conservation of volume

Hi,

In your question, are you talking about the reaction where

2 volumes of Hydrogen reacts with 1 volume of Oxygen and forming 1 volume of Water?

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H - H             O = O                  >                   H - O - H 

H - H                                                              H - O - H

This is the chemical reaction.

The volume that is taken up by molecules depends on the number of moles of the molecules (number of molecules exist). 

Before reaction, we have 2 mol of Hydrogen molecules (taking up 2 volumes) and 1 mol of Oxygen molecules (taking up 1 volume).

After they react, they form 2 mol of Water molecule (water vapor) which takes up 2 volume (since it is 2 mol of molecules).

You can imagine as they combine together, they are being closer together compared to when they are alone, which results in the "missing volume" that you mentioned.

* You can refer to 

Balancing chemical equation and Stoichiometry for more explanation and other examples. The link is below.

https://www.khanacademy.org/science/chemistry/chemical-reactions-stoichiome#empirical-molecular-formula