ELKCTRU-AIOTIVE SERIES OF THE MliTALS. 25 1 



centration of acid, the concentration of the hydrate must be a 

 maximum. The actual composition of the mixture at wliich this 

 occurs depends on tlie formula of the hydrate. Addition of either 

 pure acid or 'water to acid of such a composition aneans a decrease 

 in the coiicciitraiioii of the hydrate, although not necessarily a 

 decrease in the actual amount of hydrate formed. 



On one side of this maximum we have practically a solution 

 of hydrate in water, and on the other side a solution of hydrate 

 in pure acid. I'lie two media may be thus quite different, and 

 hence a concentrated acid may show quite different properties to 

 a dilute one. 



From this standpoint it is no surprise that a concentrated 

 acid may not act on a metal, whereas the addition of water may 

 cause the action to proceed. 



The matter may be considered a little further. Suppose that 

 combination with the solvent precedes ionisation. This is a view 

 that is being adopted by many adherents of the ionic theory. 

 Ionisation can then take place only when the dissolved substance 

 can combine to a greater or less extent with the solvent. The 

 converse is, of course, by no means true. Where there is com- 

 bination between solvent and solute this does not necessitate 

 ionisation. 



If we assume that it is the compound which ionises, i.e., 

 the hydrate, then the law of mass action requires that the can- 

 centration of ions shall be proportional to the concentration of 

 hydrate in the solution. Hence in such cases since the concentra- 

 tion of the hydrate reaches a maximum at some point, the concen- 

 tnation of the ions must also pass through a maximum. If this 

 is so. then all such acids must possess a maximum specific elec- 

 trical conductivity. As we faiow, this is the case with all the 

 strong acids. It is of interest to note that Gibson* has em- 

 phasised the fact that the strong acids, particularly the halogen 

 acids, have very dift"erent properties, accordi'.ig as their concen- 

 tration is greater or less than that corresponding to maximum 

 specific conductivity. 



Consider now the solution of nitric acid which Inns the 

 maximum ionic concentration, i.e., the maximum concentration of 

 hydrate. Addition of more pure acid means the dilution of 

 the system with pure acid. We have now practically a solution 

 of hydrate in pure acid, and since the pure acid is nothing like 

 so good an ionising mediami as water, the concentration of ions 

 must decrease very rapidly. In such a medium the tendency of 

 a metal to assume the ionic condition will be very much less than 

 in an aqueous solution, and in fact there will soon come a point 

 where the metal is no longer able to send ions into the solution. 

 From there oniward the acifl has no action on the metal. 



If the concentration of the acid be sti'll further increased 

 the metal miay assume the passive state. This is, however, quit^ 

 another matter, and one which is not yet quite sati sf actor il\>''^^^lC>0^ 

 plained. /^S^^--— -^r-^J^ ^ 



MLISRARYj 



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* Trans. Boyal Soc. Ediii. / ^O /a •^k.o-^^ 



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