Nineteenth-Century Advances in Understanding Gases, Culminating in William Ramsey's Discovery of Inert Gases in the 1890s

Overview

The nineteenth century saw many advances in our understanding of gases, their behavior, and their uses. Charles, Gay-Lussac, Faraday, Avogadro, and others contributed to this knowledge, formulating some of the basic laws that are known to govern the behavior of gases under a variety of physical and chemical conditions. This knowledge, the ability to liquefy and to compress gases, and other advances have led, in turn, to a great many inventions and discoveries that have profoundly impacted the life of virtually everyone on Earth.

Background

It was not until the early seventeenth century that scientists began to realize that gases exist as a separate form of matter. In the mid seventeenth century (1660-1662) several scientists independently determined that, as the pressure on a gas changes, so does its volume. This fact, now called Boyle's law (or, in continental Europe, Mariotte's law after Edmé Mariotte, who discovered it independently) states that PV = K, a constant, for any gas. That is, the absolute pressure P multiplied by the volume V is constant. This means that, if a gas is compressed (decreasing the volume) the pressure must increase by the same relative percentage so that this equation remains constant. Put another way, reducing the volume by one half must cause the pressure to double.

During the remainder of the seventeenth and eighteenth centuries the primary work done in these areas was in the area of separating and identifying various gases from air. During this time oxygen, nitrogen, and hydrogen were among the gases isolated and identified as separate chemical elements.

The early part of the nineteenth century saw a renewed interest in the physical relationships that govern the behavior of gases as well as ways to manipulate them. Charles' law (also known as Gay-Lussac's law) states the relationship between gas temperature and pressure, noting that pressure increases as temperature is raised. Avogadro combined Boyle and Charles' laws into a single equation, the Ideal Gas Law, which is stated PV = nRT. In this law, n refers to the number of moles (or molecules) of gas present, R is the universal gas constant, and T is the temperature of the gas on the absolute scale, starting at absolute zero. This law, while it does not work at extremes of temperature or pressure, is nonetheless useful and accurate under most conditions and is still widely used by physicists and chemists today.

In addition to understanding the physics and chemistry of gases, scientists were learning to manipulate gases. In 1823 Faraday liquefied chlorine and, soon afterwards, liquefied carbon dioxide and hydrogen chloride. Others learned to compress gases, distill off specific gases from liquid air, and more.

As these manipulations were taking place, others were identifying new gases, notable for their apparent refusal to participate in chemical reactions. Sir William Ramsay first isolated argon from liquefied air in 1894. In quick succession, Ramsay also identified helium (detected in 1868 through spectroscopic observation of the Sun), neon, krypton, and xenon and, with the identification of radon in 1900, all the known noble gases. Ramsay later won the Nobel Prize in Chemistry (1904) for these discoveries.

Impact

The impact of these discoveries falls into two general areas: 1. Scientific study of gases led to many advances, particularly in the studies of thermodynamics, the periodic table, atomic structure, and 2. Industrial and commercial inventions and advances, including compressed and liquefied gases for industrial use, liquefied gases for rocket propulsion, development of refrigeration and air conditioning, and lights using ionized noble gases.

Each of these has had far-reaching consequences that are described in more detail in the following paragraphs.

The work of Avogadro, Charles, and Gay-Lussac all noted the relationship between the pressure and temperature of a gas. All of these scientists realized that this relationship depended on some absolute temperature, that gas temperature and pressure depended in some way on the average speed of gas particles (either atoms or molecules), and that there must exist a temperature at which these motions must stop. William Thomson (later Lord Kelvin) determined the temperature at which all molecular motions must stop, the temperature now known as absolute zero because it is thought to be impossible for colder temperatures to exist. Recent investigations at such low temperatures have led to the creation of the Bose-Einstein Condensate, a long-sought condition in which large numbers of atoms act as a single large atom. This, in turn, has shed new light on the properties of matter under extremely cold conditions and has resulted in the creation of "atomic lasers." It may also lead to new, highly accurate clocks that might replace current atomic clocks by being both smaller and more accurate.

