58 PROPERTIES AND MAKE-UP OF MATTER 



SUMMARY 



Anything that occupies space is matter. Matter is known 

 to us in three forms solids, liquids, and gases. Matter 

 has certain properties, such as extension, inertia, and gravi- 

 tation. The laws of inertia and gravitation explain so per- 

 fectly the movements of the heavenly bodies that their 

 courses may be accurately foretold. 



All matter consists of particles called molecules, too small 

 to be seen with the most powerful microscope. Molecules 

 may be divided into smaller particles called atoms. If the 

 molecules of a substance may be broken up into two or more 

 kinds of atoms, the substance is called a compound ; if not, 

 it is called an element. There are about eighty elements 

 known to scientists. All other substances are compounds. 



When molecules of a substance gain atoms, lose atoms, or 

 exchange atoms with molecules of other substances, a chem- 

 ical change is said to occur. Any other kind of change in 

 matter is a physical change. If when we combine two sub- 

 stances, the molecules remain unchanged, we have a mixture ; 

 if atoms of different kinds unite into molecules, we have a 

 chemical compound. 



Acids, bases, and salts are most important chemical com- 

 pounds. Acids exist in many familiar substances. Many 

 acids are liquid. Dilute solutions of common acids taste 

 sour. Acids turn blue litmus paper red. Bases are in some 

 ways just the opposite of acids. Most bases are solid and 

 dilute solutions of them taste bitter. They turn red litmus 

 paper blue. 



Strong acids and bases are injurious to flesh or to common 

 substances. The process of combining an acid and a base is 

 called neutralization, and the result is water and a salt. A 



