CHEMICAL EQUATIONS, TYPES OF CHEMICAL CHANGE, ETC. 123 



many compounds ; it is, for instance, a constituent of all the different 

 hydroxides (formerly called hydrates), such as potassium hydroxide, 

 KOH, calcium hydroxide, Ca(OH) 2 , etc. 



According to the number of points of attraction left unprovided 

 for in a radical, we distinguish univalent, bivalent, trivalent, and 

 quadrivalent radicals. 



Carbon is a quadrivalent element forming with the univalent hy- 

 drogen the saturated compound CH 4 . By removal of one, two, or 

 three hydrogen atoms the radicals CH 3 ', CH/', CH'", are formed. 



Constitutional or graphic formulas. Though it is impossible 

 to examine the structure of a molecule by means of a microscope, yet 

 we may obtain some information of the atomic arrangement within 

 the molecules by a study of the formation and decomposition which 

 they undergo under different conditions. 



Such investigations lead to the conclusion that molecules are not 

 merely clusters of atoms held together irregularly, but that the 

 atoms are arranged systematically and occupy a definite position 

 within the molecules of each individual substance. 



In order to represent figuratively our views regarding the atomic 

 arrangement the so-called constitutional or graphic formulas are often 

 used. Thus, while sulphuric acid is represented by the molecular 

 formula H 2 SO 4 , we may assign to it the graphic formula SO/'. (OH) 2 , 

 which indicates that sulphuric acid is made up of the bivalent radi- 

 cal SO 2 and of two univalent radicals OH. In order to give a yet 

 fuller expression of our views regarding the linkage of the atoms, sul- 

 phuric acid may be graphically represented thus : 



O H 



01 vv, 



-H 



QUESTIONS. What are chemical equations and how do they serve as a basis 

 for calculations? Mention the principal types of chemical change, with exam- 

 ples. Define reversible actions and chemical equilibrium. How may a revers- 

 ible action be made to run to completion in one direction? Give an example. 

 What is the law of mass action? Define an acid, and state the general proper? 

 ties of basic and neutral substances. By what means can they be recognized? 

 Distinguish between mono-, di-, and tri-basic acids. What are salts and how 

 are they formed? Define neutral, acid, and double salts. Explain the term 

 radical or residue. 



