Sometimes when matter is combined,_a chemical occurs causing molecules in the mixture to change. For example, mixing baking soda and vinegar together causes a reaction that cannot be_ ; a new substance is made. Even though the original matter (baking soda and vinegar) has changed, the mass of the new matter will remain the same. This phenomenon is known as " _", which states that matter can be changed chemically, but the total_ of the products will remain the same. Another example of how this law works is when steel wool_ . This happens when iron chemically reacts with_ and changes form. In a closed system, the mass of these two elements (uncombined) would be equal to the mass of the rusted product (combined). In a closed system, a chemical reaction creates a new substance when the atoms of the original substances reassemble in new arrangements.