The law of Conservation of Mass states that the mass of matter before and after a chemical change remains the same. The number of atoms also remains the same. The only thing that changes is the arrangement of the atoms.
For example, the mass of hydrogen and oxygen before they combine to make water is the same as the mass of the water that they form. Furthermore, the number of atoms in the hydrogen and oxygen is the same as the number of atoms in the water.
The only change is in the arrangement of the atoms. Before the reaction, the hydrogen and oxygen atoms are not bonded to one another. However, after the reaction the oxygen and hydrogen are bonded to one another.
