188 NON-METALS AND THEIR COMBINATIONS. 



Borax may be looked upon as containing sodium metaborate and boric 

 oxide, 2NaBO 2 + B,O 3 . When it is heated with basic oxides, these unite with 

 the excess of B 2 O 3 > forming a fused mixed metaborate. This action explains 

 the use of borax on hot metal surfaces which are to be welded. Some of these 

 metaborates have distinctive colors. For this reason borax beads are used as 

 tests for certain metals (see under Sodium Borate). 



Boric acid is such a weak acid that its solution has only a slight action on 

 litmus-paper, and it is displaced from its salts by nearly every other acid. The 

 borates of the alkali metals only are easily soluble in water, the others being 

 either insoluble or nearly so. Hence, when a solution of an alkali borate (but 

 not of free boric acid) is added to a solution of a saltof other metals, a precipitate 

 is obtained. Alkali borates show a strong alkaline reaction to litmus because 

 they are partly hydrolyzed in solution to free alkali and boric acid. It will be 

 ?een that boric acid is very much like carbonic acid in behavior. 



Boric acid and borax are practically the only compounds of boron that are 

 used. Both are used in medicine and as preservatives. When powdered they 

 look much alike, but can be distinguished by the fact that the acid is soluble 

 in alcohol while borax is not, and that borax has a marked alkaline reaction to 

 litmus, and when held in a Bunsen flame on platinum wire gives a yellow color, 

 while free boric acid gives a green color. 



Tests for boric acid and borates. 



1. When borax is heated on the loop of a platinum wire in a Bunsen 

 flame it first puffs up very much, and then gradually melts into a 

 transparent, colorless bead. If the bead is moistened with concen- 

 trated sulphuric acid and heated again, a green color is produced. 

 Boric acid also melts into a colorless bead. Note any difference in 

 color of the flame. 



2. Mix in a porcelain dish some borax with 2 c.c. of concentrated 

 sulphuric acid, add about 10 c.c. of alcohol, and ignite. The flame 

 has a mantle of green color, which is best seen by alternately extin- 

 guishing and relighting the alcohol. Eepeat the experiment, omitting 

 the acid ; no green color is seen. Free boric acid is volatilized with 

 alcohol, but not its salts. 



QUESTIONS. How is carbon found in nature? State the physical and 

 chemical properties of carbon in its three allotropic modifications. Mention 

 three different processes by which carbon dioxide is generated in nature, and 

 some processes by which it is generated by artificial means. State the physical 

 and chemical properties of carbon dioxide. Explain the process of respiration 

 from a chemical point of view. What is the percentage of carbon dioxide in 

 atmospheric air, and why does its amount not increase ? State the composition 

 of carbonic acid and of a carbonate. How can they be recognized by analyt- 

 ical methods? Under what circumstances will carbon monoxide form, and 

 how does it act when inhaled? What is destructive distillation, and what 

 gases are generally formed during that process? Explain the structure and 

 luminosity of flames. 



