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I have learnt that both hydrogen and oxygen exist as molecules. If so how was $\ce {H2O}$ formed?

In water there is two hydrogen molecule and one oxygen atoms but how is this possible? Oxygen only exists as a molecule in our earth then how does the molecule oxygen react with molecule hydrogen to form water.

Moreover, if water is formed by combining of hydrogen and oxygen molecules, then the chemical formula of water should be $\ce{H2O2}$, shouldn't it?

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    $\begingroup$ So what if oxygen exists as molecules? Molecules can be broken. $\endgroup$ – Ivan Neretin Mar 29 '18 at 6:05
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    $\begingroup$ If you look at Standard Gibbs free energy of formation of $\ce {H2O}$ and $\ce {H2O2}$, you would find some reasoning (for reasonable values, see: !en.wikipedia.org/wiki/Standard_Gibbs_free_energy_of_formation). $\endgroup$ – Mathew Mahindaratne Mar 29 '18 at 7:00
  • $\begingroup$ I didn't understand Standard Gibbs free energy of formation of H2O and H2O2 $\endgroup$ – Akash. B Mar 29 '18 at 7:27
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Water does not have two oxygen atoms (has one) and one hydrogen (has two) hence the molecular formula of water is H2O. Water forms when there is enough energy to break the molecular bonds within the Hydrogen and Oxygen diatoms, they then combine and form water. As Mathew said, look at the Standard Gibbs free energy of formation. It will explain that H2O is much more favourable to be formed than H2O2.

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