Avogadro's Hypothesis

Science creates knowledge using two processes: 

  • by making observations (to collect empirical evidence and create scientific laws
  • by explaining the reason for those observations (to create and test hypotheses) 

Occasionally both processes are carried out by the same person; often however, the observations have occurred long before a satisfactory explanation for them is thought up. 

An hypothesis is a tentative (as opposed to a theory which is well tested) explanation for observed events. An hypothesis is not a prediction, but it must allow you to make predictions which can be tested by experiment. When the results of those experiments are as predicted, it lends support to the hypothesis as a good explanation, and its eventual acceptance as a theory. If the results are not as predicted, the hypothesis must be modified, or replaced with a better explanation. No statement is an hypothesis unless it suggests a cause for an effect, and unless it has the possibility of being wrong. 

Which of the following is not a characteristic of an hypothesis?
it lets us make predictions that can be experimentally tested
like a theory, it may be revised
when it is well established it will become a scientific law
it suggests a cause - effect relationship to explain why some event occurs

Truly revolutionary hypotheses, the kind that invalidate established beliefs, have often been initially rejected. This is especially true when a new idea contradicts that of well known scientists. Even scientists whose ideas were initially rejected accept this as the way science should work. For science is also full of examples of revolutionary ideas that did not stand up under the scrutiny of testing and verification. The non-discovery of cold fusion is probably one of the best known examples of this process in recent scientific history. Skepticism is one of the scientist's main tools

The late 1700's and early 1800's was a period of scientific revolution. Old ideas were crumbling, and new ones coming into place. It was an exciting time for chemistry. New concepts flowed out to explain the behavior of a new universe unfolding from Lavosier's combustion theory. New elements were being discovered, and new observations made that required explanation. Among those observations was the work of Joseph Louis Gay-Lussac (1778-1850), whose own father was arrested in 1793, just one year before Lavoisier lost his head to the guillotine in the French Revolution.  Antoine Lavoisier

Electrolysis of water One of the easiest ways to measure a gas is by volume (provided that we keep the temperature and pressure the same). In 1800 two English chemists, William Nicholson and Anthony Carlise had demonstrated that water could be decomposed by an electric current into two volumes of Hydrogen gas, and one volume of Oxygen -- a simple whole number ratio of 2:1.
Following up on this idea, Gay-Lussac observed that when gases combined in chemical reactions they did so in ratios, by volume, that were always simple whole numbers.  The following table shows this for a few reactions. 

Combining Volumes of Gases

Volume of Gas 1

Volume of Gas 2

Ratio of
Combining Volumes



















gaylussac.gif (42851 bytes)

This observation - simple whole number combining ratios by volume - became known as the "Law of combining volumes". These results certainly suggested that the formulas for water might be H2O, for ammonia NH3, and for Nitrogen dioxide NO2.

Consider the following combining gas volumes:  Chlorine 50 mL   Oxygen 25 mL
What formula is suggested by the combining ratio of these gases?
Cl2 ClO2  Cl5O2.5   Cl50O25     none of them

However, this observation was not accepted by everyone. John Dalton for one, considered Gay-Lussac's work wrong.  He believed water contained just one atom of hydrogen and one atom of oxygen ( HO.gif (170 bytes) in the symbols of the day).  In today's symbols the forumla would be HO . If this was the right formula for water, then both Gay-Lussac's law of combining volumes and Lavoisier's law of conservation of mass could not be right. This is illustrated in this diagram. 

Combing hydrogen and oxygen 
Obviously something must be wrong. If two volumes of hydrogen H1.gif (139 bytes) react with one volume of oxygen O1.gif (136 bytes) as shown to produce Dalton's HO HO.gif (170 bytes) molecules, then there would have to be some hydrogen left over. This would be a violation of the Law of Conservation of mass, and totally contradictory to Dalton's atomic theory. Dalton's response was to declare that only one volume of hydrogen reacted, and that Gay-Lussac's work was inaccurate.

Since we don't think water is HO today, Dalton's idea must have been all wet!   His conclusion that Gay-Lussac was wrong was not agreed to by everyone, including an Italian chemist, Amedeo Avogadro. 

avogadro.jpg (6155 bytes)Lorenzo Romano Amedeo Carlo Avogadro di Quareqa e di Carreto - Avogadro for short - was born in Turin, Italy in 1776, into a family of church lawyers. He too studied ecclesiastical law, earning his Bachelor's degree at the age of 16, and a Doctorate by 20. After three years he decided that a career in law was not to his bent, and he began to study science and mathematics in earnest. In 1809 he was appointed professor of Natural Philosopy at the Royal College of Vercelli in isolated Northern Italy. In 1811 he proposed his now famous hypothesis that equal volumes of gases, at the same temperature and pressure, contain equal numbers of molecules

The reaction between Hydrogen and Oxygen according to Avogadro is shown in the following diagram.  Notice that each container has the same number of molecules in it.  The ratio of the volumes is 2 hydrogen to 1 oxygen to 2 water, and there is nothing left over.

 Combining of hydrogen and oxygen

This proposal, published in the French Journal de Physique went neglected by the European scientific community. Partly because it was published in an obscure journal, partly because it totally contradicted the opinions of John Dalton, it took fifty years before Avogadro's work was recognized. 

Avogadro made the distinction between atoms and molecules, which today seems clear. However, Dalton rejected Avogadro's hypothesis because Dalton believed that atoms of the same kind could not combine. Since it was believed that atoms were held together by an electrical force, only unlike atoms would be attracted together, and like atoms should repel. Therefore it seemed impossible for a molecule of oxygen, O2, to exist. Avogadro's work, even if it was read appears not to have been understood, and was pushed into the dark recesses of chemistry libraries and ignored. Avogadro continued to teach at the university of Turin, when it was not closed because of the political upheavals going on in Italy at the time, and died in 1854, an unknown figure. 

Which of the following statements is true?
Equal volumes of gases contain equal numbers of particles
Molecules such as O2 cannot exist because their like particles would repel
According to Dalton the formula for ammonia would be NH
When Avogadro made up his hypothesis, he was a lawyer for the Catholic church

Without a clear distinction between atoms and molecules, European chemistry blundered ahead. By 1860 enough data had been collected on atomic weights to clearly indicate that something was seriously wrong. The world's first international conference on Chemistry was called at Karlsruhe, Germany in 1860 to try and resolve the issues. At this meeting, Stanislao Cannizaro, Avogadro's countryman, younger by fifty years, made a strong speech in which he explained the ideas of Avogadro that he had discovered two years earlier. He distributed copies of his "Sketch of a Course of Chemical Philosophy" which clearly showed the importance of Avogadro's principle, resolving the confusion with atomic and molecular weights. Very shortly, Avogadro's work was recognized for its brilliant deduction. 

With a clear understanding of how to measure atomic and molecular weights established, Chemistry proceeded rapidly. Less than a decade later, Dmitri Mendeleev who had also attended the Karlsruhe conference was able to use the newly established atomic weights to develop his periodic table. While chemistry is generally agreed to have started with Lavoisier, it could well be argued that modern chemistry started after the Karlruhe conference and the acceptance of Avogadro's hypothesis. 

In recognition of his idea, the number of fundamental particles in a mole of substance is called Avogadro's number. Although Avogadro had no idea what the number of particles in equal volumes of gases was, and did nothing to measure it, his hypothesis did lead to the eventual determination of this number, 6.02 x 1023 


Copyright 1998 - 2008 David Dice