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13. Why should sulphur compounds be removed from illuminating gas? phos- 



phorus from steel ? dust from sulphur dioxide in the contact process for 

 making sulphuric acid ? moisture from materials for making baking 

 powders ? calcium and magnesium chlorides from table salt ? 



14. a) Why does the admission of air at the base of the Bunsen burner make the 



flame non-luminous ? 



b) Why is the temperature of a flame of hydrogen burning in oxygen higher 



than that of hydrogen burning in air ? 



c) Why is a mixture of fine coal dust and air explosive ? 



15. a) What property has helium which makes it better than hydrogen for filling 



balloons ? 



b) How does commercial copper compare in purity with other metals 



as commercially supplied ? Describe briefly the refining of copper. 



c) Mention three substances commonly used for extinguishing fire. Indi- 



cate the one that you consider best adapted to extinguishing burning 

 gasoline. 



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