Two or more reactants combined by forming bonds to form a new compound with a new molecular formula. Now let's look at our hypothetical example. In our example, we have two reactants, A and B react combined to produce a new product, a new compound, namely AB. Now let's look at a very common example. Whenever 1 mol of solid burns with 1 mol of diatomic oxygen in the gas state, it produces 1 mol of carbon dioxide in the gas state. Now, this is a very common combination reaction.