Element Argon Ar, Noble Gas
|The nitrogen obtained from the air differs from the " artificial," i.e. obtained from chemical compounds, in having a somewhat greater density. This at first puzzling phenomenon was finally explained (Rayleigh and Ramsay, 1894) by the fact that in atmospheric nitrogen another gas is contained which resembles nitrogen in its disinclination to form chemical compounds, and indeed, in this respect, is considerably its superior. |
By converting the nitrogen of the air into non-gaseous compounds, the other constituent, which has been called Argon, can be obtained pure. For this purpose there may be employed, for example, the property of nitrogen of combining with oxygen under the influence of the electric discharge. The nitrogen peroxide thus formed is absorbed by caustic soda, and by adding the necessary amount of oxygen the reaction can be continued till all the nitrogen is used up. The excess of oxygen can then be easily removed by means of heated copper or phosphorus. The same end is attained by the use of certain metals, e.g. magnesium or lithium, which readily absorb nitrogen at a red heat. A mixture of lime, magnesium, and some sodium has been found very suitable.
The residual gas is colourless, odourless, and tasteless, and has, in accordance with its density, the molar weight 40. It is, therefore, considerably more dense than nitrogen and oxygen. In the air it forms the 0*009 part by volume and the 0.012 part by weight, and the ratio of it to the other constituents of the air is not subject to any appreciable variations.
Since the gas does not form any compounds with other elements, no combining weight, properly speaking, can be assigned to it. On the basis of the law of Gay-Lussac, it may, however, be assumed that if it did form any compounds, these must be formed with other gases in simple ratios by volume, and that, therefore, the normal weight 40, or some fraction of it, must be equal to the combining weight. What this fraction is, however, cannot a priori be stated.
A decision can be here arrived at by means of the relation which has been found to exist in the case of other gases between the composition and the capacity for heat. By capacity for heat there is understood the ratio of the heat communicated to a body to the rise of temperature produced. This ratio is evidently all the greater, the greater the amount of substance subjected to the experiment. If it is referred to one mole of the substance, this special capacity for heat is called the molecular heat or molar heat of the particular substance.
If the amount of heat be measured in joules, and the changes of temperature, as usual, in centigrade degrees, the following are the molecular heats of a number of gases -
Oxygen – O2 - 21
Nitrogen – N2 - 20
Hydrogen – H2 - 20
Nitric oxide – NO - 21
Carbon monoxide – CO - 20
Hydrogen chloride HCl - 20
Carbon dioxide – CO2 - 32
Nitrogen oxide – N2O - 33
Water vapour – H2O - 28
Phosphorous chloride – PCl3 - 68
Chloroform – CHCl3 - 69
The smallest values of the molecular heats are, accordingly, 20, and are found in the case of those gases which contain two combining weights in the molar weight; it is thereby a matter of indifference whether the combined elements are like or different.
On determining the molecular heat of argon, however, the value 12 is obtained - a value, therefore, which is much smaller than that for all the gases given. This leads to the presumption that argon is still more simple in composition than these gases, i.e. that its molar and combining weights coincide, and that the formula of gaseous argon is given by the simple symbol Ar, and not Ar2.
This presumption can be tested by analogy. From the chemical behaviour of mercury, the same conclusion has been drawn; mercury vapour must also have the formula Hg and not Hg2, since the combining weight and the molar weight have both been found equal to 200. As a matter of fact, the determination of the molecular heat of mercury has yielded the value 13.
There is therefore sufficient reason for assuming the identity of the molar and combining weights of argon, and for ascribing to this element the combining weight 40, whereby the formula of gaseous argon becomes Ar.
For the rest, argon behaves similarly to the other gases. At -186°, under ordinary pressure, it becomes liquid. At -188°, it solidifies.
If electric discharges are passed through rarefied argon, a spectrum of numerous lines is obtained. According to the pressure and the electrical conditions, three different spectra are obtained, the light in the tube appearing blue, red, or white.\n
History of Argon
|Main article: History of Argon discovery |
A curious example of the omissions that sometimes come to light during the investigation of common substanpes is found in the fact that from 1785 till 1894 no complete and exhaustive examination of the homogeneity of atmospheric nitrogen was attempted. In the former year Cavendish published his Experiments on Air, in which he investigated this point as minutely as his methods and apparatus would permit; but that he recognised his own limitations is evident from the following sentence written at the conclusion of his paper: "... if there is any part of the phlogisticated air of our atmosphere which differs from the rest, and cannot be reduced to nitrous acid, we may safely conclude that it is not more than 1/120th part of the whole.
Occurrence of Argon
|Main article: Occurrence of Argon |
Argon is widely distributed in the free state in nature. As has already been mentioned, it is a constant constituent of the atmosphere. It forms 0.941 per cent, by volume of air freed from carbon dioxide and moisture, and 1.1845 per cent, by volume of atmospheric "nitrogen." The substantial accuracy of these figures has been verified by calculating the density of atmospheric nitrogen from them in conjunction with the known densities of pure argon and nitrogen. Results are thus obtained which agree closely with the values found experimentally by Rayleigh.