Law of Conservation of Mass

Law of Conservation of Mass
The law of conservation of mass states that matter is neither created nor destroyed, but can change forms. This applies to a system that is closed for matter and energy.

The Law of Conservation of Mass is a fundamental concept in chemistry, stating that mass in an isolated system is neither created nor destroyed by chemical reactions or physical transformations. According to the law, the mass of the reactants in a chemical reaction equals the mass of the products. Further, the number and type of atoms in a chemical reaction is the same before and after the reaction.

Definition and Statement of the Law of Conservation of Mass

The Law of Conservation of Mass was first articulated by Antoine Lavoisier in the late 18th century. It asserts that the total mass of a closed system remains constant over time. This principle is widely applicable in chemical reactions and also applies to other disciplines.

Applicability of the Law

The law holds true in chemical reactions under ordinary conditions. This is because chemical reactions only involve electrons and do not affect the identities of the parts of the atom.

However, the Law of Conservation of Mass does not hold in nuclear reactions, where mass can convert into energy (and vice versa) according to the principle of mass-energy equivalence as proposed by Einstein in the theory of relativity. This conversion occurs in nuclear fission and fusion reactions and some forms of radioactive decay.

Also, the law applies to isolated systems. If matter or energy enters or exits a system, mass may not be conserved.

Historical Overview

The concept of mass conservation dates back to ancient Greece. Mikhail Lomonsov, outlined the principle in 1756. Lavoisier gets credit for formalizing the law in 1773. His work disproved the then-popular theory of phlogiston, a supposed fire-like element released during combustion. Lavoisier demonstrated that combustion results from chemical reactions with oxygen, not from releasing a mysterious substance, and that the mass before and after the reaction was the same.

Examples in Chemical Reactions

Chemical reactions clearly illustrate the Law of Conservation of Mass. Chemists apply the law in balancing chemical equations.

  • Combustion: In a simple combustion reaction, such as burning methane (CH₄), the total mass of methane and oxygen equals the mass of the resulting carbon dioxide and water.
    CH4​ + 2O2 ​→ CO2​ + 2H2​O (4 H, 1 C, 4 O atoms on each side of the reaction arrow.)
  • Synthesis: When hydrogen and oxygen gases react to form water, the mass of the two gases equals the mass of the water produced.
    2H2 ​+ O2​ → 2H2​O (4 H and 2 O on both sides of the reaction arrow.)

Examples in Organisms

In biological systems, the law applies to metabolic processes. For example, in photosynthesis, plants convert carbon dioxide and water into glucose and oxygen. The total mass of carbon dioxide and water used equals the mass of glucose and oxygen produced:

6 CO2 + 6 H2O → C6H12O6 + 6 O2

On a larger scale, the law applies to the mass of a human body, which encompasses numerous chemical reactions occurring at once. If you maintain a constant weight, the mass you gain from breathing, eating, and drinking equals the mass lost through breathing, perspiration, urination, and defecation.

Examples in Ecosystems

In ecosystems, the law is evident in nutrient cycles, such as the carbon cycle. Carbon atoms are conserved as they move through different components of the ecosystem, including the atmosphere, hydrosphere, lithosphere, and biosphere. For example, the photosynthesis reaction takes carbon from the air and fixes it into a glucose molecule. Photosynthesis does not create mass, nor is any lost in the process.


  • Okuň, Lev Borisovič (2009). Energy and Mass in Relativity Theory. World Scientific. ISBN 978-981-281-412-8.
  • Pomper, Philip (1962). “Lomonosov and the Discovery of the Law of the Conservation of Matter in Chemical Transformations”. Ambix. 10 (3): 119–127. doi:10.1179/amb.1962.10.3.119
  • Whitaker, Robert D. (1975). “An historical note on the conservation of mass”. Journal of Chemical Education. 52 (10): 658. doi:10.1021/ed052p658