138 ELEMENTS OF LABORATORY WORK 



91. Changes observable when the Temperature of Copper 

 Oxide is raised in a Current of Hydrogen or Coal-gas. 

 Heat some bright copper filings, in an open vessel, to a high 

 temperature, and they become dark-coloured. This change, 

 however, is likely to be incomplete, the dark-coloured sub- 

 stance coating some still unchanged copper. Chemical changes 

 in which solids are engaged are, from the nature of solid 

 matter, unlikely to be extended to the whole of the matter, 

 unless the products of the change are removed from the region 

 of chemical activity by means of the process of diffusion. 



This substance may, however, be prepared otherwise than 

 by raising the temperature of copper in the presence of 

 oxygen. Place some of this pure prepared copper oxide in 

 a tube of hard glass, which ,is closed at one end, and has been 

 carefully weighed. Dry the tube and oxide by slightly heating 

 them, so that the water condensed on their surface may be 

 gasified ; then place the tube in a vessel of which the air is 

 kept free from water- vapour by strong hydrogen sulphate, 

 which absorbs all the water in its neighbourhood, and allow 

 it to cool. Such an arrangement is called a desiccator, and 

 may be made by placing a shallow vessel of hydrogen sulphate 

 on ground glass, and covering with a bell-jar with ground 

 edges. The object to be kept dry should be supported over 

 the hydrogen sulphate. The tube is then carefully weighed, 

 supported slantingly, and kept filled with coal-gas by means 

 of a caoutchouc tube which is connected at one end with the 

 gas supply or a supply of hydrogen, and at the other with a 

 glass tube of which the end has been drawn out. This tube 

 passes through a cork, which is very loosely fitted in the tube 

 containing the copper oxide. . By this means the air may be 

 excluded from the tube while it is being heated by a Bunsen 

 flame. It has been shown that copper at a high temperature 

 unites with the oxygen of the air. Copper oxide itself is 

 therefore unlikely to be changed when heated in the air, and 

 this is easily demonstrated. But now that hydrogen (or its 

 substitute, coal-gas), instead of air, is in contact with the 

 oxide, a change is soon visible. A vapour condenses at the 

 top of the tube, and finally disappears as the heating continues. 



