CHAPTER XI 



ACIDS, ALKALIES, AND CLEANING 



213. Acids. We learned in 5 that no two sub- 

 stances have exactly the same special, or specific, prop- 

 erties. There are, however, many substances that are 

 alike in some particular property or properties. This is 

 true of the substances that make up the class we call 

 " acids," as well as of those we call " bases," or " alkalies." 

 The word " acid " means sour; sourness is a character- 

 istic property of most acids. The juices of ripe fruits 

 pi.teA.coho, are sweet because they contain 



sugar; but when they are ex- 

 posed to the air, the juices " fer- 

 ment": their sugar is changed 

 to alcohol and carbon dioxide. 

 This particular fermentation is 

 due to yeast (cf. 129). The 

 change does not, however, stop 

 here. When the fermented 

 juices, which now contain alcohol, 

 stand longer in the air, they be- 

 come sour, and we have cider vinegar, currant vinegar, 

 wine vinegar, etc. (Fig. 192). The cause of this second 

 fermentation is another plant, known as the " vinegar 

 mould, " or " mother of vinegar. ' ' The sourness of vinegar 

 is due to acetic acid (cf. 124). In a pure form this is a 

 colorless, sharp-smelling liquid, which freezes at 16 C v 



188 



FIG. 192. 

 Quick Vinegar Process. 



