal substance.
Boyle's work was important for several reasons. His criticism of earlier alchemical systems was based on actual experimentation, and so were his own proposals. Before the publication of The Skeptical Chemist, the study of matter belonged to alchemy, which was not considered a respectable occupation for serious philosophers who considered alchemy akin to magic. Boyle's experiments and consideration of the mechanics of matter and its interaction placed the study of matter more firmly in the growing tradition of the physicists.
Boyle spent a lot of time investigating the properties of air. With the assistance of a young Oxford student, Robert Hooke (the same Robert Hooke who saw cork cells and fought Newton for recognition over his force theories), Boyle built an air pump and began to investigate how air and other substances interact. He placed birds, mice, and candles in his vacuum chamber, and determined that air was not an element, but contained a substance within it that was necessary to keep animals breathing, was consumed by fire, caused iron to rust, and copper to turn green. Although Boyle never identified oxygen as such, his air experiments inspired a number of other investigations into the nature of air and fire.
The Search for Phlogiston
Among those influenced to investigate the relationship between air and fire was Georg Stahl (1660-1734), who developed the phlogiston theory. Like Boyle, Stahl accepted the idea that matter was composed of corpuscles arranged in groups to form compounds. Stahl organized these corpuscles into three kinds of matter (fluid, oily, and fusible), plus water. Unlike the earlier Greek theorists, Paracelsus, and van Helmont, though, Stahl determined that not all bodies had to contain all four kinds of matter in combination. For example, not all bodies contained phlogiston; but those which did would burn in the atmosphere by ejecting phlogiston in flame or light. In the open air, such an object would burn until all its phlogiston was gone. In a closed container, however, the limited amount of air could hold only so much phlogiston, and once it became saturated, the object would cease to burn even if it still contained some phlogiston.
While this seems rather peculiar and even absurd to us, Stahl's theory had one important aspect which made it useful: one could burn objects and measure the changes to the burnt object (differences in weight) or to the air (differences in volume). More and more, investigators studying the properties of phlogiston used measurement and experimentation to gather data.
Joseph Black, a Scottish chemist, was actually a forerunner of Lavoisier. He was interested in the transformation of chalk into quicklime (CaO), a reaction which produced a gas, and the "slaking" of quicklime, in which water was absorbed by the quicklime. After many careful measurments, Black was able to show that the gas produced was not air. He called it "fixed air" (a gas we now identify as carbon dioxide). Black's 1756 publication of his results, Experiments upon Quicklime, demonstrated that there were gases other than air, a discovery which allowed Priestley and Lavoisier to realize that air itself might be composd of several gases, and ultimately to isolate oxygen from air.
Joseph Priestley a chemist and physicist and performed experiments on gases as well as studying electrical charged. Priestley isolated different "airs" or gases and then determined whether they would explode if sparked with an electrical charge, or support combustion. In April 1774, he isolated a colorless new gas by heating mercury oxide. He then burned a candle in the air successfully. But he was not able to dissolve this new gas in water in the same way that he had dissovled other gases. He called this new gas "dephlogisticated air." His publication on its properties were read with interest by the members of the Royal Society, and by the French chemist, Antoine Lavoisier.
Henry Cavendish was also a contemporary of Black, Priestley, and Lavoisier. Cavendish was interested in many things (he determined the value of G, the gravitational constant), but among chemists he is remembered for his discovery of "inflammable air", hydrogen, and a demonstration that inflammable air plus dephlogisticated air (oxygen) will produce water. Cavendish also spent much of his time determining the weights of equal volumes of gases. Later, when it was possible to determine that equal volumes of gases have equal numbers of molecules of the gas, Cavendish' data helped establish the ratios by which gases combine.
For example, hydrogen and oxygen (to use their modern names) combine in two volumes of hydrogen to one volume of oxygen to form water, but in 2 mass units of hydrogen to every 16 of oxygen. So by volume, the ratio is 2:1, but by weight it is 1:8. This means that a particle of hydrogen is 1/16 the weight of a particle of oxygen. By using this kind of analysis, chemists were able to determine the constitution of other compounds.
Antoine Lavoisier
[Painting in the Metropolitan Museum of Art, New York City. Photograph is mine. Notice Antoine's equipment!]
