Conservation of mass — Chemistry, 11–13 years
In a chemical change, matter is rearranged rather than created or destroyed. The total mass stays the same when nothing can escape.
Nothing simply vanishes
When substances react, their particles are rearranged into new substances. The atoms do not disappear, and new atoms do not appear from nowhere. If the reaction happens in a closed container, weighing everything before and after gives the same total mass.
Why weigh both sides?
People once saw a substance shrink, grow or leave smoke and wondered whether matter had been lost or created. Careful experiments showed that gases matter too, even when we cannot see them. This idea solves the puzzle by counting every substance, including air.
Vinegar and bicarbonate
Put 50 g of vinegar and 5 g of bicarbonate in a sealed bag. Before mixing, the total mass is 55 g. Bubbles form because carbon dioxide is produced, but it stays in the bag. After the reaction, the sealed bag still has a mass of 55 g.
The missing gas
A reasonable mistake is to say mass has been lost when a tablet dissolves or burns and the solid becomes lighter. We can see the solid change, but not the gas that leaves it. The missing mass has usually gone into the air, so an open experiment does not weigh the whole system.
Checking recipes and pollution
Engineers use conservation of mass to check chemical recipes in factories and to track pollutants in air or water. If atoms enter a process, they must leave in products or waste. This helps find leaks and estimate how much material a treatment system must handle.
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