WRITING EQUATIONS 123 



is necessary to take 2H 3 PO 4 and 3Ca(OH) 2 , in order to 

 make the equation balance. The Ca and H atoms ex- 

 change places ; there are three H atoms to be replaced. 

 Ca has a valence of 2 ; i Ca cannot replace 3!!, but 

 6H(2H 3 ) can be replaced by 3Ca, because 2H 3 has a total 

 valence of 6 and so has 3Ca. 



2 H 3 P0 4 + 3Ca(OH) 2 = Ca 3 (PO 4 ) 2 + 6H 2 O. 



3Ca atoms replace the 2H 3 and form Ca 3 (PO 4 ) 2 , a bal- 

 anced compound, because PO 4 is a radical having a valence 

 of 3, and if taken twice its total valence is 6, with which 

 3 Ca atoms can combine. The remaining H and O atoms 

 form 6H 2 O. 



i49. Impossible Reactions. Not all chemical com- 

 pounds when brought together give a chemical reaction. 

 Whether or not a reaction takes place can be determined 

 only after a careful study of the elements and their prop- 

 erties ; this often involves a more exhaustive knowledge 

 of chemistry than can be obtained from an elementary 

 study of the subject. 



In the case of BaSO 4 -{- 2HC1, no reaction can take 

 place, although an apparently correct reaction can be 

 written : 



BaSO 4 + 2HC1 = BaCl 2 + H 2 SO 4 . 



This is because BaSO 4 and HC1 are the products of the 

 reaction 



BaCl 2 + H 2 SO 4 = BaSO 4 + 2HC1. 



In the equations given at the end of this chapter a reac- 

 tion takes place in each case. 



