Conservation of mass

Science

Definition

The law of conservation of mass states that matter cannot be created or destroyed in a chemical reaction. The total mass of reactants must equal the total mass of products, which is why chemical equations must be balanced.

Examples

  • When wood burns, the mass of ash plus gases produced equals the mass of wood plus oxygen consumed
  • In the reaction 2H₂ + O₂ → 2H₂O, 4 g of hydrogen plus 32 g of oxygen yields exactly 36 g of water
  • Antoine Lavoisier demonstrated this law by carefully weighing reactants and products in sealed containers
Key Fact

Mass of reactants = Mass of products in every chemical reaction.

Related Terms

Balancing equations Stoichiometry Mole concept Reaction types

Study This Concept

Practice conservation of mass with free review games in these units:

Chemistry Unit 4: Chemical Reactions and Equations