Pressure 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). He is also sometimes credited, rightfully according to many modern scholars, 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.

These laws are also known variously as the Pressure Law or Amontons's law and Dalton's law respectively.

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

could also be expressed as

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 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. 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.

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