In short
The law of conservation of mass says that mass is neither created nor destroyed in a chemical reaction, so the total mass of the reactants equals the total mass of the products.
Atoms are rearranged in a reaction, but none are made or lost. This is why a chemical equation must be balanced. The law holds for a closed system. In an open container, a gas that escapes can make the mass appear to fall, and a gas that is taken in from the air can make it appear to rise. The law was established by Antoine Lavoisier.
of hydrogen reacts completely with of oxygen to form of water, since .
Also searched as: what law of conservation of mass, conservation of mass.
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