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Q & A: Autoignition of Hydrogen to form water

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Most recent answer: 03/23/2011
If I had a giant tank of hydrogen and I released it into the air, would water just start pouring out of the tank? How much Hydrogen would have to be exposed to the air at one time to get water as I think of it rather than some water vapor?
- Tyler (age 16)
Orlando, Florida, United States

Dear Tyler,

What an interesting experiment! Actually, if you released hydrogen from a giant tank into the air, water would not start pouring out of the tank. Hydrogen is an extremely light gas - it is much less dense than the surrounding air - thus, the hydrogen in the tank would escape and rise upward instead, in the same way helium gas makes helium balloons rise when you let go of them. Meanwhile, the space in the tank would be filled with heavier surrounding air.  

However, if the temperature of the hydrogen gas was very high (at least around 536C), it would be able to spontaneously combine with oxygen in the air to form water. This is called autoignition - a process whereby a substance spontaneously ignites without any external flame or spark. The product of this reaction always produces water vapor (and not liquid water). Why? The reaction of hydrogen and oxygen to form water releases heat, since water is more stable than unreacted hydrogen and oxygen. Thus, the water vapor produced in the reaction would definitely be at a temperature higher than 536C (because the reaction only occurs at or above 536C). This is above the boiling point of water, so the water can only form as a vapor.

But of course, you can always find a way to cool the water vapor and get liquid water. For example, you could have a large, cold, metal surface right above the whole setup. Then the water vapor produced in the experiment would condense when it rises up and touches the metal - and you could tilt the metal surface to collect the water dripping off the corners. 

Just for fun, the volume of hydrogen gas needed to produce a teaspoonful of water (about 5mL) is about 100mL, or 20 times the volume of water. (This is assuming the hydrogen gas is kept at 536C, and has a pressure of about 175 times atmospheric pressure, which is about the highest pressure possible for storing hydrogen in a normal metal tank.)

Hope this helps, and thanks for the question!

- Mae

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(published on 03/23/2011)

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