EXAMINATION OF COMMON SUBSTANCES 109 



more rapidly in damp air. It is about ITS times as dense as water. 

 It is very malleable. On account of the fact that it melts at 326 C. 

 (a low melting point for a metal) it can easily be cast in moulds. When 

 heated in air it becomes covered with a scum sometimes called " dross " 

 of a yellow colour (litharge). If the heating is continued long enough 

 all the metal is changed into the powder. If this yellow powder is heated 

 for a long time at the temperature at which lead melts, it slowly changes 

 into a red powder known as red lead. It is not an easy matter to carry 

 out the latter operation in an elementary class, but traces of the 

 formation of the red substance may be noticed during the heating of 

 lead. 



The metal is made into sheets and pipes, and is extensively employed 

 by the plumber. When melted with other metals it forms alloys such 

 as pewter, solder and type metal. 



50. EXAMINATION OF COPPER. 



i. Describe its physical properties. 



ii. Effect of heat on copper. By means of crucible tongs, hold a piece 

 of copper foil in a Bunsen flame and note the changes. Allow the copper 

 to cool, and examine the surface of the metal. 



Copper. Copper is a reddish coloured metal, with a characteristic 

 lustre. It is hard, but not so hard as iron. It is about nine times 

 as dense as water. It is very malleable and can be rolled into very 

 thin sheets. 



When heated in air, a wave of colours spreads over the metal, which 

 after a time becomes covered with a black scale, or powder. 



Copper is a good conductor of heat and electricity. Copper wire 

 is largely used for conducting electric current from one place to another. 



Brass is an alloy of copper and zinc. Gun metal an alloy of copper 

 and tin. 



51. EXAMINATION OF TIN. 



i. Describe its physical properties. 



ii. " Cry of tin." Bend a piece of tin while holding it near the ear. 

 Notice the crackling sound. 



iii. Change of weight in heating. Weigh a crucible, and place it on 

 a pipe-clay triangle over a Bunsen flame. Heat till the bottom of the 

 crucible is red hot. Weigh about a gram of tin foil (such as is used for 

 wrapping chocolates) and cut into pieces of less than a square cm. Add 

 these one at a time and note that they appear to burn. When the whole 

 lot has been added, heat the crucible strongly for some minutes. Allow 

 to cool and weigh. Note the appearance of the residue. 



