The law of conservation of mass descends automatically from the Dalton Atomic Theory, in particular from points 3 and 5.
The main points of Dalton's atomic theory were:
- Elements are made of extremely small particles called atoms.
- Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties.
- Atoms cannot be subdivided, created, or destroyed.
- Atoms of different elements combine in simple whole-number ratios to form chemical compounds.
- In chemical reactions, atoms are combined, separated, or rearranged.
As long as a chemical reaction consists of a rearrangement of atoms (point 5) and nothing changes in the atoms (point 3), the total mass - that is the sum of the masses of all the involved atoms - must remain unchanged.