What is Avogadro’s Hypothesis? Why is it Important?

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Avogadro's Hypothesis

Avogadro’s Hypothesis states that Equal volumes of all gases at the same temperature and pressure contain the same numbers of molecules.

As you can see, it is important here to be clear about the nature of the atom and the molecule. They define in the following way.

An atom of an element is the smallest particle of the element which can take part in a chemical reaction.

A molecule of an element or compound is the smallest particle of it which can exist separately.

That is, the Daltonian invisible atom is the smallest particle for participation in active chemical operations; the molecule is the smallest particle for inactive existence. The two are not necessarily the same particle in the case of any given gas, and in particular, the molecule may be polyatomic and so capable of splitting into individual atoms as required. Furthermore, this recognition of the occurrence of the two different particles, the atom, and the molecule, removed the difficulty which destroyed the atomic idea of Berzelius.

The Importance of Avogadro’s Hypothesis

As we all know, this hypothesis has been of the greatest value in the development of chemistry since about 1860. But it is a rather curious fact that its importance at first unnoticed. Full recognition of its implications, a few of which we shall now examine, due to work, not of Avogadro himself. But of another Italian, Cannizzaro, some forty-seven years after the hypothesis had been first put forward, and after Avogadro himself was dead.

The importance of the hypothesis lies in this fairly simple fact, that since it asserts that equal volumes of gases contain equal numbers of molecules.

Avogadro's Hypothesis

Avogadro’s hypothesis enables us to change over directly from a statement about volumes of gases to the same statement about one volume of any gas. We are also making a statement about a certain number of molecules of it. Also that number, by Avogadro’s Hypothesis is always the same. No matter what the gas may be. Consequently, we can change over at will, in any statement about gases, from volume to molecules and vice versa, if temperature and pressure are constant.

Avogadro’s Hypothesis Statements

So, this means that by applying the hypothesis to volume measurements of gases, we can probe right to the heart of a chemical reaction, to the actual molecules themselves. Also, It is an enormous step to change directly from an experiment statement like,

2 volume of hydrogen combine with 1 volume of oxygen giving 2 volumes of steam (temperature and pressure constant)


2 molecules of hydrogen combine with 1 molecule of oxygen giving 2 molecules of steam.

The second of these two statements goes right to the essentials of the reaction, to the very molecules themselves. So, Finally The hypothesis is important because it gives us this power to reveal the molecules themselves at work in chemical reactions.

Note: However, that it applies only to gases.

How Avogadro’s Hypothesis explains Gay-Lussac’s Law

Assumption: Temperature and Pressure are constant throughout the following paragraph.

As we all know, when gases react chemically, the reaction must take place between individual molecules of the gases. As Dalton suggested in a similar case of combination between atoms. The reactions will take place between small whole numbers of molecules of the reactants to produce small whole numbers of molecules of the products.

We have seen in the above that, employing Avogadro’s Hypothesis, we can change over directly from statements about molecules to statements about volumes. Also provided that gases only are concerned. Making this change, the last sentence of the last paragraph becomes, The reaction will take place between small whole numbers of volumes of the reactants to produce small whole numbers of volumes of the products (all being gases). So, this is what Gay-Lussac’s Law states. Hence, Avogadro’s Hypothesis has enabled us to deduce the experimentally observed law.

Hypothesis and the molecular weights of gases

The molecular weight of an element or compound is the number of times that 1 molecule of the element or compound is as heavy as 1 atom of hydrogen.

(Like atomic weight, this definition later switched from the standard H=1 to the standard O=16).

So, according to this equation, it is now necessary to find how the atom and the molecule pf hydrogen are related to one another. This will lead us to a method of determining molecular weights.

What evidence supports Avogadro’s Hypothesis

  1. One volume of hydrogen reacts with one volume of chlorine to form two volumes of hydrogen chloride. … (1)
  2. Two volumes of carbon monoxide react with one volume of oxygen to form two volumes of carbon dioxide. … (2)
Avogadro's Hypothesis

By applying the hypothesis to the statement (1), we deduce that one molecule of hydrogen combines with one molecule of chlorine to form two molecules of hydrogen chloride. Consequently, a molecule of hydrogen must contain at least two atoms of hydrogen. Also a molecule of chlorine at least two atoms also. Similarly, from statement (2), since one molecule of oxygen contributes to two molecules of carbon dioxide. It must contain at least two atoms of oxygen. Since no reactions found which required oxygen, hydrogen, or chlorine molecules to split into more than two atoms. It concluded that they were all diatomic molecules.

In the early 1800s, water was being electrolyzed into two volumes of hydrogen and one of oxygen. If molecules of hydrogen and oxygen each contain two-atom, the it follows that water must be H2O. (or possibly H4O2 or H6O3). After deciding that water was H2O, chemists able to use the combining mas ratio (2g H with 16g O) to begin a table of relative atomic masses. They arbitrarily set hydrogen at unity which made oxygen 16.


Because of the Hypothesis, the development of formulae for substances and the determination of atomic masses for elements went hand in hand. Often with quite complicated and indirect arguments needed to decide between alternatives. (such as whether red mercury oxide was HgO with an atomic mass of mercury being 200 or Hg2O with atomic mass 100.) Such determinations of formulae and atomic asses were major parts of chemistry throughout the 19th century.

So, from this article we hope that you gained a good knowledge about Avogadro’s Hypothesis and the importance of it.

Also Read – Chemical Bond in Chemistry

What do you mean by Avogadro hypothesis?

Equal volumes of all gases at the same temperature and pressure contain the same numbers of molecules.

Who invented avogadro’s number

Josef Loschmidt

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