lesson
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If you burn a 5 kg wooden log, you are left with a tiny pile of ash weighing less than 200 g. But if you heat a strip of steel wool in air, it actually gets noticeably heavier on a balance.
In 1789, French chemist Antoine Lavoisier solved this contradiction by showing that matter cannot be created or destroyed—this is the law of conservation of mass.
Because chemical reactions simply rearrange existing atoms into new combinations, the total mass of the reactants must always equal the total mass of the products.
📊Interactive diagram
So why did the wood seem to lose mass while the steel wool gained mass? The answer comes down to whether the container is closed or open.
Closed Systems
A closed system is an apparatus where no matter can enter or escape, such as a flask sealed with a rubber stopper.
When you mix solutions of calcium chloride and sodium carbonate inside a sealed flask, they react to form a cloudy white solid called a precipitate and dissolved salt water.
📊Interactive diagram
Because the stopper traps all particles inside, the total number of each type of atom remains identical, keeping the digital balance reading completely unchanged.
What happens if we run a reaction in a container without a stopper where a gas is produced?
Open Systems: Apparent Mass Loss
An open system is a setup that allows matter and energy to move freely between the reaction container and the surrounding environment.