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I can't find an answer of why the lowest temperature is -273.15ºC. Is it deduced theoretically or is it experimental?

An explanation is that when any gas volume tends to zero, the temperature will be -273.15ºC (Charles law). But shouldn't this number have some kind of error? And this would only apply to ideal gases.

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Well the real question should be why is there a °C (Celsius).

The Celsius scale is a "Centigrade" scale in that it uniformly divides the temperature range between the boiling point of water, and the freezing point of water into 100 equal parts, and then it arbitrarily calls the freezing point zero °C, and the boiling point becomes 100°C.

The Kelvin scale is referenced to the triple point of water, not the freezing point, and that Temperature is about 0.1°C (it might be 0.098°C but I am not sure about that).

Quite arbitrarily, it was decided that degrees on the Kelvin scale, should be identical in size to Celsius degrees, and experimentally the zero on the Kelvin scale (zero kelvins) is 273.16 Celsius degrees below the triple point of water, which makes it also -273.15°C

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    $\begingroup$ You say "experimentally the zero on the Kelvin scale (zero kelvins) is 273.16". But doesn't every measurement have some error +-something? $\endgroup$ – jinawee Sep 30 '13 at 19:45
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    $\begingroup$ @jinawee the kelvin (and the deg centrigrade) is defined such that the triple point of water is 273.16K. So 0k is fixed by definition, if measurements change the triple point it will just change the size of a degree. The same thing happens with 'c' and the metre $\endgroup$ – Martin Beckett Sep 30 '13 at 20:50
  • $\begingroup$ @MartinBeckett Thanks, that's what I wanted to know. Why does 'c' have so many significant figures compared to 273.16? $\endgroup$ – jinawee Oct 1 '13 at 7:30
  • $\begingroup$ @jinawee because we had an existing definition of the meter which was very accurately measured, so the new one had to fit that. If we made a new 'm' now it would probably be c=300,000,000 m/s. We did this with the inch which is now 25.4mm exactly everywhere. $\endgroup$ – Martin Beckett Oct 1 '13 at 16:35
  • $\begingroup$ I'm in general agreement with most of what the other commenters have said. Yes, experimental measurements do have uncertainties in them, and those uncertainties have to be considered. In the case of the meter; and by inference, the value of "c" it was considered important to try and maintain historical continuity. Eventually, metrology became so accurate, that it was clearly absurd, to try and measure a scratch on a metal bar. Eventually, c and its components, mu,nought and epsilon,nought were defined as absolutes with no error. So is the triple point of water, at 273.16 K $\endgroup$ – user26165 Oct 1 '13 at 18:55
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"Kelvin" and "degrees Celsius" are defined such that there are 273.16 degrees between absolute zero and the triple point temperature of water. Degrees Celsius are defined as $K - 273.15$.

These definitions have been in place since 1967.

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It is indeed defined as the temperature for which an ideal gas reaches a zero volume. Or that's how the definition is based. This number has an error altough I can't seem to find it anywhere, they probably did a lot of experiments so that the error is much smaller than the value -273.15°C that's used for 0K.

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