The law of conservation of mass states that in any chemical reaction, mass is neither created nor destroyed. This means the total mass of the reactants (starting substances) is always equal to the total mass of the products (ending substances).
Although atoms may be rearranged to form new substances during a reaction, no atoms are lost or gained—they are simply recombined. This principle was first clearly formulated by Antoine Lavoisier in the 18th century and remains a foundational concept in chemistry.
For example, when hydrogen reacts with oxygen to form water, the mass of the resulting water equals the combined mass of hydrogen and oxygen used:
- 2 g of hydrogen + 16 g of oxygen = 18 g of water
This law ensures that chemical equations must always be balanced, reflecting that matter is conserved in all physical and chemical processes.