50 CHEMISTRY FOR AGRICULTURAL STUDENTS 



88. What are the three characteristics of chemical combination? Why 



do we regard water as a chemical compound, and air as a mere 

 mixture of the constituent gases? 



89. In what proportions do mercury, carbon, and hydrogen combine 



with oxygen to form red oxide of mercury, carbonic acid gas, and 

 water respectively? 



90. For identifying chemical substances we can employ both physical and 



chemical methods. Give illustrations of each. 



