Compounds Display Constant Composition

A FIGURE 5.4 A mixture This balloon is filled with a mixture of hydrogen and oxygen gas. The relative amoimts of hydrogen and oxygen are variable. We could easily add either more hydrogen or more oxygen to the balloon. 

A FIGURE 5.5 A chemical compound This balloon is filled with water, composed of molecules that have a fixed ratio of hydrogen to oxygen. (Source JoLynn E. Funk.) 

Even though atoms combine in whole-number ratios, their mass ratios are not necessarily whole numbers. 

The first chemist to formally state the idea that elements combine in fixed proportions to form compounds was Joseph Proust (1754-1826) in the law of constant composition, which states  

All samples of a given compound have the same proportions of their 

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In addition, miscible liquid-solid systems can display phase behavior more complex than vapor-liquid systems. For example, mixtures of carbon tetrachloride and cyclohexanone form a compound from one molecule of each pure this compound (xj = 0.5) melts at -39.6°C. Below this temperature, the compound exhibits two minimum melting temperatures so the melting curve for this binary has three extrema, two minima and a maximum, and all three lie below the melting points of the pure components. Compound formation in a solid phase can also cause constant-composition melting without an extremum in temperature. This occurs in mixtures of bromine and iodine. At 40°C the compound IBr melts at constant composition, although this temperature lies between the melting points of pure iodine and pure bromine. Phase diagrams for these kinds of solid systems can be found in the book by Walas [5]. 

It has been proposed [44, 46, 48] to use Equation 1.20 for open systems, the composition and values of which vary negligibly. Further, the possibility of application of this equation was displayed for various chemical compounds melting at < 373 K and condensing at constant standard temperature T= = 298 K [46-49]. In a more strict approach, for these cases Equation 1.20 should be represented as follows ... 

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