Combustion is the burning of hydrocarbons (compounds containing carbon, hydrogen, and possibly some oxygen) in the presence of oxygen to produce water and carbon dioxide. Combustion requires three things to occur: fuel (hydrocarbons), oxygen (from the air), and a catalytic spark. Complete combustion will produce only carbon dioxide and water as the products and nothing will be leftover. Incomplete combustion will produce other byproducts like carbon monoxide or carbon soot left behind. There are instruments available that can measure the amount of carbon dioxide and water vapor produced in an enclosed chamber. So if a known amount of a hydrocarbon is placed in the chamber and burned, we can know if it has been completely converted to the theoretical amount of CO2 and water required for complete combustion.