The law of conservation of mass follows from the concept that

a. atoms are indivisible.

b. atoms of different elements have different properties.

c. matter is composed of atoms.

d. atoms can be destroyed in chemical reactions.

"The law of conservation" is about how under the same condition of pressure and tempersture the number of atoms is constant.

Also, the fact that atoms have different properties, (as indivivisible as they may seem they have different potentials for forming molecules) is true.