190 ACIDS, ALKALIES, AND CLEANING 



215. Acids and Coloring Matter. Besides having the 

 property of sourness, acids have a definite action on cer- 

 tain coloring matters, such as purple cabbage solution and 

 litmus. Acids often change the color of the dyes in our 

 clothing. Thus, " navy blue " fabrics are colored red by 

 the common acids of the laboratory. 



Litmus is the coloring matter we generally use in testing 

 for acids. It is a blue substance obtained from certain 

 plants called lichens (cf. 324). Either the solution, or 

 filter paper that has been soaked in the solution, may be 

 used. The prepared paper is called litmus paper. Blue 

 litmus is changed to red litmus by sour plant juices and 

 by other acids. A substance which is able to change blue 

 litmus to red is said to have an " acid reaction." 



216. Action of Acids with Metals. A third important 

 property of acids is that they corrode, or " eat," metals. 

 We have learned that hydrogen is made by the action of 

 some of the acids upon zinc, iron, etc. (cf. 103). When 

 the metals are so used, they are " eaten up," and disap- 

 pear. 



Some metals, such as copper, do not act with dilute 

 acids to give hydrogen (cf. 150) ; but if the metal and the 

 dilute acid are kept in contact with air, the metal is 

 gradually corroded. Here the oxygen of the air acts with 

 the metal to give the oxide of the metal (cf. 48) ; the oxide 

 then reacts with the acid. Copper and lead cannot be 

 used for cooking utensils, because they act with the air 

 and the acids of fruits and other food, producing poisonous 

 compounds. 



Copper is " eaten " very readily by dilute nitric acid. If a design is 



