Synthesized answer
Based on the provided passages, the atmosphere is classified as a mixture because its gaseous constituents retain their individual properties, unlike the constituents of a compound. The key evidence is that the density of air agrees with the mean of the densities of its constituents, taken in the proportion in which they occur [2]. This is stated to be the case with compound gases only when the constituents are present in equal proportions by volume, which air does not fulfill [2]. Furthermore, the constituents of air can be separated by physical means, specifically by diffusion, as demonstrated by Thomas Graham's discovery that gases pass through a porous partition at rates inversely proportional to the square roots of their densities [2].
This classification is simplified compared to other substances because, in the case of mixtures of gases, "each gas retains its individual properties" [1]. The passages contrast this with the difficulty of distinguishing between a mixture and a compound in other cases, such as a solution of carbon dioxide in water versus a solution of sugar in water, where analogy is necessary [1]. The passages conclude that for these reasons, "air is a…
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From the book
Again, it is usually accepted that the separation of the constituents of a mixture may be effected by mechanical, or at least by physical means ; whereas the separation of the elements from a compound require chemical treatment. Here it is difficult to draw a sharp distinction. The VOL. II. A 2 MODERN CHEMISTRY separation of carbon dioxide from soda-water by the appli- cation of heat is similar in character to the separation of sugar from water by evaporation of the water ; yet we believe that a solution of carbon dioxide in water consti- tutes a compound, while that of sugar in…
if small amounts of water-vapour, of carbon dioxide, and of ammonia, all of which vary considerably in amount, be subtracted. This can be shown by several lines of argument. First, The density of air agrees with the mean of the densities of its constituents, taken in the proportion in which they occur. Thus, the density of the mixture of atmos- pheric nitrogen and argon differs by only I part in 40,000 from that calculated from their relative weights, and the proportion in which they occur. This is the case with compound gases only when the constituents are present in equal…
For these reasons, and other similar ones, it is concluded that air is a mixture. 4 MODERN CHEMISTRY The Analysis of the Atmosphere is, however, always performed by chemical means, for the difference in physical properties of its constituents is not sufficiently marked to allow of their being utilised for purposes of separation. Many common elements unite easily with oxygen to form non-volatile compounds, when they are heated in air. One of the most convenient for this purpose is metallic copper. By passing a known volume of air over copper turnings, contained in a counter- .…
Reference has already been made in Part I. to the different processes which may be used for the isolation of elements from their compounds. But there exists a group of elements, that of which the first member is helium, which form no com- pounds, and which therefore are found only in a free state. It is, therefore, convenient to begin with these. The HELIUM Group. — These elements are all gases at the ordinary temperature of the atmosphere, and they are consequently all to be found in atmospheric air. They are colourless, even in the liquid condition, and are devoid of smell and…
owing to the combination of the oxygen which it contains with most other elements. Now, when air passed through a tube full of a mixture of caustic soda and lime, to remove carbon dioxide, and then through a U-tube containing sul- phuric acid, to deprive it of water- vapour and ammonia, is led over red-hot copper, or over some other red-hot metal which unites with oxygen, the oxygen is retained, and nitrogen with members of the helium group alone passes on. The nitrogen can be removed in one of two ways. The first plan is due to Cavendish, who attempted to prove that atmospheric…
More questions about this book
- How did the historical lack of distinction between mixtures and compounds, and John Dalton's subsequent formulation of laws, fundamentally alter the scientific approach to classifying chemical substances?
- The text states that "a mixture may be homogeneous; a compound must." Explain why homogeneity alone is not a sufficient criterion to definitively classify a substance as a compound, using examples from the text.
- Why does the author suggest that the separation of carbon dioxide from soda-water indicates a compound (carbonic acid), while the separation of sugar from water indicates a mere mixture, despite both separations involving heat? What specific chemical principle or "analogy" guides this distinction?
- Based on the challenges and nuances presented in the text, what are the two most crucial criteria you would use to distinguish between a mixture and a compound, and what are the primary difficulties or exceptions you would highlight when applying these criteria?