SOME CHEMISTRY OF EVERYDAY LIFE 183 



radical applies to any group of atoms such as OH which in 

 chemical changes appears to behave as a single atom, 

 changing from molecule to molecule of different substances 

 apparently as a group rather than as separated atoms. 



When the formula for any salt is given, it is possible to find 

 the formula for the acid from which it may be considered the 

 the salt was formed. This can be done by replacing the 

 metallic atoms of the salt by a number of hydrogen atoms 

 chemically equivalent to them. The number of hydrogen 

 atoms replaceable by a single atom of another element meas- 

 ures the valency of that atom. Thus the valency of sodium 

 in NaCl (formed from HC1) is one, and of copper in CuS 

 (formed from H 2 S) is two. The valency of an atom or 

 radical is likewise determined by the number of hydrogen 

 atoms with which it will unite. Thus the valency of Cl in 

 HC1 is one, and that of the radical SC>4 in H2S04 is two. 



In the study of Chemistry one must become able to deter- 

 mine the valency of elements in compounds. As formulae 

 of the simpler chemical compounds are met in this text 

 and elsewhere it is worth while to notice what seems to be 

 the valency of different elements and of any radicals. For 

 example, it will be noted that the valency of Ca and of S 

 is two in the compounds CuS and Ca(OH) 2 , and that of the 

 radical OH is one. 



SUMMARY 



A base is a compound of hydroxyl (OH) and any metallic atom or 

 radical. An acid is a compound containing hydrogen atoms replace- 

 able by metallic atoms or radicals. A salt is a chemical compound 

 that may be considered as formed when any of the hydrogen of an 

 acid is replaced by a metal. 



In the break-up (dissociation) of molecules when certain substances 

 dissolve, the metallic atom or group of atoms is conceived to be posi- 

 tively electrified, and the negatively electrified atom or group is non- 

 metallic in character. The behavior of hydrogen ions is the same as 

 that of the metallic ions. 



