We use 4 variables to describe the properties of a pure gas: P,V,T, n.
Recall that
P is pressure (SI unit is the Pascal or Pa)
V is volume (SI unit is m3)
T is temperature (SI unit is K)
n is number of moles (SI unit is the mole)
The early experimenters such as Boyle, Charles, and Avogadro fixed two of the variables while recording the value of the fourth as the third is varied.
Charles Law results from measurements of V vs T while holding both P and n fixed.
The following series of graphs indicates the kind of analysis that might have been made by the
French chemist, Jacques Charles (1746-1823), in arriving at the Law that bears his name.
1) Keep pressure, P, and amount of gas, n, constant. Observe trend of V decreasing as T decreases. Temperatures down to easily achievable "cold" &endash; brine solutions etc.
2) Use heroic efforts to reach lower temperatures. Eventually gas condenses to liquid at around &endash;196 °C.
3) Add a "best fit" line to the data. Points show a RANDOM rather than a SYSTEMATIC deviation from the straight line.
4) EXTRAPOLATE to where V=0. Value of T is about -273 °C.
5) Do same experiment at different pressures with the same amount of gas. All the extrapolations (dotted lines) converge on a point.
6) Same as 5) except define K= °C + 273 and plot vs new temperature variable. Kelvin or absolute temperature scale is the "natural" scale for all gas law problems.
"Yes, Virginia, there is a coldest cold!"
The new Kelvin scale with the zero of temperature defined as the extrapolation of the Charles' experiment data is the official SI scale.