True, a DECOMPOSITION reaction involves breaking a larger substance into smaller parts, hence decomposes means "breaks down."
A SYNTHESIS reaction involves the opposite, a combining or synthesizing of smaller substances into a larger one.
An example of a synthesis is combining hydrogen gas (H2) with oxygen (O2) to form water (H2O): 2H2 + O2 -- > 2H2O
The opposite reaction, in reverse, is called hydrolysis, a decomposition, literally meaning "splitting water": 2H2O -- > 2H2 + O2
