182 SMITH'S INTERMEDIATE CHEMISTRY 



hydrogen atoms carrying a positive charge (H + ) and chlorine 

 atoms carrying a negative charge (Cl~) . That part of the hydro- 

 gen chloride which remains undissociated in solution is under- 

 stood to be inactive. It plays no part in the conduction of the 

 current, it exerts a normal effect on the physical properties of the 

 solution (vapor pressure, boiling-point, freezing-point changes), 

 it does not convey to the solution any of the properties of an acid, 

 it takes no direct share in displacements or double decompositions. 

 That part of the hydrogen chloride, on the other hand, which 

 breaks up into hydrogen ions H + and chlorine ions Cl~ is active. 

 All of the special properties of the solution may therefore be 

 referred to these ions, as we shall see below. 



Ionic Equations. Of course, there is an equilibrium between 

 the undissociated and dissociated parts of an electrolyte in solu- 

 tion, and this equilibrium may be represented by the ionic equa- 

 tion: 



HC1 ^ H+ + C1-. 



lonization is, therefore, a kind of decomposition or rather 

 dissociation, and is in every sense a chemical change. In ionic 

 equations the charges upon the ions must be shown, as they are 

 essential parts of the ionic substances. 



Nad < Na+ + Cl- FeCl 3 ^ Fe+++ + 3C1- 



Ca(N0 3 ) 2 <= Ca++ + 2NO 3 - Na*S0 4 <= 2Na+ + S0 4 =. 



The number of positive charges must equal the number of negative 



charges. This is proved most simply by the fact that the solution 



of a substance like ferric chloride (FeCl 3 ) is electrically neutral, 



as a whole. Thus some ions carry one charge, like Na + and Cl~ 



and H+, others two charges, like Ca ++ and S0 4 ~, and so forth. 



'An ion is an atom, or group of atoms, carrying an electric charge, 



or a number of such charges.^ 



The positive ions are called the cations, since they move toward 



