To synthesize is to create or build. A synthesis reaction builds. That's a synthesis reaction. A chemical reaction in which a large molecule is split into several smaller molecules is called a decomposition reaction . When something breaks down, it decomposes.
Water chemically splits apart to form oxygen and hydrogen.
That's a decomposition reaction. 4NaCl is a formula for four molecules of sodium chloride. The formula for 1 molecule is NaCl. Four molecules are written 4NaCl. In a chemical reaction, the starting substances are called the reactants .