# Gas Laws: Pressure, Volume, and Temperature

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## 4.3 Absolute Temperature

Absolute zero is defined as the temperature at which all motion ceases. When viewing Figure 4.2, we might assume that we could extrapolate the plot line of the gas’s volume versus the absolute temperature until the volume is equal to zero. This was initially hypothesized in 1848 and is plotted in Figure 4.3. Figure 4.3 Extrapolation to zero volume versus temperature to determine absolute zero

Using the data available at that time, the value of absolute zero was estimated to be –266.66°C compared to the actual value of –273.15°C. In reality, we cannot reach a volume of zero because atoms and molecules do occupy a finite amount of space.

While it may seem logical that the concept of a temperature of absolute zero came from the gas laws, in reality it came from the Second Law of Thermodynamics, which defines an absolute, thermodynamic temperature that is independent of the properties of any particular thermometric body. The law involves the concept of entropy, which is a measure of the disorder of a system. In essence, it prohibits us from getting more work, which is energy, out of a system than has been put into that system. Thus, it prohibits perpetual motion machines. These terms may seem strange—and they are. They are discussed in much more detail in Chapter 5, “Thermodynamics: Energy, Heat, and Work.”