The chemical reaction is based from the number of moles of each species participating in the reaction and not by its volume.
This is how it works:
First, this is the reaction for the formation of water:
All chemical reactions are based on the number of moles (see those coefficients?) and not in their volume. Meaning 1 mole of
The answer lies in Avogadro's Hypothesis.
Avogadro's Hypothesis states that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules.
The chemical equation for the formation of water is
#"2H"_2"(g)" + "O"_2"(g)" → 2"H"_2"O(g)"#
We could also write
#"2 mL + 1 mL → 2 mL"#
#"100 mL + 50 mL → 100 mL"#
The volumes will always be in the same