Eli5 why Is water which is 66.6% Hydrogen great for firefighting even though Hydrogen is highly inflammable?
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Molecules do not have the properties of their constituent atoms. You could say that a water molecule is hydrogen that has already burned. It can’t burn again.
When hydrogen burns the final product is water. The hydrogen in water is effectively already “burned”.
a) Water isn’t 66.6% hydrogen. Hydrogen accounts for two grams of water’s eighteen-gram molar mass, so it’s more like 11.1%.
b) The hydrogens are covalently* bonded to the oxygen, so they aren’t free to participate in combustion reactions.
*They are labile though, but that doesn’t matter for this discussion.
When you burn hydrogen it’s reacting with oxygen in the air to form water. Water is burnt hydrogen.
Water isn’t inflammable. It has different components than its constituent parts.
When hydrogen burns, it reacts with oxygen to make water. You can’t burn it twice because there’s no potential energy to be released.
Take sodium, for example. It’s a highly reactive metal, and if you put it in water, it will ignite and possibly explode.
Chlorine is a highly reactive gas. It can cause severe chemical burns, and it’s very deadly.
Put them both together, and you get sodium chloride, table salt. You literally eat it every single day without fear.
Oxygen really really wants to hold on to any molecule with an electron to spare. Hydrogen has one electron to spare. So oxygen will hold very tight creating one of the strongest bonds with two hydrogen atoms because oxygen has 2 spots open. Once formed it would take a lot of energy to separate them. When water evaporates it’s collecting energy and moving to a gaseous state but is water.
A super ELI5 answer:
Lots of elements are flammable because when there’s enough energy they really want to make new friends with other elements. They find those other elements, join them in a very strong hug, and release some of their own energy in the process.
But once those elements find each other and are caught in a strong hold, they’re not often looking to hug other atoms in the same way. It usually takes very special circumstances for them to want to break up their current hug and hug someone else. Once they find a stable hug with a good friend, they’re often happy to stay there.
Hydrogen is pretty comfortable hugging oxygen. Even in the face of a high-energy fire, it still wants to hang out with oxygen.
Molecules take on different properties than the atoms that compose them. When two hydrogen atoms and one oxygen atom bond to create a water molecule, the molecule becomes stable. When hydrogen is ignited on its own, what’s happenning is that the hydrogen is cumbustoming with the o2 in the atmosphere, heat energy is released in the form of a flare explosion and water vapor is the exhausted result of the combustion.
But if you start with h20, the hydrogen which on its own would be seaking o2 to attach to, is already satisfied and as such no chemicals reaction like a flair explosion will occure.
Same reason that table salt is made of sodium (a metal that reacts violently with water) and chlorine (a deadly gas).
Because hydrogen burns when it reacts with oxygen to form water. So water is the end product.
It’s sort of like asking, “Why aren’t ashes flammable?”
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