PROPERTIES OF MATTER 37 



_ [H30+][0H-] 



[H^O]-^ 



where the brackets denote molar concentrations ot the various quan- 

 tities. It turns out that [HoO] is very much greater than either [H3O + ] 

 or [OH-] and is affected so slightly by changes in the last two that 

 [H2O] is in effect a constant. So for simplicity one may say that a new 

 constant K^^ = Kiyi^O]-, and 



A', = [H30+][0H-] 



V^alues of the dissociation constant A',„ change with temperature and 

 must be determined accordingly. There are additional effects when 

 other electrolytes are present in relatively high concentrations. How- 

 ever, this latter problem is usually minor and is quite difficult to take 

 into accurate account besides, so it is commonly neglected. 



The important implications of the dissociation, or in this case ioniza- 

 tion, constant are these. A constant fraction of the water present 

 is ionized regardless of the amount of water involved. And the addi- 

 tion of either or both ions leads to enough recombination of ions to 

 preserve the equality of A,„ with the product of the two ion concen- 

 trations. Conversely, if some of one kind of ion is removed, additional 

 water dissociates to preserve the equality. Stated in other words, if 

 the concentration of either ion is increased, the other automatically 

 decreases and vice versa. 



At 25°C., A,, = 1.008 times lO-i-*, and if [H:iO + ] = [OH-] the con- 

 centrations of both ions equal 10~' molar. All aqtieous solutions 

 having equal concentrations of hydrogen (hydronium) and hydroxyl 

 ions are said to be neutral and correspond in these concentrations to 

 pure water. Note that both are lO-'M at 25°C. But a change in tem- 

 perature changes A',„ and hence changes the values of [H;^0 + ] and 

 [OH~] at neutrality. When [H3O+] exceeds [OH-] a solution is said 

 to be acidic, and when the reverse is true the solution is alkaline. 



The dissociation reaction written for water takes no account of the 

 aggregation of the molecules discussed earlier. The effect of this aggre- 

 gation on the reaction and its constant cannot be evaluated. For- 

 tunately, the influence is constant and introduces a factor appearing 

 in K^. Perhaps a part of the change of K^^ with temperature reflects 

 a change in the structure of the water aggregates. If so, it is not yet 

 subject to experimentation. 



Acids and Bases 



A number of compounds are called acids or bases. When dissolved 

 in water, the acids increase the concentration of hydrogen ion and 



