128 JUNIOR GRADE SCIENCE 



passed. The other end of this tube, called the delivery tube, dips under 

 water in a trough. Mix together four parts of potassium chlorate and one 

 part of manganese dioxide, as in the previous experiment, and place the 

 mixture in the tube. Support the tube and delivery tube as shown in the 

 illustration. Fill several jars with water, and invert them in the trough. 

 Gently warm the tube, and place one of the jars of water over the end of 

 the delivery tube. As the oxygen is driven off, it displaces the water and 

 gradually fills the bottle. When the jar is full of oxygen, cover its mouth 

 with a greased glass plate, and lift it out of the trough. In this way fill 

 five or six jars with oxygen. 



Caution. Be careful not to take away the. burner from under the hard glass 

 tube before removing the delivery tube from the trough. 



Preparation of oxygen from potassium chlorate. As the quantity 

 of oxygen obtained by heating red oxide of mercury is comparatively 

 small, and the oxide of mercury is expensive, a more convenient source 

 of the gas is the white crystalline powder called potassium chlorate. 



If this white crystalline compound is heated, in the same way as the 

 red oxide of mercury, it melts and gives off bubbles of oxygen, and after 

 all the oxygen has been given off a white substance like table salt is 

 left behind. 



By heating, the potassium chlorate is broken up into two things, 

 a gas and a white substance like common salt, which is called potassium 

 chloride. 



POTASSIUM gives when POTASSIUM 



CHLORATE heated CHLORIDE ^nd OXYGEN. 



Use of oxygen mixture. By adopting a slightly different method, 

 oxygen oan be obtained more readily and easily, for it has been found 

 that by mixing the potassium chlorate with certain other substances, 

 as, for instance, a black compound, manganese dioxide, the oxygen 

 from the chlorate comes off more easily and at a lower temperature. 

 This mixture may be called oxygen mixture. If after all the oxygen 

 has been driven out of the mixture the residue left behind is boiled up 

 with water in a flask and the turbid liquid filtered, the manganese 

 dioxide remains unchanged on the filter paper. 



59. PEOPEETIES OF OXYGEN, 



The jars of oxygen prepared as described in Expt. 38 iii. are required. 



i. Physical properties of oxygen. Take one of the jars of oxygen (one 

 of those collected last should be chosen). Notice everything you can about 

 the contents of the jar. The gas in the jar may be slightly cloudy, and 

 have a sharp odour, but if left standing over water for a time, both these 

 disappear. Try the taste by breathing some of the gas. See if the gas has 

 any effect on moist litmus paper. 



