James Richard Fromm
The atmosphere or air which surrounds us is an almost uniform gas mixture of roughly 80% nitrogen and 20% oxygen, with traces of other gases. Once Van Helmont had shown that different gases existed, as discussed in a different section, the chemists of the eighteenth century quickly characterized four: carbon dioxide, nitrogen, hydrogen, and oxygen.
The first gas prepared and truly characterized as a pure substance was carbon dioxide, CO2(g). Carbon dioxide was carefully studied around 1750 by Joseph Black (1728 - 1799, Edinburgh), who named it fixed air. Black, a medical man, presented his studies in 1754 as a doctoral dissertation entitled "On the Acid Humour Arising from Food, and Magnesia Alba". Magnesia alba, or white magnesia, is a complex compound whose structure is xMgCO3.yMg(OH)2.zH2O; the values of x, y and z can vary. Suspensions of this material are still sold, under the name of milk of magnesia, as a remedy for acid stomach. Black realized that the reaction which took place on heating magnesia alba were analogous to those which took place on heating limestone. That is, the reaction:
magnesia alba quicklime + fixed air (on heating)
limestone + acid a salt + fixed air
quicklime + acid a salt
We now know that the reactions which take place are actually:
xMgCO3.yMg(OH)2.zH2O (x+y)MgO +(y+z)H2O + yCO2
xMgCO3.yMg(OH)2.zH2O + (2x+2y)HCl (2x+2y+z)H2O + xCO2 + (x+y)MgCl2
MgO + 2HCl MgCl2 + 2H2O and CaCO3 CaO + CO2
CaCO3 + 2HCl CaCl2 + 2H2O + CO2
CaO + 2HCl CaCl2 + H2O
The alkalies then known included these materials and comparatively few others; all existed both as a "mild form" and a "caustic form". The older names and modern formulas for these are given in the table below.
|Vegetable Alkali (potash), mile form||K2CO3|
|Vegetable Alkali (potash), caustic form||KOH|
|Marine Alkali (soda), mild form||Na2CO3|
|Marine Alkali (soda), caustic form||NaOH|
|Volatile Alkali (ammonia), mild form||(NH4)2CO3|
|Volatile Alkali (ammonia), caustic form||NH4OH, actually NH3 + H2O|
Only recently has it been realized that ammonium hydroxide, NH4OH, actually exists as hydrated ammonia, NH3.H2O, rather than as a hydroxide species. Black could use some of them to obtain information about fixed air. Fixed air is, or behaves as, an acid. Since:
limestone quicklime + fixed air, and
quicklime + mild alkali limestone + caustic alkali, then
mild alkali caustic alkali + fixed air.
Although fixed air had no commercial use when it was discovered, carbonated beverages or "mephitic juleps" were first prepared by Joseph Priestley, the discoverer of oxygen, in London in 1772. The taste of carbonated water is due partly to the evolution of carbon dioxide from its solution but mostly to the formation of carbonates and hydrogen carbonates in solution.
Most of our atmosphere is made up of the diatomic gas nitrogen, N2. The credit for the discovery of nitrogen is unknown, although it was clearly distinguished around 1772. In addition to studies by Cavendish and by Priestley at this time, work was done in 1770 - 1773 by Scheele and by one of Black's students, Daniel Rutherford, in his thesis of 1772. Scheele noted that air could be trapped and a part of the air removed by a chemical reaction, such as absorption by linseed oil or reaction with moist iron filings; the remaining air will not support combustion. Daniel Rutherford, who later became a professor of botany, permitted mice to breathe air until suffocation; the fixed air produced by them was removed by caustic potash. The remaining air, which does not support life or combustion, is not fixed air because it does not give a precipitate with limewater. However, like fixed air, it does not support life or combustion and so it was called dead air.
The diatomic gas we know as hydrogen, H2(g), was once called inflammable air. Credit for the discovery of hydrogen goes to Henry Cavendish (London, 1731 - 1810) who described it in his work "On Factitious Airs" published in 1766. By factitious airs Cavendish meant airs, or gases, produced by chemical art rather than found in nature. Hydrogen gas is not a normal constituent of the atmosphere of the earth. The gas was produced by reacting an acid with any of several metals:
H2SO4 or HCl + Zn or Fe or Sn inflammable air + a salt
This gas had a very low density, which Cavendish measured. Cavendish was also responsible for development of a method for measuring the weight of fixed air present by absorbing it on pearl-ash (KOH) which was weighed before and after. This method is still used in the analysis of organic compounds, as we shall see in the following sections.
From many points of view the most important gas we know is the diatomic gas oxygen, O2(g). Oxygen, which makes up about one-fifth of our atmosphere, was given the early names of fire air and dephlogisticated air because it has such an obvious and central role in combustion. Credit for the discovery and preparation of oxygen is generally given to Joseph Priestley (England, 1733 - 1804), who prepared it by heating the red oxide of mercury, HgO, in 1774. In modern terms the reaction is:
HgO + heat Hg + 0.5 O2 (g),
but this was not Priestley's interpretation. He considered air to acquire or or lose phlogiston; thus:
air + X phlogisticated air (nitrogen, N2)
air - X dephlogisticated air (oxygen, O2)
The measure of phlogiston gained or lost required a method for measurement of the degree of phlogistication of air, or a measure of the goodness of air. The method used by Priestley to measure this was its slow reaction with nitrogen dioxide, NO2:
4NO2(g) + O2(g) 2N2O5(g)
N2O5(g) + H2O 2HNO3
The dinitrogen pentoxide gas formed immediately dissolves in the water above which the gas is confined. Thus, the relative decrease in volume on reaction with nitrogen dioxide gas is a measure of the goodness, or the degree of phlogistication, of a sample of air.
The work of chemists from Boyle onward has been based on the idea that a chemical element is a pure substance which can not be broken down into any simpler substances by chemical means. Although we now know that nuclear reactions can actually convert one element into another, these lie outside the normal province of chemistry. Most of the substances found on earth are mixtures containing several different substances. Mixtures can be separated by physical means into the different pure substances which are their components. The trinity of principles of the alchemists, mercury, sulfur, and salt, could all be prepared as pure substances, as could water and metals. Reactions of pure metals with air produced pure metal oxides, and reactions of these oxides with water produced other pure compounds. Through the seventeenth century, it became clear that most pure substances are not elements but compounds. Compounds could be made or synthesized from their pure component elements, and in many cases broken back down to elements again or analyzed. The number of pure compounds known grew quickly, and the number of elements known grew more slowly, through both the seventeenth and eighteenth centuries.
Pure substances, both elements and compounds, engage in well-defined chemical reactions with each other to produce new pure substances or, in many cases, mixtures of substances. Pure substances which engage in chemical reactions came to be known as reagents. Substances of the highest available purity, which are the most desirable for the quantitative study of chemical reactions, became known as reagent-grade materials. The specifications for modern reagent-grade chemicals are published in most countries by their national governmental standards office or by their national society of chemists because they are so important.