17 julio, 2024

Law of conservation of matter: what it is and examples

What is the law of conservation of matter?

The law of conservation of matter or mass It is one that establishes that in any chemical reaction, matter is neither created nor destroyed. This law is based on the fact that atoms are indivisible particles in this type of reaction; while in nuclear reactions the atoms fragment, which is why they are not considered chemical reactions.

If atoms are not destroyed, then when an element or compound reacts the number of atoms before and after the reaction must be kept constant; which translates into a constant amount of mass between the reactants and products involved.

This is always the case if there is no leak that causes loss of matter; but if the reactor is hermetically sealed, no atom «disappears», and therefore the mass charged must equal the mass after the reaction.

If the product is solid, on the other hand, its mass will be equal to the sum of the reactants involved in its formation. Similarly, it occurs with liquid or gaseous products, but it is more prone to errors when measuring their resulting masses.

This law was born from experiments of past centuries, strengthened by the contributions of several famous chemists, such as Antoine Lavoisier.

Consider the reaction between A and B2 to form AB2 (top image). According to the law of conservation of matter, the mass of AB2 must be equal to the sum of the masses of A and B2, respectively. So if 37 g of A reacts with 13 g of B2, the product AB2 must weigh 50 g.

Therefore, in a chemical equation, the mass of the reactants (A and B2) must always be equal to the mass of the products (AB2).

An example very similar to the one just described is the formation of metallic oxides, such as rust or rust. Rust is heavier than iron (although it may not look like it), since the metal reacted with a mass of oxygen to create rust.

How does this law apply to a chemical equation?

The law of conservation of mass is of transcendental importance in stoichiometry, the latter being defined as the calculation of the quantitative relationships between reactants and products present in a chemical reaction.

The principles of stoichiometry were enunciated in 1792 by Jeremiah Benjamin Richter (1762-1807), who defined it as the science that measures the quantitative proportions or mass ratios of the chemical elements that are involved in a reaction.

In a chemical reaction there is a modification of the substances involved in it. It is observed that the reactants or reactants are consumed to originate the products.

During the chemical reaction there are bond breaks between the atoms, as well as the formation of new bonds; but the number of atoms involved in the reaction remains unchanged. This is what is known as the law of conservation of matter.

Basic principles

This Law implies two basic principles:

-The total number of atoms of each type is the same in the reactants (before the reaction) and in the products (after the reaction).

-The total sum of the electrical charges before and after the reaction remains constant.

This is because the number of subatomic particles remains constant. These particles are neutrons with no electrical charge, protons with a positive charge (+), and electrons with a negative charge (-). So the electric charge does not change during a reaction.

chemical equation

Having said the above, when representing a chemical reaction by means of an equation (like the one in the main image), the basic principles must be respected. The chemical equation uses symbols or representations of the different elements or atoms, and how they are grouped into molecules before or after the reaction.

The following equation will be used again as an example:

A + B2 => AB2

The subscript is a number that is placed to the right of the elements (B2 and AB2) at the bottom, indicating the number of atoms of an element present in a molecule. This number cannot be changed without the production of a new molecule, different from the original.

The stoichiometric coefficient (1, in the case of A and the rest of the species) is a number that is placed on the left side of the atoms or molecules, indicative of the number of them involved in a reaction.

In a chemical equation, if the reaction is irreversible, a single arrow is placed, indicating the direction of the reaction. If the reaction is reversible, there are two arrows pointing in the opposite direction. To the left of the arrows are the reactants or reactants (A and B2), while to the right are the products (AB2).


Balancing a chemical equation is a procedure that allows to equalize the number of atoms of the chemical elements present in the reactants with those of the products.

In other words, the number of atoms of each element has to be the same on the reactants side (before the arrow) and on the product side of the reaction (after the arrow).

It is said that when a reaction is balanced, the Law of Mass Action is being respected.

Therefore, it is essential to balance the number of atoms and the electrical charges on both sides of the arrow in a chemical equation. Likewise, the sum of the masses of the reactants must be equal to the sum of the masses of the products.

In the case of the represented equation, it is already balanced (equal number of A and B on both sides of the arrow).

Experiments proving the law

metal incineration

Lavoiser, observing the incineration of metals such as lead and tin in closed containers with a limited entry of air, noticed that the metals were covered with a calcinate; and also, that the weight of the metal at a given moment of heating was equal to the initial one.

