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Why isn't water flammable?


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In a water molecule, oxygen is in the -2 oxidation state and the hydrogens are each in the +1 oxidation state. These elements are quite energetically happy in this combination. Also, the O-H bond is quite strong. A hydrogen can be ripped of to give [ce] OH^-[/ce] or the equivalent radical but then that O-H bond is much more difficult to break.

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Like ashes, water is already "burnt" and so at a very low energy level. All that energy released when burning hydrogen with oxygen would have to be put back in to separate them back into hydrogen and oxygen. However you can burn magnesium in water, and the magnesium will steal the oxygen from the water and release the hydrogen.

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However you can burn magnesium in water, and the magnesium will steal the oxygen from the water and release the hydrogen.

 

This happens with alkali metals as well. And since the reaction is exothermic, the liberated hydrogen is ignited and burns to create more water.

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This happens with alkali metals as well. And since the reaction is exothermic, the liberated hydrogen is ignited and burns to create more water.

 

I'm pretty sure that's what makes the "pop" test work. You light a flame near the supposed hydrogen source and see if you can hear a "pop". It only works with relatively large quantities of hydrogen. It's a simple experiment and was actually performed by one of my science teachers.

 

It also should be noted that a number of the alkali metals react in a very nasty way to water. Some of the more inert ones produce bubbles. Which are probably hydrogen and if there's enough bubbles, it might be worthwhile doing the pop test.

 

On a slightly different note, I remember the same science teacher erecting blast shields before placing a small amount of sodium in a beacker of water. The sodium fizzed and hissed, dancing across the surface of the water (sodium actually seems to float). Probably producing hydrogen and sodium oxide. I didn't see a flame (supposedly indicative of burning hydrogen), but neither did I see a hydrogen flame when the pop test was performed.

Edited by Samm
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I think the pop is because Hydrogen burns fairly quickly and will happen of you have an accumulation of it when the flame is introduced With an alkali, once the combustion starts it will go continually; they get more reactive as you move down the periodic table. If one produced bubble without inducing combustion it would have to be Lithium. I can personally attest to the violence of Rubidium reactions.

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I think the pop is because Hydrogen burns fairly quickly and will happen of you have an accumulation of it when the flame is introduced With an alkali, once the combustion starts it will go continually; they get more reactive as you move down the periodic table. If one produced bubble without inducing combustion it would have to be Lithium. I can personally attest to the violence of Rubidium reactions.

 

I can say, I didn't know the hydrogen reacted continually, during some of the more energetic alkali-water reactions. Yeah, the hydrogen definitely combusts quickly during the pop test, I would almost describe it as a small explosion.

Edited by Samm
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  • 5 weeks later...

Just a point you made earlier Samm....you don;t sodium oxide; you get sodium hydroxide. Thats why there called the alkalie metals...they react with water to give an alkaline solution.

 

Nad yes, lithium hardly reacts where as ceasium is EXTREMLEY reactive with sodium, potassium and rhubidium filling the gaps in between.

Edited by Horza2002
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