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Can a spark ignite hydrogen? Also, what about the tip of a cigarette? I've heard that hydrogen is very explosive. Thanks

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Can a spark ignite hydrogen? Also, what about the tip of a cigarette?

Yes, if the $\ce{H2(g)}$ concentration in oxygen or air is greater than about 4% not only will it burn, but most likely explode. You should avoid all heat sources when dealing with it, including direct sunlight. Just so you know, hydrogen safety datasheets include words such as "extremely flammable" and "DANGER" in red capital letters.

Besides its high flammability, another thing that makes it so dangerous is that you can barely see its flames when there's a high oxygen concentration, as seen below. It also reacts violently (A.K.A. explodes) with most oxidizing compounds, so you should be careful where you are storing it.

A rocket engine at full thrust. There's a barely visible bluish flame below it, that's the hydrogen-oxygen mix used as fuel. enter image description here

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Actually hydrogen is not at all explosive. The mixture of hydrogen with oxygen (also air) is. As you know hydrogen is stored as liquid (high pressure cylinders) and it's perfectly stable. You can mix it with any amount of argon heat it as high as you want and nothing will happened. OK, if you reach temperature as high as on the Sun then it will explode, but that's an nuclear fusion (i.e. physics not chemistry). On the other side the mixture of hydrogen with oxygen can very easily by ignited by an electric spark or a cigarette.

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    $\begingroup$ Even at the Sun's core temperature Hydrogen is incredibly stable. After all, the Sun hasn't burned even half its Hydrogen after all these billions of years, and that's despite the presence of C,N and O catalysts. $\endgroup$
    – MSalters
    Aug 3 '14 at 22:28
  • $\begingroup$ This is slightly misleading. If you ha a room completely filled with hydrogen then, yes, a spark would not ignite the hydrogen. But few such rooms exist. What if you have a balloon filled with pure hydrogen? Then that is highly flammable and can be ignited with a spark despite the hydrogen being pure. It won't explode but it will burn. The same balloon filled with an oxygen/hydrogen mixture will explode violently. $\endgroup$
    – matt_black
    Mar 25 at 10:13
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Hydrogen is flammable only when mixed with oxygen, hence this can be mixed with compressed oxygen in petrol engines (hydrogen gas & oxygen gas) to be used as fuel.

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