The breakthrough in the definition of chemical elements came through the studies of Antoine Laviosier (1743 -1794), a lawyer with a serious hobby in chemical investigations. Lavoisier believed that advances in his field could be made only if he achieved greater precision in his measurements, so when he received an inheritance from his father, Laviosier built his own laboratory. He was able to design special equipment for each experiment, so that his containers themselves would not react with the chemicals he was studying. His insistance on experimental accuracy was perhaps even a more enduring legacy to chemistry than his identification of many chemical elements.
Lavoiser never learned English, but his wife did, and she translated papers presented to the Royal Society so that her husband could keep up with the new discoveries by the English chemists Joseph Priestley and Henry Cavendish. She also ran a salon, or weekly gathering of intellectuals, for all of Lavoisier's friends, where they could meet and discuss their theories and experiments. Lavoisier was interested in all aspects of chemistry: the nature of acids and bases, the identification of the elements, the composition of compounds and their properties.
Like his contemporaries, most of Lavisier's experiments were investigations into the nature of air. He performed numerous experiments in which he measured the compounds going into a reaction (the reactants) and the products of the reaction. As a result of his experiments, he decided that phlogiston did not exist, and that in all reactions, the total amount of matter was conserved, even if it changed form. Finally, he published a new list of elements which differed from all his predecessors by its length:
| Light | Suphur | Antimony | Mercury | Lime |
| Caloric | Phosphorus | Arsenic | Molybdena | Magnesia |
| Oxygen | Charcoal | Bismuth | Nickel | Barytes |
| Azote | Muriatic radical | Cobalt | Platina | Argilla |
| Hydrogen | Fluroic radical | Copper | Silver | Alumina |
| Boracic radical | Gold | Tin | Silex | |
| Iron | Tungsten | Silica | ||
| Lead | Zinc | |||
| Manganese |
A number of Lavoisier's elements are now known to be compounds; for example, lime is a combination fo calcium and oxygen, and muriatic radical is muriatic acid. But his list broke the down the tendency to look for a very few simple elements as the basis for all matter.
Unfortunately for the advancement of chemistry, Lavoisier drew some of his income from investments in a company that collected taxes. As a result of his connection with both the aristocracy and a corrupt corporation (which he actually seems to have tried to reform), Laviosier was guillotined during the French Revolution.
We have now seen how Lavoisier built on the work of Stahl, Priestley, and earlier experimenters in creating his list of chemical elements and their compounds and reactions. Several other important discoveries were also necessary to establish a coherent approach to chemcial reactions. One was the realization that compounds had fixed proportions, like a recipe for a cake. Another was the resurgence of the idea that matter could be broken down into descrete particles, or atoms. We've already used this concept in the science section last week; now we see how it was proposed by John Dalton and gradually accepted during the 19th century.
John Dalton's Atomism
The idea that matter can be broken down into indivisible bits with specific characteristics goes all the way back to Democritus and Leucippus, early Greek philosophers who lived before Aristotle. Aristotle and his contemporaries could not observe atoms, and so sought other explanations for the properties of matter. But during the eighteenth century, the atomic theory became popular and was supported by Newton, Descartes, and Leibniz, although without any serious experimental evidence. The one who was able to bring together the discoveries of Lavoisier, Priestley, and Black with the atomist theories of Descartes and Leibniz was a modest, color-blind Quaker by the name of John Dalton.
Dalton resurrected the idea that minute individual particles make up all matter. Each type of particle or atom had specific characteristics which depended on the atom's shape and size. Atoms of the same type were of the same element and behaved the same way. All atoms of gold, for example, have the same specific characteristics (weight, ability to combine with other elemental atoms). All atoms of oxygen share the same specific characteristics also, but these are different from the characteristics for atoms of gold or other elements. This theory has some important ramifications.
Dalton's law of partial pressures follows naturally from the idea that gases are made up of individual particles which act independently. If a sample volume has two gases, the total pressure on the container is the sum of the pressures exerted by each of the gases, just as though the other was not present: PTOT = PGas1 + PGas2
Dalton's major contribution was the realization of the consequences of assuming atoms and multiple elements as the basis for all matter. Although published in the early 1800s, Dalton's original observations on how elements combine are so well attested that we now call them the three basic laws of chemical composition and apply them to all macro-atomic (bigger than the atom) or chemical reactions.
