| Part of a series on | ||||
| Continuum mechanics | ||||
|---|---|---|---|---|
Laws
| ||||
 | This articlemay lack focus ormay be aboutmore than one topic. In particular, it covers multiple laws with the same name. Pleasehelp improve this article, possibly bysplitting the article or organising adisambiguation page. There might be further discussion about this on thetalk page.(May 2025) |
Gay-Lussac's law usually refers toJoseph-Louis Gay-Lussac's law of combining volumes ofgases, discovered in 1808 and published in 1809.[1] However, it sometimes refers to the proportionality of thevolume of a gas to itsabsolute temperature at constantpressure. The latter law was published by Gay-Lussac in 1802,[2] but in the article in which he described his work, he cited earlier unpublished work from the 1780s byJacques Charles. Consequently, the volume-temperature proportionality is usually known asCharles's law.
The law of combining volumes states that when gases chemically react together, they do so in amounts by volume which bear small whole-number ratios (the volumes calculated at the same temperature and pressure).
The ratio between the volumes of the reactant gases and the gaseous products can be expressed in simplewhole numbers.
For example, Gay-Lussac found that two volumes of hydrogen react with one volume of oxygen to form two volumes of gaseous water. Expressed concretely, 100 mL of hydrogen combine with 50 mL of oxygen to give 100 mL of water vapor: Hydrogen(100 mL) + Oxygen(50 mL) = Water(100 mL). Thus, the volumes of hydrogen and oxygen which combine (i.e., 100mL and 50mL) bear a simple ratio of 2:1, as also is the case for the ratio of product water vapor to reactant oxygen.
Based on Gay-Lussac's results,Amedeo Avogadro hypothesized in 1811 that, at the same temperature and pressure, equal volumes of gases (of whatever kind) contain equal numbers of molecules (Avogadro's law). He pointed out that if this hypothesis is true, then the previously stated result
could also be expressed as
The law of combining volumes of gases was announced publicly byJoseph Louis Gay-Lussac on the last day of 1808, and published in 1809.[3][4] Since there was no direct evidence for Avogadro's molecular theory, very few chemists adopted Avogadro's hypothesis as generally valid until the Italian chemistStanislao Cannizzaro argued convincingly for it during theFirst International Chemical Congress in 1860.[5]
In the 17th centuryGuillaume Amontons discovered a regular relationship between the pressure and temperature of a gas at constant volume. Some introductory physics textbooks still define the pressure-temperature relationship as Gay-Lussac's law.[6][7][8] 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.[9] Given the relative technology available to both men, Amontons could only work with air as a gas, whereas Gay-Lussac was able to experiment with multiple types of common gases, such as oxygen, nitrogen, and hydrogen.[10]
Regarding the volume-temperature relationship, Gay-Lussac attributed his findings toJacques Charles because he used much of Charles's unpublished data from 1787 – hence, the law became known asCharles's law or thelaw of Charles and Gay-Lussac.[11]
Amontons's,Charles', andBoyle's law form thecombined gas law. These three gas laws in combination withAvogadro's law can be generalized by theideal gas law.
Gay-Lussac used the formula acquired from 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 ofabsolute zero was approximately 266.66 °C below 0 °C.[12] The value of the rate of expansion α is approximately the same for all gases and this is also sometimes referred to asGay-Lussac's law. See the introduction to this article, andCharles's law.
