If pure hydrogen and pure oxygen are both very flammable, why is water not?
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Burning means reacting with oxygen and making heat. The reaction for hydrogen burning is
2 H2 + O2 –> 2 H2O
**Water is hydrogen that has already burnt.** Water is like the “ash” of hydrogen. It’s the product formed by hydrogen and oxygen burning, which is why it itself doesn’t want to burn.
And in a more general sense, what other people are saying is true and really important: the properties of a material come from the arrangement of atoms, not just the atoms. Water doesn’t act like hydrogen or oxygen because it *isn’t* hydrogen or oxygen, it’s water!
Eg sodium is a silvery metal that explodes on contact with water. Chlorine is a toxic yellow green gas. Combine one atom of sodium and one of chlorine and you have sodium chloride – *table salt* which you eat every day. Just because it contains sodium and chlorine atoms, you can’t expect compounds to act like the atoms would by themselves. Forming the compound changes the properties.
The properties of a compound can be *completely different* than the properties of the individual elements it’s made from…because they are different things.
Sodium explodes if it touches water. Chlorine murders everyone who breaths it. Mix the explosive rock and the deadly gas, and you get…table salt.
Sodium is a highly reactive metal.
Chlorine is a highly toxic gas.
Surely when combined they’d make a chemical weapon right? Nope. Regular old table salt. Chemistry is much more complicated than that.
And to answer the question more directly, H2O is actually the product of burning hydrogen and oxygen, it doesn’t really want to burn again.
Ultra-ELI5 version: Because water isn’t hydrogen or oxygen.
Point of clarification though, oxygen is not flammable. You could pass a stream of oxygen through a fire where all of the fuel had been consumed already, and it would not increase the temperature at all.
Oxygen will certainly enable combustion (being an oxidizer), but it is not itself a fuel that is consumed.
Extremely simply put, chemistry isn’t like mixology.
You mix vodka and soda, you end up with a mixture of vodka and soda. That’s not how it works when bonding atoms into molecules.
“Mix” atoms to create a new molecule, and you’ll end up with an entirely new material that doesn’t have the properties of it’s constituent parts, at all.
Carbon arranged in different ways can give you graphite (pencil lead), coal, or literally diamonds. Add hydrogen to the carbon just so and you get gasoline.
So take your pencil lead, add hydrogen just so, and you get fuel for your car.
See my point? They aren’t remotely similar.
This is why when you see things like “Subway bread is only one molecule off from a yoga mat!” that’s a meaningless comparison, because one molecule could easily account for the difference between edible bread and an inedible yoga mat.
The difference between water and muriatic acid is just “one molecule” too.
So it’s folly to think of water as a “mixture” of oxygen and hydrogen for this reason.
Same reason ash isn’t flammable, while wood is. Literally. Light hydrogen on fire, and you get two things: an explosion, and water.
What makes them flammable is that those atoms are highly reactive. They want to connect with other atoms very badly; they love hugs. With water, they are already connected to each other already and are nice and cozy hugging. They like to stay hugging and are actually hard to pull apart, so there is little chance they will run off to hug other atoms.
I’ll put it in a simpler way that somewhat ignores the chemistry of the reaction. Pure oxygen and hydrogen can be said to have a high potential energy. Like a weight lifted high above your head. When you drop the weight, it lands on the ground and has less potential energy. The weight will never be raised again unless you pick it up. The universe likes these sort of things to happen in one direction only. High energy to low energy. Likewise, oxygen and hydrogen react to make water, and water has a lower potential energy than the gasses alone. You can break them up again, but you need to “do the heavy lifting” to make it happen. 💪
It would be impossible to answer a question that is based off of incorrect information.
Oxygen is NOT flammable. It is a catalyst for fire. You could fill a concrete room with 100% oxygen and light a match in the middle. The match will burn very quickly. But the air will not ignite/explode.
Flamability is the measurement of excitement. When hydrogen and oxygen are single, they are full of energy and exciment. When they are bonded or married, their excitements wane down.
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