How come hydrogen ......?

how come hydrogen is a explosive gas and oxigen can start fires but them put together makes water that helps put out fire????

Answer:
Your question is actually a bit more complicated than the other answers have given it credit for.

The other posters are correct to the extent that they describe the difference between dihydrogen (H2) or dioxygen (O2) and water (H2O). The whole point of distinguishing between elements and compounds is that different substances are *different* -- that the physical and chemical properties of water are not just the additive sum of those of hydrogen and oxygen. H2 is explosive precisely because when exposed to O2 it forms an unstable mixture that can undergo a highly exothermic reaction to form H2O, a very different and very stable compound. O2 can start fires precisely because it can form more thermodynamically stable compounds with nearly every other element on the periodic table, again by undergoing often violently exothermic reactions (called, as you said, "fires").

So O2 and H2 react with each other violently, and thereby form a more stable (and so far less reactive) material, namely water. Which is why water won't burn any more, it's already *been* burnt. It's already the most stable compound that can be formed from hydrogen and oxygen, so throwing it on a fire doesn't cause any further reaction.

OK, so that's why water itself doesn't burn. But nobody has addressed the second part of your question -- why water will actually extinguish a fire, and stop something *else* from burning.

Although O2 can react with organic material (e.g. paper wood coal gasoline methane propane and anything else you might feel like setting on fire today) in an exothermic reaction to form more stable products (ie CO2 and water), it doesn't usually do that without help. Paper doesn't explode upon contact with air. A balloon filled with methane and oxygen just sits there. Until you light a match and start the reaction going: the fire requires some heat to start. Fortunately, the fire also releases heat, so it's self-activating once it gets going, which means that once a fire begins, it'll keep burning until you run out of fuel -- *unless* you can remove that heat and take it away.

Water can absorb a huge amount of heat, especially if it boils off. This isn't a chemical change, it's just liquid phase H2O turning into gas phase H2O. Throw water on a fire -- water absorbs heat, heat water boils, hot gas phase water steams off and removes the heat, burning fuel cools down, there's no longer enough heat to initiate the reaction with O2, the fire goes out.

However, the CO2 in a fire extinguisher is better. Not only does it not burn (same reason as for water), and not only does it absorb heat, removing the activating energy for the reaction, it also smothers the flames by creating a cushion of dense gas around the fire, effectively preventing O2 from getting to the fuel. It denies the chemical reaction one of the reactants, so the reaction simply stops.
When combined, the final molecule is a energenically stable.
Chemistry is intriguing ...
because water is a compound and compounds have different properties than their constituents. same thing with iron sulfide
iron is attracted to a magnet
but iron sulfide isn't.
Because water's a complete combustion product, keyword "complete"
Because when combined the outer energy level becomes filled so they become stable. Seperately there energy levels are not so the are unstable.

Some with Salt (NaCl)
Alone Chlorine is poisinous, in salt it is not.
They react strongly together and give a stable product. This product is very stable due to hydrogen bounds particularly strong in water.

One characteristic of water is its very high specific heat and this serves to extinguish fire . You need comparatively to other products a very high energy to rise the temperature of water for a given value. So ,if you throw water on a fire a great quantity of the energy serve to rise the temperature of water and to evacuate heat.
Hydrogen reacts with oxygen because they are a thermodynamically unstable mixture.
Water on the other hand extinguishes fire because it blocks air (consequently oxygen) away from the combustion area.No air no fire. I don't think this has to do with water being a product of combustion .
When two individual atoms have different chemical properties combine, this compound has different properties as well. Compounds do not follow genetics.
because they are crazy!
Another way of looking at combustion is oxidation. For example, we burn CH4 = we oxidise C, as the end product is CO2.

That is why everything on earth need oxygen to burn. By burning hydrogen, we are effectively oxidising it to give H2O. (oxidation state of H increases from 0 to +1) As H2O is the fully oxidised form of hydrogen, it is very stable and can be used to put out fire.

*If you notice, CO2 which is the fully oxidised form of carbon, is also used to put out fire. Similarly, SiO2 can also be used to put out fire.

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