Chemistry · Ch 1 — Some Basic Concepts of Chemistry
Law of Conservation of Mass
Law of Conservation of Mass
The Law of Conservation of Mass
The law of conservation of mass was put forward by the French chemist Antoine Lavoisier in 1789. Before Lavoisier, chemistry was largely qualitative — alchemists and early chemists had not yet developed the habit of precise measurement. Lavoisier changed that. He designed careful, quantitative experiments, especially on combustion reactions, and weighed everything — every reactant and every product — with extraordinary care.
What he found was striking. In every chemical reaction he studied, the total mass of the products was exactly equal to the total mass of the reactants. Nothing was lost; nothing was gained. The mass remained unchanged throughout the process.
This led to the fundamental statement: In all physical and chemical changes, there is no net change in mass during the process. Matter can neither be created nor destroyed. This is the law of conservation of mass.
The law of conservation of mass states: Mass can neither be created nor destroyed in a chemical reaction. The total mass of the reactants equals the total mass of the products.
The Experimental Foundation
Lavoisier's work was revolutionary not because he guessed this idea — others before him had speculated about the indestructibility of matter — but because he proved it through exact measurement. He used sealed apparatus, carefully controlled conditions, and precise balances. For example, when he burned phosphorus or mercury in a closed container, the total mass before and after the reaction was identical. If the container was opened, the mass changed because gases escaped or entered — but in a closed system, mass was always conserved.
This was the first clear, experimentally verified quantitative law in chemistry. It laid the groundwork for everything that followed: the law of definite proportions, Dalton's atomic theory, and the entire edifice of stoichiometry.
Mathematical Formulation
If we have a chemical reaction:
Then the law of conservation of mass states:
For a reaction with reactants and products:
where are the masses of the individual reactants and are the masses of the individual products.
A common mistake is to think the law applies only to solids or liquids. It applies to all substances involved in a reaction — including gases. If a gas is produced (like carbon dioxide from burning charcoal) and escapes, the remaining solid appears to lose mass. But if you collect and weigh that gas, the total mass is conserved. The law holds only for a closed system — one where nothing enters or leaves.