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More about the equation
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    H2 + O2 -> H2O

Although the above equation does correctly show that hydrogen and oxygen combine to form water, it is not entirely correct because the number of atoms on each side of the arrow is not equal. The equation is more properly written as...

    2H2 + O2 -> 2H2O

The equation now shows that two hydrogen molecules combine with one oxygen molecule to form two water molecules. Note that there are four hydrogen atoms and two oxygen atoms on each side of the equation.

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