Ontario Grade 9 Science Class Practice Exam

The law of conservation of mass states that in a closed system, the total mass remains constant during a chemical reaction. This means that the mass of the reactants (the substances that undergo the reaction) will be equal to the mass of the products (the substances that are formed) after the reaction has taken place.

This principle is foundational in chemistry because it establishes that matter cannot be created or destroyed in a chemical reaction; it can only change forms. For example, when hydrogen gas reacts with oxygen gas to form water, the total mass of the hydrogen and oxygen before the reaction equals the mass of the water produced. Understanding this law is crucial for balancing chemical equations and studying stoichiometry, which involves the calculation of reactants and products in chemical reactions.

The incorrect options misrepresent the relationship between mass and chemical reactions either by suggesting creation or destruction of mass, which contradicts the core tenet of conservation, or implying that mass changes form but doesn't remain equal, which also contradicts the requirement for mass to balance before and after a reaction.