Here's why:
* Atoms are not created or destroyed: Chemical reactions only involve the rearranging of atoms into different molecules. The atoms themselves remain the same.
* Mass is a fundamental property of matter: Since the atoms are simply rearranged, the total mass of the reactants (starting materials) will always equal the total mass of the products (substances formed).
Example:
Imagine burning wood. You might think that the wood disappears, but it's actually transformed into ashes, gases (like carbon dioxide), and other byproducts. The total mass of the ashes, gases, and any other products will be the same as the mass of the original wood.
Important Note: While the total mass remains constant, the *volume* of the substances involved can change. For example, when you combine hydrogen and oxygen to make water, the volume of the water will be much less than the combined volume of the hydrogen and oxygen.
