Avogadro's law

Principle

In 1811, based on the similar behaviour of (ideal) gases when subjected to temperature and pressure increases (see gas laws) and on Gay-Lussac's law of volumes, Avogadro formulated the hypothesis that under the same pressure and temperature conditions, identical volumes must contain the same number of gas particles (molecules, atoms). If that conjecture by Avogadro is correct, then a quantity of gas composed of six parts by volume of CO and three parts by volume of O₂ will react to form six parts by volume of CO₂ if the pressure and temperature before the reaction are the same as the pressure and temperature after the reaction. When heated up to a little over 100°C, a gas mixture composed of six parts by volume of H₂ and three parts by volume of O₂ must also form six parts by volume of water vapour, and a mixture composed of five parts by volume of H₂ and five parts by volume of Cl₂ must form ten parts by volume of hydrogen chloride gas. The correctness of this hypothesis is examined experimentally based on the reactions mentioned.

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