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Constitution of Ozone

Although Schonbein was quite definite in his views of ozone as a distinct substance, the general confusion with hydrogen peroxide vapour retarded the development of the subject. Andrews and Tait, in their formation of ozone by the action of the electric discharge on pure oxygen, demonstrated that ozone was an allotropic form of oxygen of higher molecular weight. Soret then discovered that ozone is completely absorbed by turpentine, and was able to demonstrate that the decrease in volume during the ozonisation of oxygen is approximately one-half the decrease observed when the ozone is subsequently absorbed by turpentine; assuming that the ratio is actually 1:2, it is easily seen that the total loss in oxygen is 1½ times the volume of ozone, indicating for ozone a molecular weight 1½ times that of oxygen. The correctness of the assumption as to the ratio of the two volume changes was demonstrated later by Brodie whose experiments yielded the figures 1:202, and so indicated in a convincing manner that the chemical change in the formation of ozone was to be represented

3O2 = 2O3.

Soret also confirmed his earlier result by an independent investigation based on the relative velocity of diffusion of mixtures of oxygen with ozone, carbon dioxide, and chlorine respectively, in which he was able to show that the vapour density of ozone was a little higher than that of carbon dioxide, but distinctly lower than that of chlorine. This experiment has been repeated with greater accuracy by Ladenburg who used a gas containing 84.4 per cent, of ozone obtained by evaporation of liquefied ozone, and, from the velocity of diffusion, was able to calculate a density 1.3698 referred to oxygen which leads to the value 1.469 instead of the theoretical 1.5 for pure ozone of molecular formula O3.

Recent determinations of the vapour density of pure ozone confirm the value 48 for the molecular weight. No tendency to associate to higher molecules has been observed either in the pure liquid or the gas.

The greater molecular complexity of ozone relative to oxygen is also distinctly discernible in the ratio of the specific heat at constant pressure and constant volume. For ozone this ratio has the value 1.29 which approximates closely to that expected of a triatomic gas, whilst for oxygen the value is 1.404.

Various suggestions have been made as to the structure of the ozone molecule, the most favoured being

According to Bruhl, the last representation is in closest agreement with the specific refraction of the gas, and it also gives at least as easy an explanation as the others of the readiness with which the ozone molecule eliminates one atom leaving a stable molecule of oxygen.

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