No material disappears without new material appearing. When stuff in the world changes form, it does not vanish; it becomes something else, somewhere else, in some other arrangement. This is the fourth foundational assumption.
Watch a candle burn. Watch wood char in a fire. Watch ice melt and water steam. In every case, what looks like loss is transformation. The candle becomes wax-on-the-table, soot-in-the-air, and warmth-in-the-room. The ice becomes liquid water, the liquid water becomes vapor; nothing is gone, only relocated and renamed. The disappearance of one material is always paired with the appearance of another.
If materials could simply vanish, there would be no chemistry, only an inventory of what currently exists and what currently does not. The discipline of the field is built on the discipline of the assumption: if you saw a material before and not after, you have not finished looking. The new material is somewhere. Find it.
This assumption sets up the next. Once we accept that materials transform rather than vanish, the natural follow-up is to ask whether the totals stay the same. They do, and that is the fifth assumption: total material is conserved. The two assumptions together are what make chemistry quantitative. We can balance equations because the books always balance.
Question
When a candle burns down to nothing, where did the wax go?
Reveal Merrill's answer →Hide answer
Into the air, the table, and the room as warmth. None of it is gone, only relocated and renamed. Some of it is now soot in the air, some is now water vapor, some is now warmth. The disappearance of the wax is paired with the appearance of these other materials. If you saw something before and not after, you have not finished looking.