## why every gas would exert zero pressure at -273.15 degree celsius

I was reading about the experiment of constant volume gas thermometer today. When we extrapolate the presuure -temperature lines, the lines would meet at point -273.15 degree celsius, which is zero presure. I was wondering whether it is a matter of coincidence or some science behind it. The lines could have intercepted at some non-zero pressure. Please help me understanding this.
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 Quote by saurabhjain I was reading about the experiment of constant volume gas thermometer today. When we extrapolate the presuure -temperature lines, the lines would meet at point -273.15 degree celsius, which is zero presure. I was wondering whether it is a matter of coincidence or some science behind it. The lines could have intercepted at some non-zero pressure. Please help me understanding this.
That extrapolation is based on the assumption that the ideal gas scenario works all the way to that value. We know that this isn't true, because at some point, there will be a discontinuity in the form of a phase transition (try extrapolating the behavior of steam across 100 C).

Zz.
 But, still, it is no coincidence. The absolute temperature scale is defined so that intersection occurs at 0 K (absolute zero). The celcius scale is defined by the triple point of water, I believe, and that gives 0 Celcius at 273.15 Kelvin. Thus, you measure absolute zero at -273.15 C.

 Tags thermodynamics