8 SMITH'S INTERMEDIATE CHEMISTRY 



more substances, without ceasing to exist, may be changed 

 into one or more new and entirely different substances. When 

 such an alteration occurs, it is called a chemical change or reac- 

 tion. To learn more about this most remarkable fact, we shall 

 take up in the next chapter some examples of a simple and long 

 familiar nature. 



Definition of Law. In science, a law, or generalization or 

 rule, is a statement describing some general fact or constant 

 mode of behavior. Its uses are to condense a* great many similar 

 facts into one statement, and thus to make the whole set of facts 

 more easy to remember. 



Constituents. As we have seen, we speak of the substances 

 in a mixture as the components. When we wish to refer to the 

 forms of matter which are chemically united in a compound, we 

 call them the constituents (Lat., standing together) of the com- 

 pound substance. Thus, iron and oxygen are the constituents of 

 rust. 



The chemist separates (p. 13) the components of a mixture, 

 for that is all that is necessary. He liberates the constituents of a 

 compound, however, because they are bound together in chemical 

 combination. 



The names given to compounds are usually devised so as to 

 indicate the nature of the constituents. Thus, iron-rust is oxide 

 of iron (or ferric oxide, from Lat., ferrum, iron). The yellowish 

 powder obtained when lead is heated in air is lead oxide or oxide 

 of lead, and the white powder similarly derived from tin is oxide 

 of tin. 



A Condensed Form of Statement. We may represent a 

 chemical combination, or indeed any kind of chemical change, in a 

 condensed form, thus: 



Iron + Oxygen > Oxide of iron (ferric oxide) 



