Conservation of Mass

The total mass of materials before a change equals the total mass of materials after, provided nothing has escaped the accounting. That is the whole principle, and it does a remarkable amount of work. It is the quantitative face of the conservation assumptions, and it is what lets a chemist treat a reaction as an arithmetic problem.

This principle is not an observation one makes directly. It is derived. The fourth assumption says no material disappears; the fifth says the total amount of material in the world is constant. If both of those are accepted, then the mass of what we had before must be the mass of what we have after. Nothing can have vanished into nothing. Nothing can have appeared from nothing. The books, if the counting is honest, must balance.

The principle became a principle of chemistry in the late 18th century, when Antoine Lavoisier put his scales where his theory was. Before that, people had reasonable intuitions about matter being conserved, but the intuition had not been forced to account for cases that seemed to contradict it. When metals rusted, they appeared to gain weight. When wood burned, it appeared to lose weight. Lavoisier weighed both situations with care and showed that the gain and the loss were not violations of conservation at all; they were the mass of air combining with the metal, or the mass of gases rising off the burning wood. Weigh the whole system, and the totals hold.

Once the principle is accepted, it becomes a tool. If you know the mass of what went in, and you know the mass of one product, you can calculate the mass of the other. If your totals do not match across a reaction, you know something has escaped the pan or condensed onto it; you have not disproved conservation, you have only caught yourself in an incomplete measurement. Conservation of mass is the arithmetic floor on which quantitative chemistry stands. Everything the field does with numbers assumes this holds.