How it was decided that the freezing point of water in the Kelvin scale of temperature should be 273.15 K? [closed]








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While discussing about the different temperature scales, our teacher told us about the freezing point and boiling point of water in different scales. I asked my teacher that how these numbers were decided, like why it was decided that 273.15 should be the freezing point of water in Kelvin scale and not some other number, but could not get a satisfactory answer. So, here I am requesting if anyone could tell me the reasons for giving the different specific numbers in different scales for the same temperature, in the simplest way possible.




**History: the Celsius scale was defined first; the zero on the Kelvin scale is dependent on that definition**


It may seem awkward and annoying that the zero point on the Kelvin scale (and therefore the freezing point of water) isn't an exact number, but it is a consequence of history.


There used to be a variety of different temperature scale in the early history of chemistry. The Fahrenheit scale annoyingly used by a small number of countries to confuse science students, is one legacy of that era. And this matters because of how the scales were defined.


The original definition of the celsius scale was based on the freezing point and boiling point of pure water under very carefully defined conditions. The difference between the two temperatures was divided into 100 for convenience (it is certainly more convenient than the Fahrenheit scale which divides that interval into 180 parts though the original definition of the endpoints was more complicated and convoluted). The Kelvin scale came later but uses exactly the same interval as the celsius scale. The bp was set at 100 celsius and the freezing point at 0 celsius.


But this definition occured long before we knew there *was* an absolute lower limit for temperature. And it was based on empirical observations. When chemists and physicists got to grips with thermodynamics and started to explore really low temperatures, it was too late to define a new scale with different definitions. So when experiments revealed that there was a lower limit, absolute zero, they could only measure what that number was on the celsius scale as it made little sense to devise a new scale and mess with all the existing definitions.


Hence the awkward value of absolute zero and the melting point of water. The Kelvin scale, for convenience, simply adds the empirical value of absolute zero to the celsius temperature, which minimises any need to redefine anything else and therefore all normal temperature measuring devices.


So the freezing point of water was not *decided* to be 273.15, it was decided that it was 0 in celsius and, when absolute zero was measured and the new Kelvin scale set, that *demanded* that the freezing point of water had that value in the new scale.


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