THE RECOGNITION OF SUBSTANCES, ETC. 377 



spicuous, such as needle-shape or cubical formation of the particles, 

 must be noted. 



The COLOR, if any, is significant. If yellow, the specimen 

 may be sulphur (p. 249), if black, carbon (p. 330), if black and 

 crystalline, with violet vapor, iodine (p. 203). Most substances 

 are colorless. 



The odor, if any, must be noted. Many salts of ammonium 

 (carbonate, sulphide, etc.) smell of ammonia (p. 303). Some sul- 

 phides (of sodium, potassium, ammonium, etc.) smell of hydrogen 

 sulphide (p. 253). Some acetates smell of acetic acid and hypochlo- 

 rites of hypochlorous acid (p. 223).* Some chlorides (e.g. PC1 5 

 and Aids), in moist air, give hydrogen chloride, with the odor and 

 fumes characteristic of that substance. 



Effect of Heating (Solids) . A great deal may be learned by 

 heating as much of the specimen as will fill the rounded bottom 

 of a small, dry test-tube (Fig. 92). 



The substance may fuse. Many substances, such 

 as some chlorides and most hydrates, do so. Continue 

 heating. 



A sublimate (solid deposit on the cold part of the 

 tube) may appear. Black crystals (from violet vapor) 

 are iodine: white crystals, with the limitations we have 

 set, show the substance to be a salt of ammonium (p. 305). IG ' 

 Confirm by smelling, and by adding sodium hydroxide to the 

 original substance (p. 302). (Salts of mercury sublime also.) 



A reddish-brown liquid condenses, becoming, when cold, a yel- 

 low solid. The substance was sulphur from a sulphide (such as 

 FeSs, p. 486). 



Water condenses. Hydrates, acid-salts, and some organic com- 

 pounds. Test the water with litmus paper. An acid reaction in- 



* Many experimental details, essential for the successful performance 

 by a beginner of the tests described in this chapter are here omitted. They 

 will be found in the Authors' Intermediate Outline of Elementary Chemistry. 



