Total material is conserved. The amount of stuff in a closed accounting does not change; it only moves and rearranges. No new matter is created, and no old matter is destroyed. This is the fifth foundational assumption.
The Earth system holds, to the best of our observation, a fixed amount of matter. It is not gaining matter from outside, and it is not losing matter to anywhere. What changes is the arrangement: where the matter is, what form it takes, which class of material it currently belongs to. The total is constant; the distribution is dynamic.
Conservation gives chemistry its arithmetic. If you can measure what went into a process and what came out, the totals must add up; if they do not, you have either lost something to your senses or gained something you have not accounted for. This is the principle that lets a chemist trust mass balance, that turns observation into a kind of accounting. Lavoisier built the discipline on it; every quantitative chemistry problem since has assumed it.
Conservation pairs with the fourth assumption. Together they say: nothing vanishes, nothing appears from nothing, and the books always balance. These two are the mathematical floor of macroscopic chemistry. Everything quantitative the field has to say rests on them.
Question
If conservation is true, why does my candle look like it has lost mass after burning?
Reveal Merrill's answer →Hide answer
Because the products of burning have left the candle: most as gases that have risen into the air, some as soot. The mass that was in the candle is now elsewhere. Weigh the candle, the air, and the soot together before and after, and the totals match. The candle has not lost mass; the system has not lost mass. The mass has only relocated.