In any chemical change mass is conserved.
This is one of the most fundamental laws of chemistry and is credited to Antoine Lavoisier who is often called the father of modern chemistry. His experiments in the late 1700's led him to this conclusion without any understanding of atoms or other particles we now take for granted.
Lavoisier's concept is important because it suggested to later scientists that the particles one starts with in a reaction are the same as the those one ends up with.
To us that means that only the arrangement of particles changes during a chemical reaction.