THE RECOGNITION OF SUBSTANCES, ETC. 379 



Effect of Sulphuric Acid on Solids. Fill the rounded 

 bottom (only) of a test-tube with the substance, add just enough 

 concentrated sulphuric acid to moisten the sample, and warm 

 slightly. 



A gas (effervescence) which fumes in the breath may be given 

 off. If the gas is brown or yellow, it may be bromine (bleaches 

 litmus paper) mixed with hydrogen bromide from a bromide 

 (p. 202) . It may be nitrogen tetroxide (odor) from a nitrate (p. 311). 

 If violet, with brown deposit, accompanied by an odor of hydro- 

 gen sulphide, it is iodine from an iodide (p. 204). 



If the gas fumes, but is colorless, it may be hydrogen chloride 

 from a chloride (add manganese dioxide to get chlorine, p. 142). 

 It may be hydrogen fluoride from a fluoride (a moistened glass rod 

 acquires white precipitate of silicic acid produced by decompo- 

 sition of silicon fluoride, p. 207). 



The gas does not fume. If yellow it may be chlorine from 

 bleaching powder (p. 224) or nitrous anhydride from a nitrite 

 (p. 314). A chlorate also gives a yellow gas (chlorine dioxide 

 C102) when heated with concentrated sulphuric acid. In this 

 case oxygen will have obtained in the test on page 378. Heat 

 very carefully, since chlorine dioxide is explosive (stop heating 

 when material in tube begins to crackle!). 



The gas does not fume and is colorless. An odor of sulphur 

 dioxide indicates a sulphite (p. 258). An odor of hydrogen sulphide 

 indicates a sulphide. If the gas is odorless, it may be carbon 

 monoxide (burns, leaving carbon dioxide) from a formate (p. 337), 

 or oxygen from some oxides or a peroxide, or nitrous oxide from 

 ammonium nitrate (p. 315), or carbon dioxide from a carbon- 

 ate. 



No gas evolved indicates a silicate (p. 360), sulphate (p. 270), 

 phosphate (p. 322), or a basic oxide. 



Sulphuric Acid on the Residue front p. 378. If the sub- 

 stance gave off oxygen when heated alone (p. 378), add a drop 



