General Doubt. My doubt about combustion

Question

When we give flame to a newspaper it burns readily. What we need for a fire (fuel, heat, Oxygen) is also known as the fire triangle. But why does the paper not burn when we pour boiling water over it? Water has heat and oxygen and the paper is the fuel.

Answer 1

Boiling water is not hot enough to ignite the paper. If you were to pour molten metal on the paper instead then it would burn. Note that the source of oxygen is from the surrounding air - the fact that water molecules contain oxygen atoms is irrelevant since the water molecules are not split apart at these temperatures.

Answer 2

Boiling water is not hot enough to start paper on fire, which happens around 450 degrees F. Now we note that water is noncombustible; the oxygen and hydrogen it contains are chemically locked up to one another and cannot burn. In fact, water is used to put out fires. When you pour water on a fire, it absorbs the heat of the fire and blocks off oxygen in the air from getting to the fire itself and thereby makes it go out.

Answer 3

Burning is an oxygenation reaction, where the components form bonds with oxygen atoms, releasing energy: $$ 2H_2 + O_2 \rightarrow 2H_2O,\\ C + O_2 \rightarrow CO_2,\\ 2C + O_2 \rightarrow 2CO, $$ and so on. What is a final product of the reaction - splitting water into oxygen and hydrogen requires more energy than would be released after this oxygen joins with hydrogen and carbon contained in the paper.

Furthermore, burning/combustion is really not happening between molecules, but via complex pathways (chain reactions) involving already free radicals/ions. Thus, hydrogen and oxygen in a container may co-exist for very long time without any water forming... unless there is a spark or brutal shaking which triggers the reaction, whose swiftness amounts to an explosion, almost instantly consuming all the available reactants.

Finally, as is well-know, the temperature of the burning paper is Fahrenheit 451 ($451F\approx232,8^oC$), which is higher than $212F\approx100^oC$ corresponding to boiling water - water is just not hot enough to trigger oxygenation of paper by the surrounding air.

Related: Can we call rusting of iron a combustion reaction?

Answer 4

To start a fire you need three things:

1. A fuel source - the carbon in the newspaper works fine.

2. Oxygen - but not just any oxygen, it has to be in a form where it can readily combine with the fuel - water won't work, as the oxygen it contains is too hard to separate from its hydrogen atoms, whereas the diatomic oxygen we breathe is just weakly bound enough that it can be broken up without much effort.

3. Heat - in order for the fuel to burn with the oxygen, we first need to add some heat energy to kick-start the oxidation - paper begins to combust at about 230 degrees celsius, more than twice as hot as 100 degree boiling water.

So the reason why boiling water and paper don't burn is a lack of easily-usable oxygen atoms and insufficient heat - you're missing two points of the triangle.