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Laws of Chemical Combination

Laws of Chemical Combination: Law of Conservation of Mass, Definite, Multiple Proportions, Gay Lussac’s Law of Gaseous Volumes and Avagadro’s Law

Chemistry usually involves chemical reactions where two or more elements combine to produce a single compound. The reactions involving the combination of various elements to produce a single compound are governed by the 5 laws of chemical combination.

The Five Laws of Chemical Combination are:

  1. Law of Conservation of Mass
  2. Law of Definite Proportions
  3. Law of Multiple Proportions
  4. Gay Lussac’s law of Gaseous Volumes
  5. Avagadro’s Law of Chemical Combination

Let us understand what these five laws have to say about various chemical combinations taking place in nature.

Law of Conservation of Mass

The law of conservation of mass was proposed by Antonie Lavoisier in 1789 and was based on results obtained from various combustion reactions.

According to the law of conservation of mass, “mass can neither be created nor destroyed, it can only be transformed from one form to another”.

In chemistry, the law of conservation of mass states that the total mass of the product obtained by chemical combination is equal to the sum of the mass of all reactants involved.

Example - The heating of calcium carbonate (10grams) produces calcium oxide (5.6 grams) and calcium dioxide (4.4 grams).

According to the law of conservation of mass,



Total mass of reactants = Total mass of products

Mass of calcium carbonate = Mass of calcium oxide + Mass of carbon dioxide

10.0 grams = 5.6 grams + 4.4 grams

10.0 grams = 10.0 grams

Law of Definite Proportions

Joseph Proust proposed the law of definite proportions in 1799. It is called Proust’s law or the law of constant proportions.

According to the law of definite proportions, elements are always combined in specific definite proportions only. It means that elements always combine in a specific ratio irrespective of the number of reactants taken or the source from which they were obtained.

Example - Ethyl alcohol (C2H5OH).

Irrespective of the total amount of ethyl alcohol present, its combination consists of 52% of carbon, 35% of oxygen, and 13% of hydrogen by weight. This is according to the law of constant proportions.

Law of multiple proportions

John Dalton proposed the law of multiple proportions in 1804.

According to the law of multiple proportions, Whenever the same two elements combine to form more than one compound, the various masses of an individual element that combine with the same mass of the other elements exist in the ratio of small whole numbers.

Example - Consider carbon and oxygen atoms. Carbon and oxygen atoms combine to give two different types of products - Carbon monoxide and carbon dioxide. In carbon monoxide, the amount of oxygen present for one carbon atom i.e. 12.0grams is 16.0 grams. In carbon dioxide, the amount of oxygen present for one carbon atom 12.0 grams is 32.0 grams. The ratio of the amount of carbon present in carbon monoxide to the amount of carbon present in carbon dioxide is 32:16 or 2:1.

Gay Lussac’s Law of Gaseous Volumes

Gay Lussac’s proposed the law of gaseous volumes in 1808.

According to Gay Lussac’s law of gaseous volumes, the ratio of the volumes of all gaseous reactants and products can be represented in small whole numbers. When gases react at a particular pressure and temperature, the ratio of volumes of reactants and products can easily be expressed in small whole numbers.

Example - The combination of hydrogen gas and chlorine gas is governed by Gay Lussac’s law of gaseous volumes.


H2 + Cl2 → 2 HCl

In this example, one volume of Hydrogen gas combines with one volume of chlorine gas to produce 2 volumes of HCl gas. The ratio of the volume of hydrogen gas, chlorine gas, and HCl is 1:1:2.

Avagadro’s Law of Chemical Combination

Avagadro proposed the fifth law of chemical combination in 1811.

According to Avagadro’s law of chemical combination, at a given constant temperature and pressure, the volume of gaseous molecules is directly proportional to the total number of moles. Avogadro’s law is expressed as follows.


V ∝ n

Where V is the volume of gaseous molecules,

And n is the total number of moles of the gas.

V = k × n

Where k is the proportionality constant.


Where V1 is the volume and n1 is the number of moles of one gas. And V2 is the volume and n2 is the number of moles of another gas.

Example - Blowing of helium gas into a balloon. As the total number of moles of helium gas blown into the balloon increases, the overall volume of the balloon also increases.



1. What is the Law of Chemical Combination?

  • The Law of Chemical Combination states that chemical compounds are formed when elements combine in fixed, definite proportions by mass.

2. Who formulated the Law of Chemical Combination?

  • The law was first proposed by Joseph Proust, a French chemist, in the late 18th century.

3. What does "fixed proportions" mean in the context of chemical combinations?

  • It means that the masses of elements in a compound are always in simple, whole-number ratios.

4. How does the Law of Chemical Combination relate to chemical reactions?

  • In a chemical reaction, reactants combine in fixed proportions to produce products. This law helps chemists predict the quantities of substances involved in reactions.

5. Can the Law of Chemical Combination be applied to all types of compounds?

  • Yes, the law is a fundamental principle applicable to all chemical compounds, whether they are simple or complex.

6. What is the difference between the Law of Multiple Proportions and the Law of Definite Proportions?

  • The Law of Definite Proportions states that a given compound always contains the same elements in the same fixed proportions by mass. The Law of Multiple Proportions, on the other hand, deals with different compounds formed from the same elements in different ratios.

7. How does the Law of Chemical Combination contribute to stoichiometry?

  • Stoichiometry involves calculating quantities of reactants and products in chemical reactions. The Law of Chemical Combination provides the foundational principles for these calculations.

8. Can the Law of Chemical Combination be broken in certain circumstances?

  • The law is considered a fundamental principle, and while there may be exceptions at the atomic and subatomic levels, it holds true for macroscopic chemical reactions.

9. How does the Law of Chemical Combination impact modern chemistry?

  • The law is a cornerstone of modern chemistry, forming the basis for understanding and predicting chemical reactions and stoichiometry.

10. Can the Law of Chemical Combination be visually represented?

  • Yes, chemical equations are visual representations that adhere to the Law of Chemical Combination. They show the balanced proportions of reactants and products in a chemical reaction.

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