Law of Conservation of Mass

The law of conservation of mass states that mass within a closed system remains the same over time. Discover more about the law of conservation of mass, including its importance, equations, and some examples of this law in action.

Table of Contents

What is the 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 a few assumptions from classical mechanics which define mass conservation. Later the law of conservation of mass was modified with the help of quantum mechanics and special relativity that energy and mass are one conserved quantity. In 1789, Antoine Laurent Lavoisier discovered the law of conservation of mass.

Formula of Law of Conservation of Mass

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

\(\begin{array}{l}\frac{\partial \rho }{\partial t}+\bigtriangledown (\rho v)=0\end{array} \)


  • 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 the law of conservation of mass is followed by the above reaction.

Frequently Asked Questions – FAQs


Why is there no change in mass during chemical reactions?

During a chemical reaction, atoms are neither created nor destroyed. The atoms of the reactants are just rearranged to form products. Hence, there is no change in mass in a chemical reaction.


Verify law of conservation of mass with an experiment

According to the law of conservation of mass, during any physical or chemical change, the matter is neither created nor destroyed. However, it may change from one form to another. Below, we have listed an experiment that will help you verify the law of conservation of mass.
Requirements: H-shaped tube, also known as Landolt’s tube; Sodium chloride solution; silver nitrate solution.
Procedure: Sodium chloride solution is taken in one limb of the H-tube and silver nitrate solution in the other limb as shown in the figure. Both the limbs are now sealed and weighed. Now the tubes are averted so that the solutions can mix up together and react chemically. The reaction takes place and a white precipitate of silver chloride is obtained. The tube is weighed after the reaction has taken place. The mass of the tube is found to be exactly the same as the mass obtained before inverting the tube. This experiment clearly verifies the law of conservation of mass.


If energy is neither created nor destroyed, what is the ultimate source of energy?

The ultimate source of energy in our present universe is the Big Bang. All the energy was created at the beginning of time and as the universe grew several stages of particulate matter developed, produced from that energy. By the time of the Modern Universe, the energy was distributed either into mass, or kinetic energy or chemical energy in lumps of matter, or radiant energy. The masses are classified into galaxies and stars within them. The sun is one of those stars and got the energy from the primordial Big Bang.

What happens to the mass of a burned object?

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