Matter conservation law

The law of conservation of matter , law of conservation of mass , or Lomonósov-Lavoisier law is one of the fundamental laws of the natural sciences.

It was made by Mikhail Lomonósov (1711-1765) in 1748 and independently discovered four decades later by Antoine Lavoisier (1743-1794) in 1785. It can be stated as follows:

“The mass of a system remains unchanged whatever the transformation that occurs within it”; that is, “in chemical terms, the mass of the reacting bodies is equal to the mass of the products in reaction.”

This was stated in 1748 by Mikhail Lomonosov (36). In 1785, independently, the chemist Antoine Lavoisier (42) proposed that “matter is neither created nor destroyed: it is only transformed”. This is why the law of conservation of matter is often known as the Lavoisier-Lomonosov law.

It establishes a very important point: “In any chemical reaction, the mass is conserved, that is, the total mass of the reactants is equal to the total mass of the products.”


Combustion, one of the great problems of chemistry in the 18th century, aroused Lavoisier’s interest because he was working on an essay on improving the techniques of street lighting in Paris.

He found that by heating metals such as tin and lead in closed containers with a limited amount of air, they were covered with a layer of calcine until a certain moment when it did not advance any further.

If the assembly was weighed (metal, calcined, air, etc.) after heating, the result was equal to the weight before starting the process. If the metal had gained weight as it burned, it was obvious that something in the container must have lost the same amount of mass.

That something was air. Therefore, Lavoisier demonstrated that the calcination of a metal was not the result of the loss of the mysterious phlogiston, but the gain of something very material: a part of air.

The previous experience and others made by Lavoisier; They showed that if we take into account all the substances that are part of a chemical reaction and all the products formed, the mass never varies. This is the law of conservation of matter.


Mikhail Lomonósov discovered the law of conservation of matter.

The law can be stated as “In an ordinary chemical reaction the mass remains constant, that is, the mass consumed of the reactants is equal to the mass obtained from the products.”

A caveat that must be taken into account is the existence of nuclear reactions, in which the mass does change subtly, in these cases in the sum of masses the equivalence between mass and energy must be taken into account .

This law is fundamental for a proper understanding of chemistry. It is behind the usual description of chemical reactions using the chemical equation, and the gravimetric methods of analytical chemistry .

These scientists were referring to mass matter. Later it was observed that in some nuclear reactions there is a small variation in mass. However, this variation is explained by the theory of relativity of Albert Einstein , which proposes an equivalence of mass and energy . In this way, the variation of mass in some nuclear reactions would be complemented by a variation of energy , in the opposite direction, so that if a decrease in mass is observed, it is that it was transformed into energy, and if the mass increases , is that the energy was transformed into mass.

Taking into account the law of conservation of matter, when we write a chemical equation , we must adjust it so that it complies with this law. The number of atoms in the reactants must be equal to the number of atoms in the products. Fitting the equation is accomplished by placing stoichiometric indices in front of each molecule . The stoichiometric index is a number multiplied to the atoms of the substance in front of which it is placed.


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