\[0^{\circ} \: Celsius\]
is defined to be the freezing point of water at standard pressure \[p_0=1.1325 \times 10^5 N/m^2\]
\[100^{\circ} \: Celsius\]
is defined to be the boiling point of water at standard pressure \[p_0=1.1325 \times 10^5 N/m^2\]
The Kelvin scale is more fundamental. An increase of
\[1^{\circ} Celsius\]
is the same increase as an increase of \[1^{\circ} Kelvin\]
, but the zero of the Kelvin scale is defined as the temperature at which no more energy can be extracted from a system, and is equal to \[-273.16^{\circ} Celsius\]
.When doing calculations, typically we use the Kelvin scale eg when using the ideal gas equation of state
\[1 pV=nRT\]
.