CHAPTER XVII 



MOLECULAR MIXTURES 



THE law for the pressure l of a mixture of two or 

 more gases resulted from experiments by Dalton which 

 originated in his desire to explain the mixture of gases 

 in which men were just realizing that they were living. 

 In our study, some 120 years later, we may start from 

 the concept of a molecular and atomic composition 

 and thus obtain general expressions for the molecular 

 condition of homogeneous substances. In this chapter 

 we shall consider mixtures of molecules. 



In the case of a mixture of a gas and a liquid it is 

 usual to say that the gas is dissolved by the liquid. 

 Consider the case where there is no chemical action 

 between the two kinds of molecules. Let the liquid 

 be contained in a cylinder which we then fill with gas 

 and compress by a piston. Above the liquid surface 

 there is a mixture of two aeriform substances, namely 

 the gas and the vapor of the liquid, and below the 

 surface the mixture of liquid and gas which we call a 

 solution. 2 The pressure on the piston is the sum of 



1 Dalton's law is obviously only an approximate relation. Chem- 

 ically inert gases, provided their volumes are well above the critical 

 volumes, act like perfect gases, in that the total pressure due to 

 molecular impacts is the sum of the partial pressures which the 

 molecules of each gas would exert. 



2 There is no hard and fast line between a mixture and a solution 

 when the composition of the molecules is not altered. The substance 

 of smaller amount is usually said to be dissolved in that of larger 

 amount, the solute in the solvent. 



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