Law of Conservation of Mass

What is Law of Conservation of Mass?

The law of conservation of mass states that

The mass in an isolated system can neither be created nor be destroyed but can be transformed from one form to another.

According to the law of conservation of mass, the mass of the reactants must be equal to the mass of the products for a low energy thermodynamic process.

It is believed that there are few assumptions from classical mechanics which define mass conservation. Later the law was modified with the help of quantum mechanics and special relativity that energy and mass are one conserved quantity. Antoine Laurent Lavoisier discovered the law of conservation of mass.

Formula of Law of Conservation of Mass

Conservation of mass can be expressed in the differential form using the continuity equation in fluid mechanics and continuum mechanics as:

\(\frac{\partial \rho }{\partial t}+\bigtriangledown (\rho v)=0\)


  • ρ is the density
  • t is the time
  • v is the velocity
  • ⛛ is the divergence

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Law of Conservation of Mass Examples

  • Combustion process: Burning of wood is a conservation of mass as the burning of wood involves Oxygen, Carbon dioxide, water vapor and ashes.
  • Chemical reactions: To get one molecule of H2O (water) with the molecular weight of 10, Hydrogen with molecular weight 2 is added with Oxygen whose molecular weight is 8, thereby conserving the mass.

Law of Conservation of Mass Problems

Q1. 10 grams of calcium carbonate (CaCO3) produces 3.8 grams of carbon dioxide (CO2) and 6.2 grams of calcium oxide (CaO). Represent this reaction in terms of law of conservation of mass.
Ans: According to law of conservation of mass:
Mass of reactants = Mass of products
∴ 10 gram of CaCO3 = 3.8 grams of CO2 + 6.2 grams of CaO
10 grams of reactant = 10 grams of products

Hence, it is proved that law of conservation of mass is followed by the above reaction.


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