Question

When most fuels burn, water and carbon dioxide are the two main products. Why can´t you say that water and carbon dioxide are products of all combustion reactions

Answer 1

That is only the combustion of a hydrocarbon. Rust is a combustion reaction because oxygen is added.

Fe(s) + O2(g) => FeO2(s)

Answer 2

Answer: Because some of the processes are also known as incomplete combustion.

Explanation:

Combustion reaction is defined as the chemical reaction in which a hydrocarbon reacts with oxygen gas to produce carbon dioxide gas and water molecule.  The chemical equation for this reaction follows:

$\text{hydrocarbon}+O_2\rightarrow CO_2+H_2O$

If supply of oxygen gas is limited, it is known as incomplete combustion and carbon monoxide gas is also produced as a product. The chemical equation for this reaction follows:

$\text{hydrocarbon}+O_2\text{(limited)}\rightarrow CO+CO_2+H_2O$

Thus, we cannot say that all the combustion reactions produce carbon dioxide and water as products because it depends on the supply of oxygen gas.