6 AGRICULTURAL CHEMISTRY 



bine to form compounds, it is because of the chemical 

 affinity which the elements have for one another. Some 

 elements have a stronger affinity for certain elements than 

 for others. 



10. Mechanical flixtures. When two or more sub- 

 stances mix, but fail to unite chemically, a mechanical 

 mixture is obtained. When iron and sulfur are mixed, 

 a mechanical mixture is the result, and the iron and the 

 sulfur can, by purely physical methods, be separated. 

 If, however, a mixture of iron and sulfur is heated, a 

 chemical change takes place and it is impossible by physi- 

 cal methods, as by the use of a magnet or by solvents, to 

 separate the iron from the sulfur. Compounds as well 

 as elements may form mechanical mixtures. 



n. Chemical Analysis and Synthesis. Whenever a 

 substance or a compound Is separated into simpler com- 

 pounds or elements, the process is called chemical analy- 

 sis. When only the kinds of elements or simpler com- 

 pounds are determined, the process is called qualitative 

 analysis. If the percentage amounts are determined, it 

 is called quantitative analysis. When elements or simpler 

 compounds are united, the process is called synthesis. 

 Synthesis and analysis are directly opposite processes. 

 When substances are produced in the laboratory from 

 simpler elements or compounds, it is called a synthetical 

 process. Many useful compounds are produced syntheti- 

 cally. 



12. Summary. Substances may undergo either phys- 

 ical or chemical changes. A physical change does not 



