You already know that when you burn a piece of paper, what remains is a tiny, fragile ash — much lighter than the paper you started with. It looks like mass has vanished. But if you could collect every wisp of smoke, every invisible gas that escaped into the air, and weigh them together with the ash, you would find the total mass is exactly the same as the original paper plus the oxygen that helped it burn.
That is the core intuition: nothing is lost, nothing is created — matter just changes form.
The precise statement
The Law of Conservation of Mass says:
In a closed system, the total mass of the reactants equals the total mass of the products.
A "closed system" simply means nothing enters or leaves — no gas escapes, no solid is added from outside. In a chemical reaction, atoms are rearranged into new molecules, but no atoms are destroyed and no new atoms are made. Since each atom has a fixed mass, the total mass stays constant.
∑mreactants=∑mproducts
Why it matters
This law is the foundation of all chemical equations. When you write:
CH4+2O2→CO2+2H2O
you are not just showing what reacts and what forms — you are making a mass balance. The number of carbon, hydrogen, and oxygen atoms on the left must exactly match the number on the right. If they don't, the equation is wrong.
A common mistake is to think mass is "lost" when a gas is produced (like fizzing in a reaction). That only happens if the gas escapes — the mass is still there, just not in your container. Always account for all products.
A simple example
Take the reaction between hydrogen and oxygen to make water:
2H2+O2→2H2O
- Reactants: 4 g of hydrogen + 32 g of oxygen = 36 g total
- Products: 36 g of water
Exactly equal. No mass created, none destroyed.
Historical note …