5. (a) Name four important constituents of the atmosphere, and state the 



relative amount of each. How does the air exhaled from the human 

 body differ in composition from the normal atmosphere ? 

 (b) How may sea- water be made suitable for drinking purposes ? how may 

 well-water which has been contaminated by sewage ? Explain the prin- 

 ciple in each case. 



GROUP B 

 (Omit two of the following questions. Each question counts 10.) 



6. (a) Give three different general methods for preparing salts. Write one 



equation illustrating each. 



(b) How are two of the following compounds prepared: (i) bleaching pow- 

 der; (2) sulphuric acid; (3) nitrous oxide? 



7. Describe experiments involving chemical change sufficient to distinguish 



between the following: (a) chlorine and hydrogen chloride; (b) moist 

 air and dry air; (c) pure water and water containing a soluble chloride; 

 (d) calcium carbonate and calcium phosphate; (e) freshly prepared mortar 

 and mortar from an old building. 



8. (a) How many liters of hydrogen and how many of nitrogen are necessary 



to form ten liters of ammonia gas ? State the law illustrated. 

 (6) What are the valences of the metallic elements in OsO 4 , A1 2 (SO 4 ) 3 , N 2 Mg 3 ? 



9. (a) What change takes place in the molecular condition of copper sulphate 



when it is dissolved in water ? 



(b) Describe the chemical changes which occur when the electric current is 

 passed through such a solution. 



(c) Why does an aqueous solution of sodium carbonate give an alkaline 

 reaction ? 



10. (a) What is the objection to putting fresh coal on a hot fire and closing the 



damper in the flue, especially if the lid of the stove is left off ? 



(b) Why is calcium chloride sometimes used to keep road surfaces dustless ? 



(c) State the chemical reactions taking place in three of the following pro- 

 cesses: (i) boiling water containing temporary hardness; (2) adding 

 soap to hard water; (3) bessemerizing cast iron; (4) striking a match. 



11. (a) Mention one common use of each of six of the following substances and 



in each case define the use specifically: (i) carbon monoxide; (2) sodium 

 nitrate; (3) sodium carbonate; (4) silver chloride; (5) zinc; (6) lead; 

 (7) sulphuric acid. 



(b) In connection with two of the following substances describe two instances 

 in which each has been used in your laboratory work, and explain its 

 specific action in each case: (i) chlorine; (2) carbon dioxide; (3) sulphuric 

 acid. 