The properties of the noble gases were not predicted by the periodic table, but their occurrence certainly supported the version published by Mendeleev in 1869 because they appeared at precise intervals. Their extreme unwillingness to undergo chemical reactions eventually led to theories of electron shells, suggesting that atoms react to form chemical compounds, sharing electrons so that the outermost shells contain eight electrons. This realization is one of the cornerstones of chemistry and helped form, in turn, theories of atomic structure that were so important during the revolution in physics that took place in the early twentieth century. In addition, understanding that outer electron shells contain up to eight electrons suggested that other shells may also contain specific numbers of electrons. Understanding this provided a physical explanation for the periodic table—the various rows and groups reflect the filling of electron shells in each successive element. The innermost shell, containing only two electrons, is completely filled with helium, a noble gas. The next shell, with eight electrons, is filled sequentially by the next group of elements, from lithium to neon, another noble gas. This theory has proved extremely successful in explaining the behavior of various elements, why chemical reactions take place, and why the periodic table looks as it does.

As important as these scientific advances were, the applications of research into gases are probably even more important to most people on earth. Compressed gases are used in scientific research, industry, and more. Inert gases are equally important. Argon is used in welding to flood the work area during TIG (tungsten-inert gas) and MIG (metal-inert gas) welding to reduce oxidation of the metals being welded, making a stronger weld. Argon and helium are used as inert atmospheres to keep reactive or sensitive items from chemical harm. Neon, krypton, and xenon are used for lighting, and argon is used to fill Geiger-Müller tubes that detect radiation. Argon is used to keep reactive metals such as titanium, uranium, plutonium, and zirconium from oxidizing or catching fire during processing.

Another area in which knowledge of gases has proved a boon to humanity is in the development of air conditioning and refrigeration. These technologies depend on the behavior of gases under carefully controlled variations of pressure and were not developed until 1876, when changes in gas temperature at different pressures could be predicted and quantified.

In both cases, a cold refrigerant gas passes through a heat exchanger inside the refrigerator (or freezer) unit where the cold refrigerant absorbs heat. This warms it up and passes on to the compressor. Here, the warm gas is compressed, achieving a high pressure and high temperature. In this state it is passed through a heat exchanger (which, in a household refrigerator can be seen on the back of the unit). In the heat exchanger, the refrigerant is hotter than the air around it, so it gives off heat to the atmosphere, cooling it down and liquefying it. The next step is to pass the warm liquid through a small thermal expansion valve (sometimes called a TXV). This valve is almost completely shut, so the pressure on one side is much lower than on the compressor side. Because of this, the liquid that passes through the valve turns into a gas immediately and, in accordance with the Ideal Gas Law, it cools by the same proportion. So, if pressure goes down by a factor of 10, the temperature drops by the same amount, usually to below freezing for both refrigerators and freezers.

The impact of refrigeration and air conditioning on human society has been great. Air conditioning has made it possible to inhabit many parts of the earth that were formerly inhospitable or that were very difficult to live in. Such areas include many of the tropical regions, much of Australia, and the desert areas. Human life was possible in these areas, of course, but was never comfortable. It has also made possible parts of the industrial and technological revolutions since human operators of high-temperature equipment require cool temperatures and because many pieces of electronic equipment require precisely controlled environments, regardless of exterior weather.

In addition, these technologies have been used to preserve foods more effectively. In the early twentieth century it was not uncommon for households to shop daily or, at most, every few days for fresh produce, meats, and dairy products because there was no way to keep food fresh. A great deal of food grew moldy or rotted in storage, and food could only be transported a short distance for similar reasons. The development of relatively cheap refrigeration technology has made it possible to shop less frequently, to store foods almost indefinitely in the frozen state, to ship foods around the world, and to minimize waste during shipping and storage. Indeed, some foods such as ice cream only exist in the frozen state and owe their current popularity to the existence of cheap freezers. Like advances in air conditioning, these, too, are due in large part to the greater understanding of gases that took place during the nineteenth century.

P. ANDREW KARAM

Further Reading

Atkins, P.W. The Periodic Kingdom. New York: Basic Books, 1995.