Chapter 3 · Matter and Energy
3.7Conservation of Mass: There Is No New Matter
9 min · two checks
Predict
A match burns to ash that weighs less than the match. Has mass been destroyed?
The idea
- State the law of conservation of mass.
- Explain an apparent mass loss in an open container.
In a chemical reaction, atoms are rearranged, not created or destroyed. The law of conservation of mass says the total mass of the reactants equals the total mass of the products in a closed system. That is why chemical equations must be balanced: every atom on the left has to show up on the right.
Open beakers fool the balance. A steel wool pad gains mass as it rusts if the pan is open to air, because oxygen from the air joined the iron. A fizzing tablet seems to lose mass because CO₂ drifts away. Neither case violates the law. Close the system — or account for the gas that entered or left — and the masses match. Nuclear reactions, much later, convert a tiny amount of mass into energy. Ordinary chemistry does not.
Keep these
- Closed system: mass of reactants = mass of products.
- Apparent losses are usually gases escaping, or gains are gases joining.
- Balanced equations express conservation atom by atom.