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Any websites I have found have conflicting answers.

One said that they don't actually burn underwater, rather they separate hydrogen from oxygen and burn at the surface. Another actually said that they do burn underwater because they take the oxygen source from the water. I do not have a specific definition for "burn" (the question itself is my assignment- write an essay answering that question!), so I know about as much as you do concerning the wording of the assignment.

I do know that alkali and alkaline earth metals are generally very reactive with water, and usually result in exothermic reactions.

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    $\begingroup$ Considering that you are an 8th grader, I think you already know all the answer, you are just overdoing it. I am not your teacher, but if you wrote an essay actually pointing out the vagueness of the term "burn", or that are several possible mechanism of oxidation, I would call it a very good job. $\endgroup$
    – Greg
    Commented Jul 8, 2014 at 19:34

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Your last sentence was the key. Metals like sodium and magesium react with water according to the following equations $$\ce{2Na + 2H2O -> 2NaOH +H2 + heat}$$ $$\ce{Mg + 2H2O -> Mg(OH)2 +H2 + heat}$$ Both reactions produce hydrogen gas and a lot of heat. As the hydrogen gas bubbles out of the water it mixes with atmospheric oxygen. The intense heat ignites the hydrogen-oxygen mixture and the mixture continues to burn (possibly explosively depending upon the quantities involved) as long as hydrogen continues to be generated.

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  • $\begingroup$ Magnesium, actually, reacts only slowly with water. Basically it might react enough so that ribbons can temporarily float themselves on generated hydrogen bubbles, but that's about it. Magnesium that has been set on fire continues to burn in steam. $\endgroup$
    – Oscar Lanzi
    Commented Jun 26, 2021 at 13:59
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To my knowledge, neither of these metals "burn" underwater. The word burn generally suggests that something reacts with oxygen. As others have stated above, these very active metals will react with water by displacing hydrogen as H2. The hydrogen definitely will react with oxygen assuming that enough heat is evolved to start the reaction (very likely for sodium, less so for magnesium).

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  • $\begingroup$ It may depend on language (my native language is not English), but by burning in this case one means a rapid oxidation with heat and light generation. E.g. magnesium torch can be light in normal air then a diver can take with her/him under water, and it still produces the light and all stuff. $\endgroup$
    – Greg
    Commented Jul 8, 2014 at 19:31

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