When water separates into hydrogen and oxygen gas, a decomposition reaction is taking place (one substance breaking apart or breaking down into two or more substances).
The law of conservation of mass tells us that mass is not created or destroyed during a chemical reaction, so the total mass will remain the same. This is supported by the diagram on the page, which shows that in the reactants, there are four hydrogen atoms and two oxygen atoms, and this amount is the same on the product side.
