Conservation of mass
The idea
Burn a log and the ashes weigh far less than the wood — so where did the mass go? Nowhere, in fact: it left as invisible gases. Conservation of mass says that in any change, physical or chemical, atoms are only rearranged, never created or destroyed, so the total mass before equals the total mass after. You already met this idea with mixing and melting; now it extends to full chemical reactions, where it becomes one of the most powerful bookkeeping tools in science.
The mental model is an atom headcount. Every atom that enters a change must come out the other side in some product, even if that product is a gas you cannot see. The classic misconception is believing mass is genuinely lost when something burns, fizzes, or rusts away — what is really happening is that gases carry mass into the air, or pull it in from the air, without being weighed. Seal the whole event in a closed container, weigh everything, and the books always balance to the last gram.
Worked example
You pour 120.0 g of vinegar into a cup on a scale and add 10.0 g of baking soda. The mixture fizzes vigorously. When the fizzing stops, the cup and its contents weigh 125.6 g. How much gas escaped, and was mass actually conserved?
- Total the starting mass: 120.0 g of vinegar + 10.0 g of baking soda = 130.0 g sitting on the scale before anything happens.
- Compare with the final reading: the scale shows 125.6 g, so 130.0 − 125.6 = 4.4 g of mass is no longer in the cup.
- Account for the missing 4.4 g: the fizz was carbon dioxide gas escaping into the room. Those atoms left the cup — they did not leave the universe.
- Test the law properly in your head: run the same reaction in a sealed, strong bottle and the scale would read 130.0 g the whole time, because the CO₂ would be trapped and weighed.
- Interpret: mass only seems to vanish when a product escapes the weighing. Count every product, including gases, and conservation of mass holds exactly.
Answer. About 4.4 g of carbon dioxide gas escaped; mass was conserved, since the 125.6 g in the cup plus the 4.4 g of escaped gas equals the original 130.0 g.
Check your understanding
- Why does a rusting iron nail actually gain mass over time, and where must that extra mass come from?
- How would you set up an experiment to prove mass is conserved in a reaction that produces a gas?
- What happens to the atoms in gasoline when a car burns a full tank, and how can the tank empty out without a single atom being destroyed?
- How does the particle picture of a reaction explain why total mass can never change, no matter how dramatic the reaction looks?
Build the foundations first
Conservation of mass builds on these concepts. If any feel shaky, start there.