[cha236]

Hi, I am reading the book Physical Chemistry by Levine. There is one True or false problem, that on the Celsius scale, the boiling point of water is slightly less than 100.00°C. Why this is true? I think on that scale, the boiling point of water is defined as 100.00°C.

[pgk]

The boiling point of water is defined as 100.00°C at 1 atm (sea level) that once upon a time in Europe, was equal to 1 kp/cm2 = 98066.5 Pa.
Today, SI is the official unit system in European countries. According SI, the atmospheric pressure is 1 atm = 101325 Pa = 101325 N/m2 (sea level at Paris latitude). Therefore, slight differences of boiling points that are found in the literature, depend on the source, the edition time and the country of origin.

[Enthalpy]

No trace about the technical atmosphere (kgf/cm² having been ever used to define the boiling temperature, at least there
http://en.wikipedia.org/wiki/Degree_Celcius
telling that water's boiling opint at 1 standard atmosphere (101325Pa) was used from the beginning (1744) to 1954.

But in 1954, the Celsius scale was defined by the absolute zero and by the triple point (0.01°C=273.16K), so that the boiling point doesn't define the Celsius scale any more. The present 99.9839 °C at one standard atmosphere must result from this change of scale.

At least, that explanation fits a boiling point below +100°C.

[pgk]

Of course and regardless the pressure dependence, there was no correlation between of the boiling point of water with the exact atmospheric pressure, at 1744. That happened many years, later. But in 1954, during the 10th general conference of weights and measures, the old MKS and MKSA unit systems, were abandoned and replaced by the new international unit system (SI). This created  a few minimal side effects in some fundamental units and completely omitted the empirical units. The official adoption of SI unit system by the most European states, took place during the 80s-90s

[cha236]

Thanks!