Synthesis and Decomposition Reactions

Generally, in a chemical reaction, molecules interact by breaking one set of bonds and forming a new set of bonds. 

A redox, or oxidation–reduction, reaction is a type of chemical reaction involving the partial or complete transfer of electrons. In such reactions, one reactant is oxidized and the other is reduced, with an observable change in their oxidation states. 

The oxidized element, which has lost electrons, undergoes an increase in oxidation state. The reduced element, which has gained electrons, undergoes a decrease in oxidation state.

Among the most common redox reactions are synthesis and decomposition reactions. The synthesis of proteins from different amino acids and the digestion of proteins into amino acids are important examples.

Synthesis, or combination, reactions involve the formation of bonds between reactants to create a single product. The reactants may include only elements, elements and compounds, or only compounds. 

Examples are the combination of elemental hydrogen and oxygen to create water, the addition of carbon monoxide to elemental oxygen to form carbon dioxide, and the combination of calcium oxide and water to form calcium hydroxide.

Notice that in all cases, multiple simpler reactants combined into a single complex product.

A decomposition reaction is the opposite of a synthesis reaction. In decomposition reactions, a single complex reactant breaks down into simpler products like elements, elements and compounds, or just compounds. 

Decomposition reactions require an input of some form of energy. For example, under the influence of an electric field, water breaks down to give hydrogen and oxygen.

In the presence of sunlight, hydrogen peroxide decomposes into oxygen and water. Similarly, calcium hydroxide, upon being heated, decomposes into calcium oxide and water.