Why a reaction can seem to lose mass in an open container
In ordinary chemical reactions, atoms are rearranged rather than disappearing. A reacting mixture can nevertheless become lighter on a scale if gas escapes into the room. To check mass conservation, you must account for the whole system, including material that leaves or enters the container.
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Keep track of the system boundary
A chemical equation rearranges atoms into different substances. In ordinary chemistry, total mass is conserved to normal experimental accuracy when all reactants and products are included.
An open container can give a misleading impression. If a reaction produces gas that escapes, the material remaining on the scale has less mass. That does not mean the missing atoms have ceased to exist.
The measurement follows whatever remains inside its boundary. A complete account includes escaping gas, incoming air and any other transferred material.
- Gas leaving can reduce the measured mass.
- Oxygen entering can increase a reacting solid's mass.
- A closed system keeps the material available for accounting.
This is why comparing only a solid before and after a reaction may be insufficient: some of the reaction's matter may be elsewhere.