176 OUR PHYSICAL WORLD 



We have already been using in this and preceding chapters 

 some chemical terms, and shall need to use others in later chap- 

 ters. It is a very simple matter, however, to get in mind such 

 elementary chemical concepts as are needed to understand the 

 simple chemical processes treated in this book. 



The difference between a physical change and a chemical 

 change must be apparent from the discussion of burning in the 

 preceding chapter. Heat a substance like solid ice and it 

 changes to a liquid, and this in turn to steam, a gas. These, 

 however, are merely three different physical states of water. 

 So solid sulphur may be changed to liquid and solid iron to molten 

 iron by heat. Heat sulphur still more in the air until it reaches 

 its ignition point and it burns or unites with oxygen and forms a 

 new substance, oxide of sulphur. So when iron burns in oxygen 

 or rusts slowly in the moist air, a new substance is formed, an 

 oxide of iron, with properties quite unlike iron. 



Chemists have devised a sort of shorthand for writing out 

 these reactions, and indicate the elements by the initial letter 

 of their English or sometimes their Latin names. In case two 

 or more elements begin with the same letter, it is necessary to 

 use in such cases two letters from the name; thus C is carbon; 

 Cl, chlorine; N, nitrogen; Na, sodium (Latin, natrium}. Thus 

 when sulphur burns the reaction is written: 



S+O 2 =SO 2 . 



This means that one atom of sulphur unites with two of oxygen 

 to form one molecule of sulphur dioxide. Such a statement to be 

 an equation must, of course, have equal numbers of atoms of 

 each substance on opposite sides of it. 



Most chemical substances are classed as bases, acids, or 

 salts. For our purpose we may define these simply. A base is 

 a positive substance, like a metal, combined with OH, and is 

 named a hydroxide. Thus KOH, Ca(OH) 2 , are potassium 

 hydroxide and calcium hydroxide respectively. The valence of 

 the OH radical is one, of potassium one, but of calcium, two. 



