EXAMINATION OF COMMON SUBSTANCES 111 



powder. Subtract the weight of the crucible and lid to find the weight 

 of the powder. The powder will almost certainly be found to weigh 

 more than the original piece of metal. Calculate the percentage increase 

 in weight. 



Magnesium. Magnesium is a bright white metal, which remains 

 bright in dry air, but soon tarnishes in moist air. Its density is less 

 than twice that of water (light for a metal). 



Magnesium burns in air with an intensely white light, and on this 

 account is used in photography as a source of light. When a weighed 

 quantity of it is burnt carefully, so that none of the fumes or powder 

 are lost, it is found that there is a decided increase of weight. When 

 the class results are collected it is found that the increase is the same 

 in every case, allowing for errors in experiment. Careful experiments 

 show that the increase is equal to about 66 per cent, on the weight of 

 the magnesium. This is a noteworthy fact and requires explanation. 

 This process of a metal changing into a powder by heating in air used 

 to be called calcination, and the powder was called a calx. This name 

 will be adopted until a fuller study of this question has been made. 



QUESTIONS ON CHAPTEES XVI. -XVIII. 



73. How are chemical changes in matter distinguished from physical 

 changes ? 



State, giving your reasons, whether the following changes are chemical 

 or not : 



(a) When magnesium wire is heated in a Bunsen flame. 

 (6) When platinum wire is similarly treated. 



(c) When iron rusts. 



(d) When water boils. 



74. You are given a substance to examine. State exactly what physical 

 properties you would look for. Illustrate your answer by giving examples 

 of substances possessing the various properties you mention. 



75. You are given some sulphur, chalk and bluestone, and asked to 

 determine : 



1st, whether they are soluble in water ; 

 2nd, whether crystalline or amorphous ; 

 3rd, the effect of heat on them. 



Explain how you would proceed in each case, and, if you have worked 

 such experiments, state the conclusions you have arrived at. 



76. Describe all you have observed, and state the nature of the changes 

 that take place when the following are respectively heated in a hard glass 

 tube : (a) mercuric oxide, (6) green vitriol and (c) washing-soda crystals. 



