74 NON-METALS AND THEIR COMBINATIONS. 



This part of the decomposition takes place at a comparatively low temper- 

 ature ; after it is complete, the temperature rises considerably and the decom- 

 position of the perchlorate begins : 



KC1O 4 = KC1 + 40. 



If the potassium chlorate be mixed with 30-50 per cent, of man- 

 ganese dioxide, and this mixture be heated, the liberation of oxygen 

 takes place with greater facility and at a lower temperature than by 

 heating potassium chlorate alone. Apparently, the manganese dioxide 

 takes no active part in the decomposition, as its total amount is found 

 in an unaltered condition after all potassium chlorate has been decom- 

 posed by heat. A satisfactory explanation regarding this action of 

 manganese dioxide is yet wanting. 



A third method is to heat to redness, in an iron vessel, manganese 

 dioxide (MnO 2 ), which suffers then a partial decomposition : 

 3MnO 2 = Mn 3 O 4 -f 2O. 



In this case there is liberated but one-third of the total amount of 

 oxygen present, while two-thirds remain in combination with the 

 manganese. 



Other methods of obtaining oxygen are : Decomposition of water by elec- 

 tricity, heating of dichromates, nitrates, barium dioxide, and other substances, 

 which evolve a portion of the oxygen contained in the molecules. 



Heating a concentrated solution of bleaching powder with a small quantity 

 of a cobalt salt (cobaltous chloride) furnishes a liberal supply of oxygen, the 

 calcium hypochlorite of the bleaching powder being decomposed into calcium 

 chloride and oxygen : 



Ca(ClO) 2 = CaCl 2 + 2O, 



Oxygen may be obtained at the ordinary temperature by adding water to a 

 mixture of powdered potassium ferricyanide and barium dioxide, and also by 

 the decomposition of potassium permanganate and hydrogen dioxide in the 

 presence of dilute sulphuric acid. 



Experiment 1. Generate oxygen by heating a small quantity (about 5 

 grammes) of potassium chlorate in a dry flask of about 100 c.c. capacity, to 

 which, by means of a perforated cork, a bent glass tube has been attached, 

 which leads under the surface of water contained in a dish. (Fig. 6.) Collect 

 the gas by placing over the delivery-tube large test-tubes (or other suitable ves- 

 sels) filled with water. Notice that a strip of wood, a wax candle, or any other 

 substance which burns in air, burns with greater energy in oxygen, and that 

 an extinguished taper, on which a spark yet remains, is rekindled when placed 

 in oxygen gas. Notice, also, the physical properties of the gas. How many 

 c.c. of oxygen can be obtained from 5 grammes of potassium chlorate ? 1000 c.c. 

 of oxygen weigh 1.43 grammes. 



The quantity of oxygen liberated from a given quantity of a substance may 

 be easily calculated from the atomic and molecular weights of the substance 



