156 NON-METALS AND THEIR COMBINATIONS. 



drops of a dilute acid. This is not a decisive test, since other sub- 

 stances besides hydrogen dioxide give the same test. 



2. To a test-tube half full of water, add in succession, 1 c.c. of the 

 hydrogen dioxide solution, a few drops of dilute sulphuric acid, and 

 2 drops of solution of potassium dichromate, and mix. A blue com- 

 pound, known as perchromic acid, HCrO 4 , is produced, which fades 

 after a short time. The color may be made more permanent by 

 shaking the mixture with ether, which dissolves the compound and 

 collects on the surface on standing. This is a very delicate and 

 decisive test. 



3. Acidify a few c.c. of the hydrogen dioxide solution with about 

 2 c.c. of dilute sulphuric acid, and add solution of potassium per- 

 manganate, a little at a time. The purple color vanishes quickly and 

 a gas is given off (oxygen). The permanganate is an unstable oxi- 

 dizing agent, which gives up its oxygen. This unites with oxygen 

 from the dioxide, and escapes as a gas. The reaction will be under- 

 stood when the chemistry of manganese is studied. Other substances 

 also decolorize permanganate. 



If a solution is colorless, odorless, practically neutral to litmus- 

 paper, volatilizes completely upon heating, and responds to the above 

 tests, especially number 2, it is, without doubt, hydrogen dioxide. 



QUESTIONS. Mention two processes by which hydrogen may be obtained. 

 Show by symbols the decomposition of water by potassium, and of sulphuric 

 acid by iron. State the chemical and physical properties of hydrogen. Define 

 the nascent state. What explanation is offered to account for it? State the 

 composition of water in parts by weight and by volume. Mention the most 

 common solid and gaseous constituents of natural waters. How does a mineral 

 water differ from other waters? Mention some different kinds of mineral 

 waters and their chief constituents. What are the characteristics of a good 

 drinking-water ? What are the purest natural waters, and by what process 

 may chemically pure water be obtained? State composition, mode of manu- 

 facture, and properties of hydrogen dioxide. What is the explanation of efflo- 

 rescence and deliquescence ? 