The conservation of mass is a basic principle of the universe. It means that (excepting whatever initial incident got us here) no mass is now being created or destroyed. In an ordinary chemical reaction, there is no difference between the mass of the reactants and the mass of the products. In a nuclear reaction, any mass loss is compensated by a release of energy equal to the mass loss times the speed of light squared (Einstein's famous E=mc2). This principle, of course, had already been discovered by Lavoisier, and indeed, had been stated centuries earlier by Francis Bacon (1600s) but without experimental proof. Dalton and Lavoisier were able to demonstrate that mass is always conserved.
The law of constant composition means that a compound is always formed out of the same components. Water is always made of two parts of hydrogen to one part oxygen. Changing one of the components changes the compound into something else.
For example, both water and hydrogen peroxide are composed of hydrogen and oxygen. But water is in the proportion 2 volumes of hydrogen gas to 1 volume of oxygen gas, or 2 masses of hydrogen to 16 masses of oxygen, a 1:8 proportion by weight. Hydrogen peroxide is formed from 2 masses of hydrogen to 32 masses of oxygen: a 1:16 proportion by weight. Water is always 1:8, hydrogen peroxide is always 1:16. The exact proprotions are constant for a given compound, and identify the compount.
The law of multiple proportions means that elements combine in whole-number ratios (because the atoms are particles which are combining). This makes it much easier to determine the composition of complex molecules. It also makes sense only if whole particles like atoms are combining. We never see combinations that imply half or a third of an atom is part of a compound.
The Ninteenth Century Chemists after Dalton
The French chemist Joseph Gay-Lussac, following Dalton's work, showed that volumes of gases combine in whole number ratios: one volume of nitrogen combins with three volumes of hydrogen to form two volumes of ammonia gas, for example. Amedeo Avogadro, an Italian, took this concept further, and published a theory in 1811 that if there were such a simple numerical relationship between the volumes of the combining gases and their products, there must be a simple connection between the actual numbers of particles in equal volumes of the gas. He proposed that these particles (which could be made of compounds of individual atoms) be called molecules, or little masses. Then equal volumes of gases under the same conditions of temperature and pressure would contain the same number of molecules.
Avogadro's proposal was not particularly well received by his contemporaries, however; his work remained obscure and unused for nearly half a century, until it was rediscovered and used in the 1860s, after Avogadro's death. The chemists of the period did honor Avogadro by naming a physical constant for him. Avogadro's number, 6.02 * 1023, is the number of molecules or atoms in a mole of substance, where a mole is equal to the gram weight matching the atomic weight of an element. For example, the atomic weight of hydrogen is one; so one gram of hydrogen has one mole of hydrogen molecuels. The atomic weight of oxygen is eight, so eight grams of oxygen contain one mole of oxygen atoms. One gram of hydrogen gas 6.02 * 1023 hydrogen atoms in it; eight grams of oxygen also has 6.02 * 1023 atoms, this time of oxygen, it them.
Other chemists like William Prout, Jons Berzelius, and Humphrey Davy devised thousands of chemical experiments to determine which of the remaining substances were elements or compounds, and if they were compounds, what was the chemical composition of each. A major breakthrough occured in 1859, when Gustav Kirchhoff and R. W. Bunsen (of Bunsen burner fame) showed that there was a connection between the spectrum, or pattern of light, given when matter burned and certain kinds of matter. This made it possible to identify components of matter by analyzing the light given off by a sample raised to a temperature where it would glow. Using this method, Bunsen was able to identify hitherto unexpected elements, including thallium, indium, and gallium.
With a wealth of data before him, the Russian chemist Dmitri Mendeléev (or Mendelyeef) began to organize the elements into a sequence of properties, which he divided into families and periods. He left gaps where his properties predicted an element for which non was yet known — and so predicted the existence of gallium. The result was the first periodic table of elements, in which each "family" of elements with the same characteristics was organized in a column, while the rows were organized into elements which increased in mass from left to right. It was a wonderful discovery, with one major flaw: it left out an entire family.
The final piece of the table was the discover of the Scottish chemist William Ramsay, who performed his experiments in the last decade of the 19th century. Ramsay became interested in helium, the substance Cavendish had identified as an inert gas nearly a century earlier. Cavendish never claimed hydrogen was an element, and had gone on to other researches, but Ramsay became interested in several new and similar gases, all of which refused to form compounds.
Discussion Questions
- What kinds of characteristics do we associate with matter now that are different from those used in the ancient world or the medieval world?
- What patterns of behavior in matter became "laws of chemistry"?
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