The Law of Conservation of Mass
"Mass cannot be created or destroyed in a chemical reaction."
During a chemical reaction, atoms rearrange themselves to form new substances. No atoms appear out of nowhere, and none disappear. This means:
This was first clearly stated by French chemist Antoine Lavoisier in the 1780s using careful experiments with sealed containers.
Why Does Mass Seem to Change Sometimes?
In open containers:
- Burning (combustion): oxygen from the air is added, so the products can be heavier than the original fuel. Conversely, the carbon dioxide and water vapour formed escape into the air, making the solid residue lighter.
- Thermal decomposition of a carbonate: CO₂ gas escapes, so the solid remaining is lighter — but the total mass (including the gas) is the same.
If you heat calcium carbonate in an open dish and only weigh the solid before and after, it looks like mass was lost. But the "missing" mass is the carbon dioxide gas that escaped into the air. Collect and weigh the gas too — total mass is conserved.
Balanced Chemical Equations
Conservation of mass is shown in balanced equations: the same number of each type of atom appears on both sides.
Example: burning methane
Count: 1 C, 4 H, 4 O on each side. ✓
Example: iron + sulfur
1 Fe and 1 S on each side. ✓
Calculating Masses in Reactions
Using conservation of mass, if you know most of the masses in a reaction, you can calculate the rest.
Example: 10 g of calcium carbonate is heated. 5.6 g of calcium oxide is made. What mass of carbon dioxide was produced?
Always check your balanced equations by counting atoms on both sides. If they don't match, adjust the numbers in front of the formulae (coefficients) — never change the formulae themselves.
Real world: Rusting iron appears to gain mass
Leave an iron nail exposed to damp air and it gains mass — the rust (iron oxide) is heavier than the original iron because oxygen atoms from the air have bonded with the iron atoms. The total mass is conserved: the mass gained by the nail equals the mass of oxygen taken from the air. 4Fe + 3O₂ → 2Fe₂O₃ — every oxygen atom is accounted for on both sides of the equation.
Real world: Burning wood appears to lose mass
A log on a fire seems to lose mass as it burns down to a small pile of ash. In reality, most of the mass has been converted to CO₂ and H₂O vapour, which escape into the air. If you could collect all the gases and add their mass to the ash, it would equal the original mass of the log plus the oxygen it consumed. Mass is conserved — it is just hard to see the gases leaving.
Real world: Weighing reactions in a sealed flask
Chemists prove conservation of mass by conducting reactions in sealed flasks on a balance. Mixing baking soda (NaHCO₃) and vinegar (acetic acid) in a sealed bag: the CO₂ produced stays in the bag. The balance reading does not change before and after the reaction. But if you open the bag and let the CO₂ escape, the mass appears to drop — confirming that the "missing" mass was the gas all along.
Where do you see this in real life? Rusting iron gains mass (absorbs oxygen), burning wood appears to lose mass (CO₂ and water vapour escape), baking soda and vinegar in a sealed bag (mass unchanged) — conservation of mass is at work in every reaction around you, even when the evidence seems to contradict it.