Gay-Lussac's law states that the pressure of a given mass of gas varies directly with the absolute temperature of the gas, when the volume is kept constant.
Mathematically, it can be written as: P/T=constant,
Gay-Lussac is most often recognized for the Pressure Law which established that the pressure of an enclosed gas is directly proportional to its temperature and which he was the first to formulate (c. 1808). He is also sometimes credited  with being the first to publish convincing evidence that shows the relationship between the pressure and temperature of a fixed mass of gas kept at a constant volume.
Law of combining volumesEdit
The law of combining volumes states that, when gases react together to form other gases, and when all volumes are measured at the same temperature and pressure
The ratio between the volumes of the reactant gases and the gaseous products can be expressed in simple whole numbers.
For example, Gay-Lussac found that 2 volumes of hydrogen and 1 volume of oxygen would react to form 2 volumes of gaseous water. Based on Gay-Lussac's results, Amedeo Avogadro theorized that, at the same temperature and pressure, equal volumes of gas contain equal numbers of molecules (Avogadro's law). This hypothesis meant that the previously stated result
- 2 volumes of hydrogen + 1 volume of oxygen = 1 volume of gaseous water
could also be expressed as
- 2 molecules of hydrogen + 1 molecule of oxygen = 1 molecule of water.
The law of combining gases was made public by Joseph Louis Gay-Lussac in 1808. Avogadro's hypothesis, however, was not initially accepted by chemists until the Italian chemist Stanislao Cannizzaro was able to convince the First International Chemical Congress in 1860.
This law is often referred to as Gay-Lussac's law of pressure–temperature, between 1800 and 1802, discovered the relationship between the pressure and temperature of a fixed mass of gas kept at a constant volume. Gay Lussac discovered this while building an "air thermometer".
If a gas's temperature increases, then so does its pressure if the mass and volume of the gas are held constant. The law has a particularly simple mathematical form if the temperature is measured on an absolute scale, such as in kelvins. The law can then be expressed mathematically as
- P is the pressure of the gas,
- T is the temperature of the gas (measured in kelvins),
- k is a constant.
This law holds true because temperature is a measure of the average kinetic energy of a substance; as the kinetic energy of a gas increases, its particles collide with the container walls more rapidly, thereby exerting increased pressure.
For comparing the same substance under two different sets of conditions, the law can be written as:
Because Amontons discovered the law beforehand, Gay-Lussac's name is now generally associated within chemistry with the law of combining volumes discussed in the section above. Some introductory physics textbooks still define the pressure-temperature relationship as Gay-Lussac's law. Gay-Lussac primarily investigated the relationship between volume and temperature and published it in 1802, but his work did cover some comparison between pressure and temperature. Given the relative technology available to both men, Amontons was only able to work with air as a gas, where Gay-Lussac was able to experiment with multiple types of common gases, such as oxygen, nitrogen, and hydrogen. Gay-Lussac did attribute his findings to Jacques Charles because he used much of Charles's unpublished data from 1787 – hence, the law became known as Charles's law or the Law of Charles and Gay-Lussac.
Expansion of GasesEdit
Gay-Lussac used the formula ΔV/V = αΔT to define the rate of expansion α for gases. For air he found a relative expansion ΔV/V = 37.50% and obtained a value of α = 37.50%/100°C = 1/266.66°C which indicated that the value of absolute zero was approximately 266.66°C below 0°C . The value of the rate of expansion α is approximately the same for all gases and this is also sometimes referred to as Gay-Lussac's Law.
- Avogadro's law – Relationship between volume and number of moles of a gas at constant temperature and pressure.
- Boyle's law – Relationship between pressure and volume in a gas at constant temperature
- Charles's law – Relationship between volume and temperature of a gas at constant pressure
- Combined gas law
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- See also: Fontenelle, B. B. (1743) "Sur une nouvelle proprieté de l'air, et une nouvelle construction de Thermométre" (On a new property of the air and a new construction of thermometer), Histoire de l'Academie royale des sciences, 1–8.
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- Gay-Lussac (1802), "Recherches sur la dilatation des gaz et des vapeurs" (Researches on the expansion of gases and vapors), Annales de Chimie 43: 137–175. On page 157, Gay-Lussac mentions the unpublished findings of Charles: "Avant d'aller plus loin, je dois prévenir que quoique j'eusse reconnu un grand nombre de fois que les gaz oxigène, azote, hydrogène et acide carbonique, et l'air atmosphérique se dilatent également depuis 0° jusqu'a 80°, le cit. Charles avait remarqué depuis 15 ans la même propriété dans ces gaz ; mais n'avant jamais publié ses résultats, c'est par le plus grand hasard que je les ai connus." (Before going further, I should inform [you] that although I had recognized many times that the gases oxygen, nitrogen, hydrogen, and carbonic acid [i.e., carbon dioxide], and atmospheric air also expand from 0° to 80°, citizen Charles had noticed 15 years ago the same property in these gases; but having never published his results, it is by the merest chance that I knew of them.) Available in English at: Le Moyne College.
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