Here's why:
* Atoms are rearranged: In a chemical reaction, the atoms within the reactants (starting materials) are rearranged to form new products.
* No atoms are created or destroyed: The total number of atoms of each element remains the same before and after the reaction.
* Mass is conserved: Since the number of atoms doesn't change, the total mass of the reactants must equal the total mass of the products.
Example:
Consider the reaction of hydrogen gas (H2) with oxygen gas (O2) to form water (H2O):
2H2 + O2 → 2H2O
* Reactants: 2 hydrogen molecules + 1 oxygen molecule
* Products: 2 water molecules
The number of hydrogen and oxygen atoms is the same on both sides of the equation. Therefore, the mass of the reactants (hydrogen and oxygen) is equal to the mass of the product (water).
Important Note: While the *amount* of matter remains constant, the *form* of matter changes during a chemical reaction.
