Gay Lussac's Law

Gay-Lussac's law can refer to several discoveries made by French chemist Joseph Louis Gay-Lussac (1778–1850) and other scientists in the late 18th and early 19th centuries pertaining to thermal expansion of gasses and the relationship between temperature, volume, and pressure.
He 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).^[1] He is also sometimes credited, rightfully according to many modern scholars,^[2][3][4] with being the first to publish convincing evidence that, in Gay-Lussac’s words, "All gases have the same mean thermal expansivity at constant pressure over the same range of temperature", or when heated, a wide variety of gases respond in the same predictable way.^[3


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 = 2 volumes of gaseous water

could also be expressed as

2 molecules of hydrogen + 1 molecule of oxygen = 2 molecules of water.

The law of combining gases was made public by Joseph Louis Gay-Lussac in 1808.^[6][7] 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.^[8]

Pressure-temperature law

This law is often referred to as Amontons's law of pressure–temperature after Guillaume Amontons, who, between 1700 and 1702, discovered the relationship between the pressure and temperature of a fixed mass of gas' kept at a constant volume.^[9][10][11] Amontons discovered this while building an "air thermometer".

The pressure of a gas of fixed mass and fixed volume is directly proportional to the gas's absolute temperature.

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

${\displaystyle {P}\propto {T},}$

or

${\displaystyle {\frac {P}{T}}=k,}$

where:

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:

${\displaystyle {\frac {P_{1}}{T_{1}}}={\frac {P_{2}}{T_{2}}}\qquad {\text{or}}\qquad P_{1}T_{2}=P_{2}T_{1}.}$

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.^[12][13] 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.^[14] 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.^[15] 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.^[16] However, in recent years the term has fallen out of favor.
Gay-Lussac's (Amontons') law, Charles's law, and Boyle's law form the combined gas law. These three gas laws in combination with Avogadro's law can be generalized by the ideal gas law.