Explain the following with example: 

Tetravalency of carbon 

Explain the following with example: 

Substitution reaction 

Explain the following with example: 

Addition reaction 

Explain the following with example: 

Saponification 

Explain the following with example: 

Combustion 

A student heated a mixture of fat and sodium hydroxide solution. He added this mixture to an aqueous solution of sodium chloride. A soft, white solid substance was produced. 

(i) Name the soft, white solid. 
(ii) What is the name of the process the student carried out? 
(iii) Why did he add the mixture to the solution of sodium chloride?