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## Animated Charles and Gay-Lussac's Law

#### Glenn Research Center

A text version of this slide is also available.

Air is a gas which has various properties that we can observe with our senses, including the gas pressure, temperature (T), mass, and the volume (V) that contains the gas. Careful, scientific observation has determined that these variables are related to one another and that the values of these properties determine the state of the gas.

The relationship between temperature and volume, at a constant number of moles and pressure, is called Charles and Gay-Lussac's Law in honor of the two French scientists who first investigated this relationship. Charles did the original work, which was verified by Gay-Lussac. They observed that if the pressure is held constant, the volume is equal to a constant times the temperature.

In a scientific manner, we can fix any two of the four primary properties and study the nature of the relationship between the other two by varying one and observing the variation of the other. This slide shows a schematic "gas lab" in which we can illustrate the variation of the gas properties. In the lab a theoretical gas is confined in a blue container. The volume of the gas is shown in yellow and is determined by the position of a red piston. The volume can be changed by moving the red piston using the red screw at the top of the piston. The number of moles of the gas is indicated by the number of small black "molecules" in the volume. The number of moles can be changed by injecting or withdrawing molecules using the pump at the left. The pressure can be changed by adding or removing green weights from the top of the red piston, and the temperature can be changed by heating the container with the "torch" at the bottom. There are two probes inserted into the bottom of the container to measure the pressure and the temperature and display the results on the gages.

As the heat is added at the bottom of the vessel, the temperature increases and the volume increases. The relationship is plotted on the graph at the right.