How is synthesis and decomposition reaction related to each

They are kind of opposite processes. Chemical synthesis is execution of chemical reactions to make products. Like you take two substances, you put them together, they react and you have new a substance. For example.  here Sodium and Chlorine react to make a new substance which is Sodium Chloride.

Chemical decomposition is process of seperating a substance to different substances. There is a substance by a reaction this substance becomes two substances, Like : XY -> X+Y For example : 

Ithink this is what you meant. the temperature at which a liquid begins to boil is called the boiling point. the boiling point of water is 212°f or 100°c.  and at that point will start to turn into steam (gas)