Edit: wow okay, I did not expect to wake up to THIS. Of course my most popular post would be a dumb stoner question.
Thankyou so much for the awards and the answers, I can sleep a little easier now
In: Chemistry
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It requires energy to break chemical bonds, and it releases energy when we form new chemical bonds.
You can lower the amount of energy required to break the bonds of the hydrogen and the oxygen by pressurizing them, but it still requires that thermal energy /a spark to get the process to occur.
To all posters: Please remember that ELI5 is for *explanations* to questions, not just answers.
This means that any response to the OP should detail *why* you can’t just stick hydrogen and oxygen together and get water.
Unfortuately, a shockingly large number of posts have consisted of little more than “hydrogen is hard to get.” This answers a similar question to OP’s, but doesn’t explain the chemistry behind their question. We are a volunteer mod team, and unfortunately, popular posts like this can start to overwhelm us. Please do your part and make sure you check out this part of rule 3 specifically!
> Answers” are not the same thing as “explanations”. An explanation contains more detail. Generally an explanation has 3 components; a context, mechanism, and an impact, while an answer will leave 1 or more of those to be inferred by the reader. This is why very short comments are automatically removed; a user can absolutely ask for an automatic removal to be reviewed.
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We totally can by burning hydrogen. Burning is the process of combining a chemical with oxygen and releasing heat at the same time. The reason why it burns is because two hydrogen and one oxygen molecule contain more internal energy than a single water molecule, so that energy goes away in the form of heat and light.
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A single atom of Hydrogen has one proton in the nucleus, and one electron, and is electrically neutral. But the quantum physics that govern electron behavior around an atomic nucleus allow two electrons in that energy level (or shell). In fact, two electrons is a more stable state than one. So a Hydrogen atom pairs up with a buddy, and they share the two electrons in the shell, to form a Hydrogen molecule – H*_2_*.
Oxygen is similar – it has 6 outer electrons in a shell that can allow 8, so Oxygen atoms buddy up to form Oxygen molecules, where each atom has 8 electrons part of the time. So while oxygen atoms and hydrogen atoms are both really reactive on their own, when they are in molecules they are quite stable.
If you mix Oxygen molecules and Hydrogen molecules, nothing will happen – it’s still stable. Throw a spark in there, and you get a **BOOM**. All it takes is one molecule of oxygen to be disrupted into oxygen atoms, and those free atoms will rip the Hydrogen molecules apart to form something stable (water – H*_2_*O). This is more stable than the original molecules, and chemical binding energy in the form of heat is released – this helps further molecules break up and react. A disconnected atom of hydrogen or oxygen is called a *free radical* which can initiate further molecular breakup and then gets released to trigger more reactions. This is why exploding Hydrogen/Oxygen is so fast and energetic, and was used for rocket fuel.
In the book *The Martian*, the protagonist needs water for the potato plants he is growing. He has Oxygen, but he makes Hydrogen from a rocket fuel called Hydrazine and a catalyst that releases hydrogen and nitrogen. He has to ensure the hydrogen keeps burning to make water to prevent hydrogen explosions.
Other people have said yes you can, but I don’t think they’re getting to the meat of your question. If you have molecular oxygen and hydrogen, you have to break their bonds before you create oxygen-hydrogen bonds. This takes energy, usually found in the form of a flame. If you have atomic oxygen and hydrogen, you won’t have to break those bonds first. But you still can’t just put loose atomic hydrogen and oxygen in a balloon and precipitate water. This is because every reaction has an **activation energy**. In other words, you have to give the reaction a little “kick” for it to actually happen. Now, this “kick” can take the form of really any energy, It could be kinetic (physically mash them together), thermal (flame or just elevated temperature), pressure (like if you mixed them in a bag and squeezed it really hard). It could also turn out that whatever environment you choose could have enough energy by itself to facilitate this process.
For example, rust. The process of oxidizing iron is considered spontaneous because it can happen in your common ambient environment. You can accelerate it with moisture, and even more so by salt water but it’ll rust on its own if you give it time. On the other hand, the thermite reaction is not spontaneous. Igniting rust with aluminum powder gives you a very intense, self sustaining burn giving you iron with aluminum oxide. However this will not happen in ambient conditions because it requires a massive kick to start, something on the order of a hotly burning bunsen burner.
E: add example
You know how you can’t just blow air on wood and it will start a fire? You need to light it to start a fire.
In chemistry, there’s this concept called activation energy. In order for chemicals to react, they need to overcome this energy barrier in order to reach the new state, and this is typically done with heat. And in some cases with electricity (since what holds molecules together are electrons and electricity is using an electric potential to move electrons, we can induce a chemical reaction through an electric potential alone).
So you need to smosh hydrogen and oxygen together hard enough in order to make water. And the key word is hard enough. If you place hydrogen and oxygen molecules together without the activation energy, it won’t react.
As others said, we can, and I believe that in space we sometimes do, but: why should we?
Water is extremely abundant and easy to transport (unlike hydrogen, which likes to just go slowly seep through solid metal while damaging it – “Hydrogen Embrittlement”). In fact, even the ISS gets its oxygen shipped in the form of water. 18 kg of water consist of 16 kg of oxygen and 2 kg of hydrogen, and unlike oxygen, water doesn’t need to be stored in pressure vessels, doesn’t make everything flammable, etc.
Fuel cells produce water from hydrogen and oxygen as a byproduct (their main purpose is providing electricity), but at least on earth, we rarely have a reason to capture and use that water.
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