Since an increase in weight is observed when a metal is incinerated, Lavoiser thought that the excess weight observed could be explained by a certain mass of something being extracted from the air during incineration. For this reason the mass remained constant.

This conclusion, which could be considered to have a weak scientific basis, is not such, given the knowledge that Lavoiser had about the existence of oxygen at the time he enunciated his Law (1785).

oxygen release

Oxygen was discovered by Carl Willhelm Scheele in 1772. Later, Joseph Priesley discovered it independently, and published the results of his research, three years before Scheele published his results on this same gas.

Priesley heated mercury monoxide and collected a gas that increased the brightness of the flame. In addition, when the mice were placed in a container with the gas, they became more active. Priesley called this dephlogisticated gas.

Priesley reported his observations to Antoine Lavoiser (1775), who repeated his experiments showing that the gas was found in air and in water. Lavoiser recognized the gas as a new element, naming it oxygen.

When Lavoisier used as an argument to state his law, that the excess mass observed in the incineration of metals was due to something that was extracted from the air, he was thinking of oxygen, an element that combines with metals during incineration.

Examples (practical exercises)

Decomposition of mercury monoxide

If 232.6 of mercury monoxide (HgO) is heated, it decomposes into mercury (Hg) and molecular oxygen (O2). Based on the law of conservation of mass and the atomic weights: (Hg = 206.6 g/mol) and (O = 16 g/mol), state the mass of Hg and O2 that is formed.

HgO => Hg + O2

232.6g 206.6g 32g

The calculations are very straightforward, since exactly one mole of HgO is being broken down.

Incineration of a magnesium ribbon

A 1.2 g magnesium ribbon was incinerated in a closed vessel containing 4 g oxygen. After the reaction, 3.2 g of unreacted oxygen remained. How much magnesium oxide was formed?

The first thing to calculate is the mass of oxygen that reacted. This can be easily calculated, using a subtraction:

Mass of O2 that reacted = initial mass of O2 – final mass of O2

(4 – 3.2) g O2

0.8g of O2

Based on the law of conservation of mass, the mass of MgO formed can be calculated.

Mass of MgO = mass of Mg + mass of O


2.0g MgO

Calcium hydroxide

A mass of 14 g of calcium oxide (CaO) reacted with 3.6 g of water (H2O), which was totally consumed in the reaction to form 14.8 g of calcium hydroxide, Ca(OH)2:

How much calcium oxide reacted to form calcium hydroxide?

How much calcium oxide was left over?

The reaction can be schematized by the following equation:

CaO + H2O => Ca(OH)2

The equation is balanced. Therefore, it obeys the law of conservation of mass.

Mass of CaO involved in the reaction = mass of Ca(OH)2 – mass of H2O

14.8g – 3.6g

11.2g CaO

Therefore, the CaO that did not react (the one that is left over) is calculated by subtracting:

Mass of excess CaO = mass present in the reaction – mass that took part in the reaction.

14 g of CaO – 11.2 g of CaO

2.8g CaO

Copper oxide

How much copper oxide (CuO) will be formed when 11 g of copper (Cu) completely reacts with oxygen (O2)? How much oxygen is needed in the reaction?

The first step is to balance the equation. The balanced equation is as follows:

2Cu + O2 => 2CuO

The equation is balanced, so it complies with the law of conservation of mass.

The atomic weight of Cu is 63.5 g/mol, and the molecular weight of CuO is 79.5 g/mol.

It is necessary to determine how much CuO is formed from the complete oxidation of the 11 g of Cu:

Mass CuO = (11 g Cu) ∙ (1mol Cu/63.5 g Cu) ∙ (2 mol CuO/2mol Cu) ∙ (79.5 g CuO/ mol CuO)

Mass of CuO formed = 13.77 g

Therefore, the difference in the masses between CuO and Cu gives the amount of oxygen involved in the reaction:

Oxygen mass = 13.77 g – 11 g

1.77g O2

Sodium chloride formation

A mass of chlorine (Cl2) of 2.47 g was reacted with sufficient sodium (Na) to form 3.82 g of sodium chloride (NaCl). How much Na reacted?

Balanced equation:

2Na + Cl2 => 2NaCl

According to the law of conservation of mass:

Mass of Na = mass of NaCl – mass Cl2

3.82g – 2.47g

1.35g Na


National Polytechnic Institute. (nd). Law of conservation of mass. CGFIE. Recovered from: aev.cgfie.ipn.mx
Law of Conservation of Mass. Retrieved from: thoughtco.com

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