Final answer:
The total mass of substances before a chemical change is always equal to the total mass of the substances after the change, as stated by the law of conservation of mass.
Step-by-step explanation:
According to the law of conservation of mass, the total mass of substances before a change is always equal to the total mass of the substances after the change. This law indicates that matter cannot be created or destroyed in a chemical reaction. Therefore, during any chemical process, the mass of the reactants (the starting substances) will always remain equal to the mass of the products (the substances that are formed).
For example, when charcoal burns in oxygen, the mass of the charcoal and oxygen together is equal to the mass of the carbon dioxide, water vapor, and ash produced. This principle is fundamental to modern chemistry and confirms that in an isolated system, mass is neither created nor destroyed, but is simply rearranged or changes form.