IX. SOME CHEMISTRY OF EVERYDAY LIFE 



A REVIEW OF SOME CHEMICAL CHANGES 



Every substance has some distinguishing characteristics, 

 or properties, by which it is known as different from all other 

 substances, and by means of which when once known it may 

 always be recognized. Physics and Chemistry are some- 

 times spoken of as " fundamental" sciences because they are 

 so largely concerned with the properties of bodies by means 

 of which the bodies are identified. 



When a chemical change occurs in a body the properties 

 by which it was before known disappear and other properties, 

 often widely different ones, become manifest. It is in this 

 way that a chemical change is known to have taken place. 

 The original substance has ceased to exist, and new sub- 

 stances take its place. The matter continues to occupy 

 space, however, and its mass remains the same. The amount 

 of the new material as determined by weight is exactly the same 

 as that from which the new substances were formed. The 

 chemical properties of a body may be considered those that 

 are made apparent through chemical changes, while physical 

 properties involve no change in the nature of the substance. 



To understand better chemical changes in matter it is 

 assumed to be true that the molecules of a body have phys- 

 ical properties the same as those of the body as a whole, 

 and that the molecules themselves are made up of still smaller 

 particles called atoms. These atoms have their own charac- 

 teristic weight values. Atoms of the same kind are believed 

 to have the same weights, while different kinds of atoms have 



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