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